| Message ID | 87tvzmqwoi.fsf@xmission.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 9/28/2017 3:34 PM, Eric W. Biederman wrote: > It looks like once upon a time a long time ago selinux copied code > from cap_inode_removexattr and cap_inode_setxattr into > selinux_inode_setotherxattr. However the code has now diverged and > selinux is implementing a policy that is quite different than > cap_inode_setxattr and cap_inode_removexattr especially when it comes > to the security.capable xattr. What leads you to believe that this isn't intentional? It's most likely the case that this change occurred as part of the first round module stacking change. What behavior do you see that you're unhappy with? > > To keep things working Which "things"? How are they not "working"? > and to make the comments in security/security.c > correct when the xattr is securit.capable, call cap_inode_setxattr > or cap_inode_removexattr as appropriate. > > I suspect there is a larger conversation to be had here but this > is enough to keep selinux from implementing a non-sense hard coded > policy that breaks other parts of the kernel. Specifics, please. Since I can't guess what problem you've encountered I can't tell if it's here, in the infrastructure, or in your perception of what constitutes "broken". > > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> > --- > security/selinux/hooks.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index f5d304736852..edf4bd292dc7 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct dentry *dentry, const char *name, > u32 newsid, sid = current_sid(); > int rc = 0; > > + if (strcmp(name, XATTR_NAME_CAPS) == 0) > + return cap_inode_setxattr(dentry, name, value, size, flags); > + No. Don't even think of contemplating considering embedding the cap attribute check in the SELinux code. cap_inode_setxattr() is called in the infrastructure. > if (strcmp(name, XATTR_NAME_SELINUX)) > return selinux_inode_setotherxattr(dentry, name); > > @@ -3282,6 +3285,9 @@ static int selinux_inode_listxattr(struct dentry *dentry) > > static int selinux_inode_removexattr(struct dentry *dentry, const char *name) > { > + if (strcmp(name, XATTR_NAME_CAPS) == 0) > + return cap_inode_removexattr(dentry, name); > + > if (strcmp(name, XATTR_NAME_SELINUX)) > return selinux_inode_setotherxattr(dentry, name); > . -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, 2017-09-28 at 17:34 -0500, Eric W. Biederman wrote: > It looks like once upon a time a long time ago selinux copied code > from cap_inode_removexattr and cap_inode_setxattr into > selinux_inode_setotherxattr. However the code has now diverged and > selinux is implementing a policy that is quite different than > cap_inode_setxattr and cap_inode_removexattr especially when it comes > to the security.capable xattr. > > To keep things working and to make the comments in > security/security.c > correct when the xattr is securit.capable, call cap_inode_setxattr > or cap_inode_removexattr as appropriate. > > I suspect there is a larger conversation to be had here but this > is enough to keep selinux from implementing a non-sense hard coded > policy that breaks other parts of the kernel. Originally SELinux called the cap functions directly since there was no stacking support in the infrastructure and one had to manually stack a secondary module internally. inode_setxattr and inode_removexattr however were special cases because the cap functions would check CAP_SYS_ADMIN for any non-capability attributes in the security.* namespace, and we don't want to impose that requirement on setting security.selinux. Thus, we inlined the capabilities logic into the selinux hook functions and adapted it appropriately. When the stacking support was introduced, it had to also special case these hooks so that only the primary module's hook is used for the same reason; otherwise, the kernel would end up applying a CAP_SYS_ADMIN check on setting security.selinux. Your change below is almost but not quite right since it only calls the cap functions when setting the capability attribute; the residual problem is that it will then skip the SELinux FILE__SETATTR (file setattr) permission check when setting those attributes, which we want to retain. So you need to only return early if cap_inode_setxattr()/removexattr() return an error; otherwise, you need to proceed to the SELinux check, and you can then delete the duplicated logic from selinux_inode_setotherxattr(). At which point it just becomes a call to dentry_has_perm() and you can just inline that into selinux_inode_setxattr() and selinux_inode_removexattr(). > > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> > --- > security/selinux/hooks.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index f5d304736852..edf4bd292dc7 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct dentry > *dentry, const char *name, > u32 newsid, sid = current_sid(); > int rc = 0; > > + if (strcmp(name, XATTR_NAME_CAPS) == 0) > + return cap_inode_setxattr(dentry, name, value, size, > flags); > + > if (strcmp(name, XATTR_NAME_SELINUX)) > return selinux_inode_setotherxattr(dentry, name); > > @@ -3282,6 +3285,9 @@ static int selinux_inode_listxattr(struct > dentry *dentry) > > static int selinux_inode_removexattr(struct dentry *dentry, const > char *name) > { > + if (strcmp(name, XATTR_NAME_CAPS) == 0) > + return cap_inode_removexattr(dentry, name); > + > if (strcmp(name, XATTR_NAME_SELINUX)) > return selinux_inode_setotherxattr(dentry, name); > -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: > On 9/28/2017 3:34 PM, Eric W. Biederman wrote: > > It looks like once upon a time a long time ago selinux copied code > > from cap_inode_removexattr and cap_inode_setxattr into > > selinux_inode_setotherxattr. However the code has now diverged and > > selinux is implementing a policy that is quite different than > > cap_inode_setxattr and cap_inode_removexattr especially when it > > comes > > to the security.capable xattr. > > What leads you to believe that this isn't intentional? > It's most likely the case that this change occurred as > part of the first round module stacking change. What behavior > do you see that you're unhappy with? > > > > > To keep things working > > Which "things"? How are they not "working"? > > > and to make the comments in security/security.c > > correct when the xattr is securit.capable, call cap_inode_setxattr > > or cap_inode_removexattr as appropriate. > > > > I suspect there is a larger conversation to be had here but this > > is enough to keep selinux from implementing a non-sense hard coded > > policy that breaks other parts of the kernel. > > Specifics, please. Since I can't guess what problem you've > encountered I can't tell if it's here, in the infrastructure, > or in your perception of what constitutes "broken". > > > > > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> > > --- > > security/selinux/hooks.c | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > > index f5d304736852..edf4bd292dc7 100644 > > --- a/security/selinux/hooks.c > > +++ b/security/selinux/hooks.c > > @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct > > dentry *dentry, const char *name, > > u32 newsid, sid = current_sid(); > > int rc = 0; > > > > + if (strcmp(name, XATTR_NAME_CAPS) == 0) > > + return cap_inode_setxattr(dentry, name, value, > > size, flags); > > + > > No. Don't even think of contemplating considering embedding the cap > attribute check in the SELinux code. cap_inode_setxattr() is called > in > the infrastructure. Except that it isn't, not if any other security module is enabled and implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when setting security.selinux or security.SMACK*. An alternative approach to fixing this would be to change the cap functions to only apply their checks if setting the capability attribute and defer any checks on other security.* attributes to either the security framework or the other security modules. Then the framework could always call all the modules on the inode_setxattr and inode_removexattr hooks as with other hooks. The security framework would then need to ensure that a check is still applied when setting security.* attributes if it isn't already handled by one of the enabled security modules, as you don't want unprivileged userspace to be able to set arbitrary security.foo attributes or to set up security.selinux or security.SMACK* attributes if those modules happen to be disabled. > > > > if (strcmp(name, XATTR_NAME_SELINUX)) > > return selinux_inode_setotherxattr(dentry, name); > > > > @@ -3282,6 +3285,9 @@ static int selinux_inode_listxattr(struct > > dentry *dentry) > > > > static int selinux_inode_removexattr(struct dentry *dentry, const > > char *name) > > { > > + if (strcmp(name, XATTR_NAME_CAPS) == 0) > > + return cap_inode_removexattr(dentry, name); > > + > > if (strcmp(name, XATTR_NAME_SELINUX)) > > return selinux_inode_setotherxattr(dentry, name); > > > > > . -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 9/29/2017 7:18 AM, Stephen Smalley wrote: > On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: >> On 9/28/2017 3:34 PM, Eric W. Biederman wrote: >>> It looks like once upon a time a long time ago selinux copied code >>> from cap_inode_removexattr and cap_inode_setxattr into >>> selinux_inode_setotherxattr. However the code has now diverged and >>> selinux is implementing a policy that is quite different than >>> cap_inode_setxattr and cap_inode_removexattr especially when it >>> comes >>> to the security.capable xattr. >> What leads you to believe that this isn't intentional? >> It's most likely the case that this change occurred as >> part of the first round module stacking change. What behavior >> do you see that you're unhappy with? >> >>> To keep things working >> Which "things"? How are they not "working"? >> >>> and to make the comments in security/security.c >>> correct when the xattr is securit.capable, call cap_inode_setxattr >>> or cap_inode_removexattr as appropriate. >>> >>> I suspect there is a larger conversation to be had here but this >>> is enough to keep selinux from implementing a non-sense hard coded >>> policy that breaks other parts of the kernel. >> Specifics, please. Since I can't guess what problem you've >> encountered I can't tell if it's here, in the infrastructure, >> or in your perception of what constitutes "broken". >> >>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>> --- >>> security/selinux/hooks.c | 6 ++++++ >>> 1 file changed, 6 insertions(+) >>> >>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>> index f5d304736852..edf4bd292dc7 100644 >>> --- a/security/selinux/hooks.c >>> +++ b/security/selinux/hooks.c >>> @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct >>> dentry *dentry, const char *name, >>> u32 newsid, sid = current_sid(); >>> int rc = 0; >>> >>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>> + return cap_inode_setxattr(dentry, name, value, >>> size, flags); >>> + >> No. Don't even think of contemplating considering embedding the cap >> attribute check in the SELinux code. cap_inode_setxattr() is called >> in >> the infrastructure. > Except that it isn't, not if any other security module is enabled and > implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when > setting security.selinux or security.SMACK*. OK. Yes, this bit of the infrastructure is some of the worst I've done in a long time. This is a case where we already need special case stacking infrastructure. It looks like we'll have to separate setting the cap attribute from checking the cap state in order to make this work. In any case, the security_inode_setxattr() code is where the change belongs. There will likely be fallout changes in the modules, including the cap module. > An alternative approach to fixing this would be to change the cap > functions to only apply their checks if setting the capability > attribute and defer any checks on other security.* attributes to either > the security framework or the other security modules. Then the > framework could always call all the modules on the inode_setxattr and > inode_removexattr hooks as with other hooks. The security framework > would then need to ensure that a check is still applied when setting > security.* attributes if it isn't already handled by one of the enabled > security modules, as you don't want unprivileged userspace to be able > to set arbitrary security.foo attributes or to set up security.selinux > or security.SMACK* attributes if those modules happen to be disabled. Agreed. This isn't a two line change. Grumble. I can guess at what the problem might be, but I hate making assumptions when I go to fix a problem. I will start looking at a patch, but it would really help if I could say for sure what I'm out to accomplish. It may be obvious to the casual observer, but that description has not been applied to me very often. > >> >> >>> if (strcmp(name, XATTR_NAME_SELINUX)) >>> return selinux_inode_setotherxattr(dentry, name); >>> >>> @@ -3282,6 +3285,9 @@ static int selinux_inode_listxattr(struct >>> dentry *dentry) >>> >>> static int selinux_inode_removexattr(struct dentry *dentry, const >>> char *name) >>> { >>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>> + return cap_inode_removexattr(dentry, name); >>> + >>> if (strcmp(name, XATTR_NAME_SELINUX)) >>> return selinux_inode_setotherxattr(dentry, name); >>> >> >> . > -- > To unsubscribe from this list: send the line "unsubscribe linux-security-module" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > . -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Casey Schaufler <casey@schaufler-ca.com> writes: > On 9/29/2017 7:18 AM, Stephen Smalley wrote: >> On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: >>> On 9/28/2017 3:34 PM, Eric W. Biederman wrote: >>>> It looks like once upon a time a long time ago selinux copied code >>>> from cap_inode_removexattr and cap_inode_setxattr into >>>> selinux_inode_setotherxattr. However the code has now diverged and >>>> selinux is implementing a policy that is quite different than >>>> cap_inode_setxattr and cap_inode_removexattr especially when it >>>> comes >>>> to the security.capable xattr. >>> What leads you to believe that this isn't intentional? >>> It's most likely the case that this change occurred as >>> part of the first round module stacking change. What behavior >>> do you see that you're unhappy with? >>> >>>> To keep things working >>> Which "things"? How are they not "working"? >>> >>>> and to make the comments in security/security.c >>>> correct when the xattr is securit.capable, call cap_inode_setxattr >>>> or cap_inode_removexattr as appropriate. >>>> >>>> I suspect there is a larger conversation to be had here but this >>>> is enough to keep selinux from implementing a non-sense hard coded >>>> policy that breaks other parts of the kernel. >>> Specifics, please. Since I can't guess what problem you've >>> encountered I can't tell if it's here, in the infrastructure, >>> or in your perception of what constitutes "broken". >>> >>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>>> --- >>>> security/selinux/hooks.c | 6 ++++++ >>>> 1 file changed, 6 insertions(+) >>>> >>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>>> index f5d304736852..edf4bd292dc7 100644 >>>> --- a/security/selinux/hooks.c >>>> +++ b/security/selinux/hooks.c >>>> @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct >>>> dentry *dentry, const char *name, >>>> u32 newsid, sid = current_sid(); >>>> int rc = 0; >>>> >>>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>>> + return cap_inode_setxattr(dentry, name, value, >>>> size, flags); >>>> + >>> No. Don't even think of contemplating considering embedding the cap >>> attribute check in the SELinux code. cap_inode_setxattr() is called >>> in >>> the infrastructure. >> Except that it isn't, not if any other security module is enabled and >> implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when >> setting security.selinux or security.SMACK*. > > OK. Yes, this bit of the infrastructure is some of the > worst I've done in a long time. This is a case where we > already need special case stacking infrastructure. It looks > like we'll have to separate setting the cap attribute from > checking the cap state in order to make this work. In any > case, the security_inode_setxattr() code is where the change > belongs. There will likely be fallout changes in the modules, > including the cap module. > > >> An alternative approach to fixing this would be to change the cap >> functions to only apply their checks if setting the capability >> attribute and defer any checks on other security.* attributes to either >> the security framework or the other security modules. Then the >> framework could always call all the modules on the inode_setxattr and >> inode_removexattr hooks as with other hooks. The security framework >> would then need to ensure that a check is still applied when setting >> security.* attributes if it isn't already handled by one of the enabled >> security modules, as you don't want unprivileged userspace to be able >> to set arbitrary security.foo attributes or to set up security.selinux >> or security.SMACK* attributes if those modules happen to be disabled. > > Agreed. This isn't a two line change. Grumble. > > I can guess at what the problem might be, but I hate making > assumptions when I go to fix a problem. I will start looking > at a patch, but it would really help if I could say for sure > what I'm out to accomplish. It may be obvious to the casual > observer, but that description has not been applied to me very > often. Apologies for the delayed reply. I am looking at security_inode_setxattr. For setting attributes in the security.* the generic code in fs/xattr.c applies no permission checks. Each security module that implements an xattr in security.* then imposes it's own policy on it's own attribute. For smack the basic rule is smack_privileged(CAP_MAC_ADMIN). For selinux the basic rule is inode_or_owner_capable(inode). For commoncap the basic rule is capable_wrt_inode_uidgid(inode, CAP_SETFCAP). commoncap also applies a default policity to setting security.* xattrs. ns_capable(dentry->d_sb->s_userns, CAP_SYS_ADMIN). smack reuses that default policy by calling cap_inode_setxattr if it isn't a smack security.* xattr. selinux has what looks like an old copy of the commoncap checks for the security.* in selinux_inode_setotherxattr. Testing for capable(CAP_SETFCAP) for security.capable and capable(CAP_SYS_ADMIN) for the others. With the added complication that selinux calls selinux_inode_setotherxattr also for the remove_xattr case. So fixing this in selinux_inode_setotherxattr is not appropriate. I believe selinux also has general policy hooks it applies to all invocations of setxattr. So I think to really fix this we need to separate the cases of is this your security modules attribute from general policy checks added by the security modules. Perhaps something like this for security_inode_setxattr: Hmm. Looking at least ima also has the distinction between protecting it's own xattr writes and running generaly security module policy on xattr writes. int security_inode_setxattr(struct dentry *dentry, const char *name, const void *value, size_t size, int flags) { int ret = 0; if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) return 0; if (strncmp(name, XATTR_SECURITY_PREFIX, sizeof(XATTR_SECURITY_PREFIX) - 1) == 0) { /* Call the security modules and see if they all return * -EOPNOTSUPP if so apply the default permission * check of ns_capable(dentry->d_sb->s_user_ns, CAP_SYS_ADMIN) * otherwise if one of the security modules supports * this attribute (signaled by returning something other * -EOPNOTSUPP) then set ret to that result. * * The security modules include at least smack, selinux, * commoncap, ima, and evm. */ ret = magic_inode_protect_setxattr(dentry, name, value, size); } if (ret) return ret; /* Run all of the security module policy against this setxattr call */ return magic_inode_policy_setxattr(dentry, name, value, size); } Eric -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 9/30/2017 9:22 AM, Eric W. Biederman wrote: > Casey Schaufler <casey@schaufler-ca.com> writes: > >> On 9/29/2017 7:18 AM, Stephen Smalley wrote: >>> On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: >>>> On 9/28/2017 3:34 PM, Eric W. Biederman wrote: >>>>> It looks like once upon a time a long time ago selinux copied code >>>>> from cap_inode_removexattr and cap_inode_setxattr into >>>>> selinux_inode_setotherxattr. However the code has now diverged and >>>>> selinux is implementing a policy that is quite different than >>>>> cap_inode_setxattr and cap_inode_removexattr especially when it >>>>> comes >>>>> to the security.capable xattr. >>>> What leads you to believe that this isn't intentional? >>>> It's most likely the case that this change occurred as >>>> part of the first round module stacking change. What behavior >>>> do you see that you're unhappy with? >>>> >>>>> To keep things working >>>> Which "things"? How are they not "working"? >>>> >>>>> and to make the comments in security/security.c >>>>> correct when the xattr is securit.capable, call cap_inode_setxattr >>>>> or cap_inode_removexattr as appropriate. >>>>> >>>>> I suspect there is a larger conversation to be had here but this >>>>> is enough to keep selinux from implementing a non-sense hard coded >>>>> policy that breaks other parts of the kernel. >>>> Specifics, please. Since I can't guess what problem you've >>>> encountered I can't tell if it's here, in the infrastructure, >>>> or in your perception of what constitutes "broken". >>>> >>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>>>> --- >>>>> security/selinux/hooks.c | 6 ++++++ >>>>> 1 file changed, 6 insertions(+) >>>>> >>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>>>> index f5d304736852..edf4bd292dc7 100644 >>>>> --- a/security/selinux/hooks.c >>>>> +++ b/security/selinux/hooks.c >>>>> @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct >>>>> dentry *dentry, const char *name, >>>>> u32 newsid, sid = current_sid(); >>>>> int rc = 0; >>>>> >>>>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>>>> + return cap_inode_setxattr(dentry, name, value, >>>>> size, flags); >>>>> + >>>> No. Don't even think of contemplating considering embedding the cap >>>> attribute check in the SELinux code. cap_inode_setxattr() is called >>>> in >>>> the infrastructure. >>> Except that it isn't, not if any other security module is enabled and >>> implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when >>> setting security.selinux or security.SMACK*. >> OK. Yes, this bit of the infrastructure is some of the >> worst I've done in a long time. This is a case where we >> already need special case stacking infrastructure. It looks >> like we'll have to separate setting the cap attribute from >> checking the cap state in order to make this work. In any >> case, the security_inode_setxattr() code is where the change >> belongs. There will likely be fallout changes in the modules, >> including the cap module. >> >> >>> An alternative approach to fixing this would be to change the cap >>> functions to only apply their checks if setting the capability >>> attribute and defer any checks on other security.* attributes to either >>> the security framework or the other security modules. Then the >>> framework could always call all the modules on the inode_setxattr and >>> inode_removexattr hooks as with other hooks. The security framework >>> would then need to ensure that a check is still applied when setting >>> security.* attributes if it isn't already handled by one of the enabled >>> security modules, as you don't want unprivileged userspace to be able >>> to set arbitrary security.foo attributes or to set up security.selinux >>> or security.SMACK* attributes if those modules happen to be disabled. >> Agreed. This isn't a two line change. Grumble. >> >> I can guess at what the problem might be, but I hate making >> assumptions when I go to fix a problem. I will start looking >> at a patch, but it would really help if I could say for sure >> what I'm out to accomplish. It may be obvious to the casual >> observer, but that description has not been applied to me very >> often. > Apologies for the delayed reply. > > I am looking at security_inode_setxattr. > > For setting attributes in the security.* the generic code in fs/xattr.c > applies no permission checks. > > Each security module that implements an xattr in security.* then imposes > it's own policy on it's own attribute. > > For smack the basic rule is smack_privileged(CAP_MAC_ADMIN). > For selinux the basic rule is inode_or_owner_capable(inode). > For commoncap the basic rule is capable_wrt_inode_uidgid(inode, CAP_SETFCAP). > > commoncap also applies a default policity to setting security.* xattrs. > ns_capable(dentry->d_sb->s_userns, CAP_SYS_ADMIN). > > smack reuses that default policy by calling cap_inode_setxattr if it > isn't a smack security.* xattr. > > selinux has what looks like an old copy of the commoncap checks for > the security.* in selinux_inode_setotherxattr. Testing for > capable(CAP_SETFCAP) for security.capable and capable(CAP_SYS_ADMIN) > for the others. > > With the added complication that selinux calls > selinux_inode_setotherxattr also for the remove_xattr case. So fixing > this in selinux_inode_setotherxattr is not appropriate. > > I believe selinux also has general policy hooks it applies to all > invocations of setxattr. > > So I think to really fix this we need to separate the cases of is this > your security modules attribute from general policy checks added by the > security modules. Perhaps something like this for > security_inode_setxattr: > > Hmm. Looking at least ima also has the distinction between protecting > it's own xattr writes and running generaly security module policy on > xattr writes. > > int security_inode_setxattr(struct dentry *dentry, const char *name, > const void *value, size_t size, int flags) > { > int ret = 0; > > if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) > return 0; > > if (strncmp(name, XATTR_SECURITY_PREFIX, > sizeof(XATTR_SECURITY_PREFIX) - 1) == 0) { > /* Call the security modules and see if they all return > * -EOPNOTSUPP if so apply the default permission > * check of ns_capable(dentry->d_sb->s_user_ns, CAP_SYS_ADMIN) > * otherwise if one of the security modules supports > * this attribute (signaled by returning something other > * -EOPNOTSUPP) then set ret to that result. > * > * The security modules include at least smack, selinux, > * commoncap, ima, and evm. > */ > ret = magic_inode_protect_setxattr(dentry, name, value, size); > } > if (ret) > return ret; > > /* Run all of the security module policy against this setxattr call */ > return magic_inode_policy_setxattr(dentry, name, value, size); > } > > Eric Yup, that's pretty much what I'm thinking. It's unfortunate that the magic_ API isn't fully implemented. There's going to be a good deal of code surgery instead. Is there an observed problem today? This is going to have to get addressed for stacking, so if there isn't a behavioral issue that impacts something real I would like to defer spending significant time on it. Do you have a case where this is not working correctly? . -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Casey Schaufler <casey@schaufler-ca.com> writes: > On 9/30/2017 9:22 AM, Eric W. Biederman wrote: >> Casey Schaufler <casey@schaufler-ca.com> writes: >> >>> On 9/29/2017 7:18 AM, Stephen Smalley wrote: >>>> On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: >>>>> On 9/28/2017 3:34 PM, Eric W. Biederman wrote: >>>>>> It looks like once upon a time a long time ago selinux copied code >>>>>> from cap_inode_removexattr and cap_inode_setxattr into >>>>>> selinux_inode_setotherxattr. However the code has now diverged and >>>>>> selinux is implementing a policy that is quite different than >>>>>> cap_inode_setxattr and cap_inode_removexattr especially when it >>>>>> comes >>>>>> to the security.capable xattr. >>>>> What leads you to believe that this isn't intentional? >>>>> It's most likely the case that this change occurred as >>>>> part of the first round module stacking change. What behavior >>>>> do you see that you're unhappy with? >>>>> >>>>>> To keep things working >>>>> Which "things"? How are they not "working"? >>>>> >>>>>> and to make the comments in security/security.c >>>>>> correct when the xattr is securit.capable, call cap_inode_setxattr >>>>>> or cap_inode_removexattr as appropriate. >>>>>> >>>>>> I suspect there is a larger conversation to be had here but this >>>>>> is enough to keep selinux from implementing a non-sense hard coded >>>>>> policy that breaks other parts of the kernel. >>>>> Specifics, please. Since I can't guess what problem you've >>>>> encountered I can't tell if it's here, in the infrastructure, >>>>> or in your perception of what constitutes "broken". >>>>> >>>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>>>>> --- >>>>>> security/selinux/hooks.c | 6 ++++++ >>>>>> 1 file changed, 6 insertions(+) >>>>>> >>>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>>>>> index f5d304736852..edf4bd292dc7 100644 >>>>>> --- a/security/selinux/hooks.c >>>>>> +++ b/security/selinux/hooks.c >>>>>> @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct >>>>>> dentry *dentry, const char *name, >>>>>> u32 newsid, sid = current_sid(); >>>>>> int rc = 0; >>>>>> >>>>>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>>>>> + return cap_inode_setxattr(dentry, name, value, >>>>>> size, flags); >>>>>> + >>>>> No. Don't even think of contemplating considering embedding the cap >>>>> attribute check in the SELinux code. cap_inode_setxattr() is called >>>>> in >>>>> the infrastructure. >>>> Except that it isn't, not if any other security module is enabled and >>>> implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when >>>> setting security.selinux or security.SMACK*. >>> OK. Yes, this bit of the infrastructure is some of the >>> worst I've done in a long time. This is a case where we >>> already need special case stacking infrastructure. It looks >>> like we'll have to separate setting the cap attribute from >>> checking the cap state in order to make this work. In any >>> case, the security_inode_setxattr() code is where the change >>> belongs. There will likely be fallout changes in the modules, >>> including the cap module. >>> >>> >>>> An alternative approach to fixing this would be to change the cap >>>> functions to only apply their checks if setting the capability >>>> attribute and defer any checks on other security.* attributes to either >>>> the security framework or the other security modules. Then the >>>> framework could always call all the modules on the inode_setxattr and >>>> inode_removexattr hooks as with other hooks. The security framework >>>> would then need to ensure that a check is still applied when setting >>>> security.* attributes if it isn't already handled by one of the enabled >>>> security modules, as you don't want unprivileged userspace to be able >>>> to set arbitrary security.foo attributes or to set up security.selinux >>>> or security.SMACK* attributes if those modules happen to be disabled. >>> Agreed. This isn't a two line change. Grumble. >>> >>> I can guess at what the problem might be, but I hate making >>> assumptions when I go to fix a problem. I will start looking >>> at a patch, but it would really help if I could say for sure >>> what I'm out to accomplish. It may be obvious to the casual >>> observer, but that description has not been applied to me very >>> often. >> Apologies for the delayed reply. >> >> I am looking at security_inode_setxattr. >> >> For setting attributes in the security.* the generic code in fs/xattr.c >> applies no permission checks. >> >> Each security module that implements an xattr in security.* then imposes >> it's own policy on it's own attribute. >> >> For smack the basic rule is smack_privileged(CAP_MAC_ADMIN). >> For selinux the basic rule is inode_or_owner_capable(inode). >> For commoncap the basic rule is capable_wrt_inode_uidgid(inode, CAP_SETFCAP). >> >> commoncap also applies a default policity to setting security.* xattrs. >> ns_capable(dentry->d_sb->s_userns, CAP_SYS_ADMIN). >> >> smack reuses that default policy by calling cap_inode_setxattr if it >> isn't a smack security.* xattr. >> >> selinux has what looks like an old copy of the commoncap checks for >> the security.* in selinux_inode_setotherxattr. Testing for >> capable(CAP_SETFCAP) for security.capable and capable(CAP_SYS_ADMIN) >> for the others. >> >> With the added complication that selinux calls >> selinux_inode_setotherxattr also for the remove_xattr case. So fixing >> this in selinux_inode_setotherxattr is not appropriate. >> >> I believe selinux also has general policy hooks it applies to all >> invocations of setxattr. >> >> So I think to really fix this we need to separate the cases of is this >> your security modules attribute from general policy checks added by the >> security modules. Perhaps something like this for >> security_inode_setxattr: >> >> Hmm. Looking at least ima also has the distinction between protecting >> it's own xattr writes and running generaly security module policy on >> xattr writes. >> >> int security_inode_setxattr(struct dentry *dentry, const char *name, >> const void *value, size_t size, int flags) >> { >> int ret = 0; >> >> if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) >> return 0; >> >> if (strncmp(name, XATTR_SECURITY_PREFIX, >> sizeof(XATTR_SECURITY_PREFIX) - 1) == 0) { >> /* Call the security modules and see if they all return >> * -EOPNOTSUPP if so apply the default permission >> * check of ns_capable(dentry->d_sb->s_user_ns, CAP_SYS_ADMIN) >> * otherwise if one of the security modules supports >> * this attribute (signaled by returning something other >> * -EOPNOTSUPP) then set ret to that result. >> * >> * The security modules include at least smack, selinux, >> * commoncap, ima, and evm. >> */ >> ret = magic_inode_protect_setxattr(dentry, name, value, size); >> } >> if (ret) >> return ret; >> >> /* Run all of the security module policy against this setxattr call */ >> return magic_inode_policy_setxattr(dentry, name, value, size); >> } >> >> Eric > > Yup, that's pretty much what I'm thinking. It's unfortunate > that the magic_ API isn't fully implemented. There's going to > be a good deal of code surgery instead. Is there an observed > problem today? This is going to have to get addressed for stacking, > so if there isn't a behavioral issue that impacts something real > I would like to defer spending significant time on it. Do you have > a case where this is not working correctly? Merged as of 4.14-rc1 is the support for user namespace root to set sercurity.capable. This fails when selinux is loaded. removexattr has the same problem and the code is a little less convoluted in that case. Not being able to set the capability when you should be able to is very noticable. Like running into a brick wall noticable. Which is where the minimal patch for selinux comes in. I think it solves the exact case in question, even if it isn't the perfect long term solution. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 9/30/2017 1:40 PM, Eric W. Biederman wrote: > Casey Schaufler <casey@schaufler-ca.com> writes: > >> On 9/30/2017 9:22 AM, Eric W. Biederman wrote: >>> Casey Schaufler <casey@schaufler-ca.com> writes: >>> >>>> On 9/29/2017 7:18 AM, Stephen Smalley wrote: >>>>> On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: >>>>>> On 9/28/2017 3:34 PM, Eric W. Biederman wrote: >>>>>>> It looks like once upon a time a long time ago selinux copied code >>>>>>> from cap_inode_removexattr and cap_inode_setxattr into >>>>>>> selinux_inode_setotherxattr. However the code has now diverged and >>>>>>> selinux is implementing a policy that is quite different than >>>>>>> cap_inode_setxattr and cap_inode_removexattr especially when it >>>>>>> comes >>>>>>> to the security.capable xattr. >>>>>> What leads you to believe that this isn't intentional? >>>>>> It's most likely the case that this change occurred as >>>>>> part of the first round module stacking change. What behavior >>>>>> do you see that you're unhappy with? >>>>>> >>>>>>> To keep things working >>>>>> Which "things"? How are they not "working"? >>>>>> >>>>>>> and to make the comments in security/security.c >>>>>>> correct when the xattr is securit.capable, call cap_inode_setxattr >>>>>>> or cap_inode_removexattr as appropriate. >>>>>>> >>>>>>> I suspect there is a larger conversation to be had here but this >>>>>>> is enough to keep selinux from implementing a non-sense hard coded >>>>>>> policy that breaks other parts of the kernel. >>>>>> Specifics, please. Since I can't guess what problem you've >>>>>> encountered I can't tell if it's here, in the infrastructure, >>>>>> or in your perception of what constitutes "broken". >>>>>> >>>>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>>>>>> --- >>>>>>> security/selinux/hooks.c | 6 ++++++ >>>>>>> 1 file changed, 6 insertions(+) >>>>>>> >>>>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>>>>>> index f5d304736852..edf4bd292dc7 100644 >>>>>>> --- a/security/selinux/hooks.c >>>>>>> +++ b/security/selinux/hooks.c >>>>>>> @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct >>>>>>> dentry *dentry, const char *name, >>>>>>> u32 newsid, sid = current_sid(); >>>>>>> int rc = 0; >>>>>>> >>>>>>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>>>>>> + return cap_inode_setxattr(dentry, name, value, >>>>>>> size, flags); >>>>>>> + >>>>>> No. Don't even think of contemplating considering embedding the cap >>>>>> attribute check in the SELinux code. cap_inode_setxattr() is called >>>>>> in >>>>>> the infrastructure. >>>>> Except that it isn't, not if any other security module is enabled and >>>>> implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when >>>>> setting security.selinux or security.SMACK*. >>>> OK. Yes, this bit of the infrastructure is some of the >>>> worst I've done in a long time. This is a case where we >>>> already need special case stacking infrastructure. It looks >>>> like we'll have to separate setting the cap attribute from >>>> checking the cap state in order to make this work. In any >>>> case, the security_inode_setxattr() code is where the change >>>> belongs. There will likely be fallout changes in the modules, >>>> including the cap module. >>>> >>>> >>>>> An alternative approach to fixing this would be to change the cap >>>>> functions to only apply their checks if setting the capability >>>>> attribute and defer any checks on other security.* attributes to either >>>>> the security framework or the other security modules. Then the >>>>> framework could always call all the modules on the inode_setxattr and >>>>> inode_removexattr hooks as with other hooks. The security framework >>>>> would then need to ensure that a check is still applied when setting >>>>> security.* attributes if it isn't already handled by one of the enabled >>>>> security modules, as you don't want unprivileged userspace to be able >>>>> to set arbitrary security.foo attributes or to set up security.selinux >>>>> or security.SMACK* attributes if those modules happen to be disabled. >>>> Agreed. This isn't a two line change. Grumble. >>>> >>>> I can guess at what the problem might be, but I hate making >>>> assumptions when I go to fix a problem. I will start looking >>>> at a patch, but it would really help if I could say for sure >>>> what I'm out to accomplish. It may be obvious to the casual >>>> observer, but that description has not been applied to me very >>>> often. >>> Apologies for the delayed reply. >>> >>> I am looking at security_inode_setxattr. >>> >>> For setting attributes in the security.* the generic code in fs/xattr.c >>> applies no permission checks. >>> >>> Each security module that implements an xattr in security.* then imposes >>> it's own policy on it's own attribute. >>> >>> For smack the basic rule is smack_privileged(CAP_MAC_ADMIN). >>> For selinux the basic rule is inode_or_owner_capable(inode). >>> For commoncap the basic rule is capable_wrt_inode_uidgid(inode, CAP_SETFCAP). >>> >>> commoncap also applies a default policity to setting security.* xattrs. >>> ns_capable(dentry->d_sb->s_userns, CAP_SYS_ADMIN). >>> >>> smack reuses that default policy by calling cap_inode_setxattr if it >>> isn't a smack security.* xattr. >>> >>> selinux has what looks like an old copy of the commoncap checks for >>> the security.* in selinux_inode_setotherxattr. Testing for >>> capable(CAP_SETFCAP) for security.capable and capable(CAP_SYS_ADMIN) >>> for the others. >>> >>> With the added complication that selinux calls >>> selinux_inode_setotherxattr also for the remove_xattr case. So fixing >>> this in selinux_inode_setotherxattr is not appropriate. >>> >>> I believe selinux also has general policy hooks it applies to all >>> invocations of setxattr. >>> >>> So I think to really fix this we need to separate the cases of is this >>> your security modules attribute from general policy checks added by the >>> security modules. Perhaps something like this for >>> security_inode_setxattr: >>> >>> Hmm. Looking at least ima also has the distinction between protecting >>> it's own xattr writes and running generaly security module policy on >>> xattr writes. >>> >>> int security_inode_setxattr(struct dentry *dentry, const char *name, >>> const void *value, size_t size, int flags) >>> { >>> int ret = 0; >>> >>> if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) >>> return 0; >>> >>> if (strncmp(name, XATTR_SECURITY_PREFIX, >>> sizeof(XATTR_SECURITY_PREFIX) - 1) == 0) { >>> /* Call the security modules and see if they all return >>> * -EOPNOTSUPP if so apply the default permission >>> * check of ns_capable(dentry->d_sb->s_user_ns, CAP_SYS_ADMIN) >>> * otherwise if one of the security modules supports >>> * this attribute (signaled by returning something other >>> * -EOPNOTSUPP) then set ret to that result. >>> * >>> * The security modules include at least smack, selinux, >>> * commoncap, ima, and evm. >>> */ >>> ret = magic_inode_protect_setxattr(dentry, name, value, size); >>> } >>> if (ret) >>> return ret; >>> >>> /* Run all of the security module policy against this setxattr call */ >>> return magic_inode_policy_setxattr(dentry, name, value, size); >>> } >>> >>> Eric >> Yup, that's pretty much what I'm thinking. It's unfortunate >> that the magic_ API isn't fully implemented. There's going to >> be a good deal of code surgery instead. Is there an observed >> problem today? This is going to have to get addressed for stacking, >> so if there isn't a behavioral issue that impacts something real >> I would like to defer spending significant time on it. Do you have >> a case where this is not working correctly? > Merged as of 4.14-rc1 is the support for user namespace root to set > sercurity.capable. This fails when selinux is loaded. OK. Is the failure unique to SELinux, or does it fail with Smack as well? > removexattr has the same problem and the code is a little less > convoluted in that case. Right. Because removexattr is a simpler situation. > Not being able to set the capability when you should be able to is > very noticable. Like running into a brick wall noticable. Ah, now you've identified the problem. Yes, I would agree that you've uncovered an undesirable behavior. > Which is where the minimal patch for selinux comes in. I think it > solves the exact case in question, even if it isn't the perfect long > term solution. If the problem is unique to SELinux I can see your logic. If it isn't, that is, if it's also a problem with any other security module, there either needs to be a fix for that/those module/s as well or a "real" fix. I'm not opposed to the SELinux short term fix if you can say that that's the only module with the problem. > > Eric > > -- > To unsubscribe from this list: send the line "unsubscribe linux-security-module" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > . -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Casey Schaufler <casey@schaufler-ca.com> writes: > On 9/30/2017 1:40 PM, Eric W. Biederman wrote: >> Casey Schaufler <casey@schaufler-ca.com> writes: >> >>> On 9/30/2017 9:22 AM, Eric W. Biederman wrote: >>>> Casey Schaufler <casey@schaufler-ca.com> writes: >>>> >>>>> On 9/29/2017 7:18 AM, Stephen Smalley wrote: >>>>>> On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: >>>>>>> On 9/28/2017 3:34 PM, Eric W. Biederman wrote: >>>>>>>> It looks like once upon a time a long time ago selinux copied code >>>>>>>> from cap_inode_removexattr and cap_inode_setxattr into >>>>>>>> selinux_inode_setotherxattr. However the code has now diverged and >>>>>>>> selinux is implementing a policy that is quite different than >>>>>>>> cap_inode_setxattr and cap_inode_removexattr especially when it >>>>>>>> comes >>>>>>>> to the security.capable xattr. >>>>>>> What leads you to believe that this isn't intentional? >>>>>>> It's most likely the case that this change occurred as >>>>>>> part of the first round module stacking change. What behavior >>>>>>> do you see that you're unhappy with? >>>>>>> >>>>>>>> To keep things working >>>>>>> Which "things"? How are they not "working"? >>>>>>> >>>>>>>> and to make the comments in security/security.c >>>>>>>> correct when the xattr is securit.capable, call cap_inode_setxattr >>>>>>>> or cap_inode_removexattr as appropriate. >>>>>>>> >>>>>>>> I suspect there is a larger conversation to be had here but this >>>>>>>> is enough to keep selinux from implementing a non-sense hard coded >>>>>>>> policy that breaks other parts of the kernel. >>>>>>> Specifics, please. Since I can't guess what problem you've >>>>>>> encountered I can't tell if it's here, in the infrastructure, >>>>>>> or in your perception of what constitutes "broken". >>>>>>> >>>>>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>>>>>>> --- >>>>>>>> security/selinux/hooks.c | 6 ++++++ >>>>>>>> 1 file changed, 6 insertions(+) >>>>>>>> >>>>>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>>>>>>> index f5d304736852..edf4bd292dc7 100644 >>>>>>>> --- a/security/selinux/hooks.c >>>>>>>> +++ b/security/selinux/hooks.c >>>>>>>> @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct >>>>>>>> dentry *dentry, const char *name, >>>>>>>> u32 newsid, sid = current_sid(); >>>>>>>> int rc = 0; >>>>>>>> >>>>>>>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>>>>>>> + return cap_inode_setxattr(dentry, name, value, >>>>>>>> size, flags); >>>>>>>> + >>>>>>> No. Don't even think of contemplating considering embedding the cap >>>>>>> attribute check in the SELinux code. cap_inode_setxattr() is called >>>>>>> in >>>>>>> the infrastructure. >>>>>> Except that it isn't, not if any other security module is enabled and >>>>>> implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when >>>>>> setting security.selinux or security.SMACK*. >>>>> OK. Yes, this bit of the infrastructure is some of the >>>>> worst I've done in a long time. This is a case where we >>>>> already need special case stacking infrastructure. It looks >>>>> like we'll have to separate setting the cap attribute from >>>>> checking the cap state in order to make this work. In any >>>>> case, the security_inode_setxattr() code is where the change >>>>> belongs. There will likely be fallout changes in the modules, >>>>> including the cap module. >>>>> >>>>> >>>>>> An alternative approach to fixing this would be to change the cap >>>>>> functions to only apply their checks if setting the capability >>>>>> attribute and defer any checks on other security.* attributes to either >>>>>> the security framework or the other security modules. Then the >>>>>> framework could always call all the modules on the inode_setxattr and >>>>>> inode_removexattr hooks as with other hooks. The security framework >>>>>> would then need to ensure that a check is still applied when setting >>>>>> security.* attributes if it isn't already handled by one of the enabled >>>>>> security modules, as you don't want unprivileged userspace to be able >>>>>> to set arbitrary security.foo attributes or to set up security.selinux >>>>>> or security.SMACK* attributes if those modules happen to be disabled. >>>>> Agreed. This isn't a two line change. Grumble. >>>>> >>>>> I can guess at what the problem might be, but I hate making >>>>> assumptions when I go to fix a problem. I will start looking >>>>> at a patch, but it would really help if I could say for sure >>>>> what I'm out to accomplish. It may be obvious to the casual >>>>> observer, but that description has not been applied to me very >>>>> often. >>>> Apologies for the delayed reply. >>>> >>>> I am looking at security_inode_setxattr. >>>> >>>> For setting attributes in the security.* the generic code in fs/xattr.c >>>> applies no permission checks. >>>> >>>> Each security module that implements an xattr in security.* then imposes >>>> it's own policy on it's own attribute. >>>> >>>> For smack the basic rule is smack_privileged(CAP_MAC_ADMIN). >>>> For selinux the basic rule is inode_or_owner_capable(inode). >>>> For commoncap the basic rule is capable_wrt_inode_uidgid(inode, CAP_SETFCAP). >>>> >>>> commoncap also applies a default policity to setting security.* xattrs. >>>> ns_capable(dentry->d_sb->s_userns, CAP_SYS_ADMIN). >>>> >>>> smack reuses that default policy by calling cap_inode_setxattr if it >>>> isn't a smack security.* xattr. >>>> >>>> selinux has what looks like an old copy of the commoncap checks for >>>> the security.* in selinux_inode_setotherxattr. Testing for >>>> capable(CAP_SETFCAP) for security.capable and capable(CAP_SYS_ADMIN) >>>> for the others. >>>> >>>> With the added complication that selinux calls >>>> selinux_inode_setotherxattr also for the remove_xattr case. So fixing >>>> this in selinux_inode_setotherxattr is not appropriate. >>>> >>>> I believe selinux also has general policy hooks it applies to all >>>> invocations of setxattr. >>>> >>>> So I think to really fix this we need to separate the cases of is this >>>> your security modules attribute from general policy checks added by the >>>> security modules. Perhaps something like this for >>>> security_inode_setxattr: >>>> >>>> Hmm. Looking at least ima also has the distinction between protecting >>>> it's own xattr writes and running generaly security module policy on >>>> xattr writes. >>>> >>>> int security_inode_setxattr(struct dentry *dentry, const char *name, >>>> const void *value, size_t size, int flags) >>>> { >>>> int ret = 0; >>>> >>>> if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) >>>> return 0; >>>> >>>> if (strncmp(name, XATTR_SECURITY_PREFIX, >>>> sizeof(XATTR_SECURITY_PREFIX) - 1) == 0) { >>>> /* Call the security modules and see if they all return >>>> * -EOPNOTSUPP if so apply the default permission >>>> * check of ns_capable(dentry->d_sb->s_user_ns, CAP_SYS_ADMIN) >>>> * otherwise if one of the security modules supports >>>> * this attribute (signaled by returning something other >>>> * -EOPNOTSUPP) then set ret to that result. >>>> * >>>> * The security modules include at least smack, selinux, >>>> * commoncap, ima, and evm. >>>> */ >>>> ret = magic_inode_protect_setxattr(dentry, name, value, size); >>>> } >>>> if (ret) >>>> return ret; >>>> >>>> /* Run all of the security module policy against this setxattr call */ >>>> return magic_inode_policy_setxattr(dentry, name, value, size); >>>> } >>>> >>>> Eric >>> Yup, that's pretty much what I'm thinking. It's unfortunate >>> that the magic_ API isn't fully implemented. There's going to >>> be a good deal of code surgery instead. Is there an observed >>> problem today? This is going to have to get addressed for stacking, >>> so if there isn't a behavioral issue that impacts something real >>> I would like to defer spending significant time on it. Do you have >>> a case where this is not working correctly? >> Merged as of 4.14-rc1 is the support for user namespace root to set >> sercurity.capable. This fails when selinux is loaded. > > OK. Is the failure unique to SELinux, or does it fail with > Smack as well? I don't have a smack configuration handy, but reading through the code smack setxattr the permission checks for all xattrs that are not smack xattrs to cap_inode_setxattr. So smack and commoncap combined will not fail. smack and selinux will result in people who should be able to set selinux xattrs not being able to. That however is less of an immediate problem. >> removexattr has the same problem and the code is a little less >> convoluted in that case. > > Right. Because removexattr is a simpler situation. > >> Not being able to set the capability when you should be able to is >> very noticable. Like running into a brick wall noticable. > > Ah, now you've identified the problem. Yes, I would agree that you've > uncovered an undesirable behavior. Apologies for not being clearer earlier, but I was still in shock from running into a brick wall. >> Which is where the minimal patch for selinux comes in. I think it >> solves the exact case in question, even if it isn't the perfect long >> term solution. > > If the problem is unique to SELinux I can see your logic. If it > isn't, that is, if it's also a problem with any other security > module, there either needs to be a fix for that/those module/s > as well or a "real" fix. > > I'm not opposed to the SELinux short term fix if you can say > that that's the only module with the problem. So far there are exactly two implementation of LSM_HOOK_INIT(inode_setxattr, ...) So as a practical case it looks like combination with selinux is the only case where the problem will be observed right now. And it makes the code at least somewhat match the comments in security_inode_setxattr. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 9/30/2017 6:02 PM, Eric W. Biederman wrote: > Casey Schaufler <casey@schaufler-ca.com> writes: > >> On 9/30/2017 1:40 PM, Eric W. Biederman wrote: >>> Casey Schaufler <casey@schaufler-ca.com> writes: >>> >>>> On 9/30/2017 9:22 AM, Eric W. Biederman wrote: >>>>> Casey Schaufler <casey@schaufler-ca.com> writes: >>>>> >>>>>> On 9/29/2017 7:18 AM, Stephen Smalley wrote: >>>>>>> On Thu, 2017-09-28 at 18:16 -0700, Casey Schaufler wrote: >>>>>>>> On 9/28/2017 3:34 PM, Eric W. Biederman wrote: >>>>>>>>> It looks like once upon a time a long time ago selinux copied code >>>>>>>>> from cap_inode_removexattr and cap_inode_setxattr into >>>>>>>>> selinux_inode_setotherxattr. However the code has now diverged and >>>>>>>>> selinux is implementing a policy that is quite different than >>>>>>>>> cap_inode_setxattr and cap_inode_removexattr especially when it >>>>>>>>> comes >>>>>>>>> to the security.capable xattr. >>>>>>>> What leads you to believe that this isn't intentional? >>>>>>>> It's most likely the case that this change occurred as >>>>>>>> part of the first round module stacking change. What behavior >>>>>>>> do you see that you're unhappy with? >>>>>>>> >>>>>>>>> To keep things working >>>>>>>> Which "things"? How are they not "working"? >>>>>>>> >>>>>>>>> and to make the comments in security/security.c >>>>>>>>> correct when the xattr is securit.capable, call cap_inode_setxattr >>>>>>>>> or cap_inode_removexattr as appropriate. >>>>>>>>> >>>>>>>>> I suspect there is a larger conversation to be had here but this >>>>>>>>> is enough to keep selinux from implementing a non-sense hard coded >>>>>>>>> policy that breaks other parts of the kernel. >>>>>>>> Specifics, please. Since I can't guess what problem you've >>>>>>>> encountered I can't tell if it's here, in the infrastructure, >>>>>>>> or in your perception of what constitutes "broken". >>>>>>>> >>>>>>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>>>>>>>> --- >>>>>>>>> security/selinux/hooks.c | 6 ++++++ >>>>>>>>> 1 file changed, 6 insertions(+) >>>>>>>>> >>>>>>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c >>>>>>>>> index f5d304736852..edf4bd292dc7 100644 >>>>>>>>> --- a/security/selinux/hooks.c >>>>>>>>> +++ b/security/selinux/hooks.c >>>>>>>>> @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct >>>>>>>>> dentry *dentry, const char *name, >>>>>>>>> u32 newsid, sid = current_sid(); >>>>>>>>> int rc = 0; >>>>>>>>> >>>>>>>>> + if (strcmp(name, XATTR_NAME_CAPS) == 0) >>>>>>>>> + return cap_inode_setxattr(dentry, name, value, >>>>>>>>> size, flags); >>>>>>>>> + >>>>>>>> No. Don't even think of contemplating considering embedding the cap >>>>>>>> attribute check in the SELinux code. cap_inode_setxattr() is called >>>>>>>> in >>>>>>>> the infrastructure. >>>>>>> Except that it isn't, not if any other security module is enabled and >>>>>>> implements those hooks, to prevent imposing CAP_SYS_ADMIN checks when >>>>>>> setting security.selinux or security.SMACK*. >>>>>> OK. Yes, this bit of the infrastructure is some of the >>>>>> worst I've done in a long time. This is a case where we >>>>>> already need special case stacking infrastructure. It looks >>>>>> like we'll have to separate setting the cap attribute from >>>>>> checking the cap state in order to make this work. In any >>>>>> case, the security_inode_setxattr() code is where the change >>>>>> belongs. There will likely be fallout changes in the modules, >>>>>> including the cap module. >>>>>> >>>>>> >>>>>>> An alternative approach to fixing this would be to change the cap >>>>>>> functions to only apply their checks if setting the capability >>>>>>> attribute and defer any checks on other security.* attributes to either >>>>>>> the security framework or the other security modules. Then the >>>>>>> framework could always call all the modules on the inode_setxattr and >>>>>>> inode_removexattr hooks as with other hooks. The security framework >>>>>>> would then need to ensure that a check is still applied when setting >>>>>>> security.* attributes if it isn't already handled by one of the enabled >>>>>>> security modules, as you don't want unprivileged userspace to be able >>>>>>> to set arbitrary security.foo attributes or to set up security.selinux >>>>>>> or security.SMACK* attributes if those modules happen to be disabled. >>>>>> Agreed. This isn't a two line change. Grumble. >>>>>> >>>>>> I can guess at what the problem might be, but I hate making >>>>>> assumptions when I go to fix a problem. I will start looking >>>>>> at a patch, but it would really help if I could say for sure >>>>>> what I'm out to accomplish. It may be obvious to the casual >>>>>> observer, but that description has not been applied to me very >>>>>> often. >>>>> Apologies for the delayed reply. >>>>> >>>>> I am looking at security_inode_setxattr. >>>>> >>>>> For setting attributes in the security.* the generic code in fs/xattr.c >>>>> applies no permission checks. >>>>> >>>>> Each security module that implements an xattr in security.* then imposes >>>>> it's own policy on it's own attribute. >>>>> >>>>> For smack the basic rule is smack_privileged(CAP_MAC_ADMIN). >>>>> For selinux the basic rule is inode_or_owner_capable(inode). >>>>> For commoncap the basic rule is capable_wrt_inode_uidgid(inode, CAP_SETFCAP). >>>>> >>>>> commoncap also applies a default policity to setting security.* xattrs. >>>>> ns_capable(dentry->d_sb->s_userns, CAP_SYS_ADMIN). >>>>> >>>>> smack reuses that default policy by calling cap_inode_setxattr if it >>>>> isn't a smack security.* xattr. >>>>> >>>>> selinux has what looks like an old copy of the commoncap checks for >>>>> the security.* in selinux_inode_setotherxattr. Testing for >>>>> capable(CAP_SETFCAP) for security.capable and capable(CAP_SYS_ADMIN) >>>>> for the others. >>>>> >>>>> With the added complication that selinux calls >>>>> selinux_inode_setotherxattr also for the remove_xattr case. So fixing >>>>> this in selinux_inode_setotherxattr is not appropriate. >>>>> >>>>> I believe selinux also has general policy hooks it applies to all >>>>> invocations of setxattr. >>>>> >>>>> So I think to really fix this we need to separate the cases of is this >>>>> your security modules attribute from general policy checks added by the >>>>> security modules. Perhaps something like this for >>>>> security_inode_setxattr: >>>>> >>>>> Hmm. Looking at least ima also has the distinction between protecting >>>>> it's own xattr writes and running generaly security module policy on >>>>> xattr writes. >>>>> >>>>> int security_inode_setxattr(struct dentry *dentry, const char *name, >>>>> const void *value, size_t size, int flags) >>>>> { >>>>> int ret = 0; >>>>> >>>>> if (unlikely(IS_PRIVATE(d_backing_inode(dentry)))) >>>>> return 0; >>>>> >>>>> if (strncmp(name, XATTR_SECURITY_PREFIX, >>>>> sizeof(XATTR_SECURITY_PREFIX) - 1) == 0) { >>>>> /* Call the security modules and see if they all return >>>>> * -EOPNOTSUPP if so apply the default permission >>>>> * check of ns_capable(dentry->d_sb->s_user_ns, CAP_SYS_ADMIN) >>>>> * otherwise if one of the security modules supports >>>>> * this attribute (signaled by returning something other >>>>> * -EOPNOTSUPP) then set ret to that result. >>>>> * >>>>> * The security modules include at least smack, selinux, >>>>> * commoncap, ima, and evm. >>>>> */ >>>>> ret = magic_inode_protect_setxattr(dentry, name, value, size); >>>>> } >>>>> if (ret) >>>>> return ret; >>>>> >>>>> /* Run all of the security module policy against this setxattr call */ >>>>> return magic_inode_policy_setxattr(dentry, name, value, size); >>>>> } >>>>> >>>>> Eric >>>> Yup, that's pretty much what I'm thinking. It's unfortunate >>>> that the magic_ API isn't fully implemented. There's going to >>>> be a good deal of code surgery instead. Is there an observed >>>> problem today? This is going to have to get addressed for stacking, >>>> so if there isn't a behavioral issue that impacts something real >>>> I would like to defer spending significant time on it. Do you have >>>> a case where this is not working correctly? >>> Merged as of 4.14-rc1 is the support for user namespace root to set >>> sercurity.capable. This fails when selinux is loaded. >> OK. Is the failure unique to SELinux, or does it fail with >> Smack as well? > I don't have a smack configuration handy, but reading through > the code smack setxattr the permission checks for all xattrs > that are not smack xattrs to cap_inode_setxattr. It's not hard to configure Smack. But, if you have a test case I can run it for you. > So smack and commoncap combined will not fail. > > smack and selinux will result in people who should be able to set > selinux xattrs not being able to. That however is less of an immediate > problem. That's not currently a problem as you can't configure them both to be enabled. > >>> removexattr has the same problem and the code is a little less >>> convoluted in that case. >> Right. Because removexattr is a simpler situation. >> >>> Not being able to set the capability when you should be able to is >>> very noticable. Like running into a brick wall noticable. >> Ah, now you've identified the problem. Yes, I would agree that you've >> uncovered an undesirable behavior. > Apologies for not being clearer earlier, but I was still in shock from > running into a brick wall. You clearly don't work in security is running into a brick wall is a shocking experience :) I'm kind of surprised that the capability changes got sent upstream without SELinux ever being tested. That's a very common configuration in the Android enabled world. > >>> Which is where the minimal patch for selinux comes in. I think it >>> solves the exact case in question, even if it isn't the perfect long >>> term solution. >> If the problem is unique to SELinux I can see your logic. If it >> isn't, that is, if it's also a problem with any other security >> module, there either needs to be a fix for that/those module/s >> as well or a "real" fix. >> >> I'm not opposed to the SELinux short term fix if you can say >> that that's the only module with the problem. > So far there are exactly two implementation of > LSM_HOOK_INIT(inode_setxattr, ...) > > So as a practical case it looks like combination with selinux is the > only case where the problem will be observed right now. And it makes > the code at least somewhat match the comments in > security_inode_setxattr. > > Eric > > > . -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Sun, Oct 01, 2017 at 11:52:29AM -0700, Casey Schaufler wrote: > I'm kind of surprised that the capability changes got sent > upstream without SELinux ever being tested. That's a very common > configuration in the Android enabled world. hm, I'm sorry about that. I'd intended years ago to get ubuntu with selinux running again, but never found enough time. Perhaps i should set up an Oracle linux VM for such testing. Would be useful in several ways.. -serge -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Casey Schaufler <casey@schaufler-ca.com> writes: > On 9/30/2017 6:02 PM, Eric W. Biederman wrote: >> I don't have a smack configuration handy, but reading through >> the code smack setxattr the permission checks for all xattrs >> that are not smack xattrs to cap_inode_setxattr. > > It's not hard to configure Smack. But, if you have a test case > I can run it for you. All I did was take /bin/ping from a RHEL or equally a fedora code base where it is setcap, and copied it with rsync as root in a user namespace and looked at the xattr. From memory: $ cd $ unshare -Ur # rsync -Xp /bin/ping ping >> So smack and commoncap combined will not fail. >> >> smack and selinux will result in people who should be able to set >> selinux xattrs not being able to. That however is less of an immediate >> problem. > > That's not currently a problem as you can't configure > them both to be enabled. Like I said not immediate. > You clearly don't work in security is running into a brick > wall is a shocking experience :) The shock was that the security code was so b0rked. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Stephen Smalley <sds@tycho.nsa.gov> writes: > On Thu, 2017-09-28 at 17:34 -0500, Eric W. Biederman wrote: >> It looks like once upon a time a long time ago selinux copied code >> from cap_inode_removexattr and cap_inode_setxattr into >> selinux_inode_setotherxattr. However the code has now diverged and >> selinux is implementing a policy that is quite different than >> cap_inode_setxattr and cap_inode_removexattr especially when it comes >> to the security.capable xattr. >> >> To keep things working and to make the comments in >> security/security.c >> correct when the xattr is securit.capable, call cap_inode_setxattr >> or cap_inode_removexattr as appropriate. >> >> I suspect there is a larger conversation to be had here but this >> is enough to keep selinux from implementing a non-sense hard coded >> policy that breaks other parts of the kernel. > > Originally SELinux called the cap functions directly since there was no > stacking support in the infrastructure and one had to manually stack a > secondary module internally. inode_setxattr and inode_removexattr > however were special cases because the cap functions would check > CAP_SYS_ADMIN for any non-capability attributes in the security.* > namespace, and we don't want to impose that requirement on setting > security.selinux. Thus, we inlined the capabilities logic into the > selinux hook functions and adapted it appropriately. When the stacking > support was introduced, it had to also special case these hooks so that > only the primary module's hook is used for the same reason; otherwise, > the kernel would end up applying a CAP_SYS_ADMIN check on setting > security.selinux. Your change below is almost but not quite right > since it only calls the cap functions when setting the capability > attribute; the residual problem is that it will then skip the SELinux > FILE__SETATTR (file setattr) permission check when setting those > attributes, which we want to retain. So you need to only return early > if cap_inode_setxattr()/removexattr() return an error; otherwise, you > need to proceed to the SELinux check, and you can then delete the > duplicated logic from selinux_inode_setotherxattr(). At which point it > just becomes a call to dentry_has_perm() and you can just inline that > into selinux_inode_setxattr() and selinux_inode_removexattr(). I will look at that. Thank you, Eric -- To unsubscribe from this list: send the line "unsubscribe linux-security-module" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index f5d304736852..edf4bd292dc7 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -3167,6 +3167,9 @@ static int selinux_inode_setxattr(struct dentry *dentry, const char *name, u32 newsid, sid = current_sid(); int rc = 0; + if (strcmp(name, XATTR_NAME_CAPS) == 0) + return cap_inode_setxattr(dentry, name, value, size, flags); + if (strcmp(name, XATTR_NAME_SELINUX)) return selinux_inode_setotherxattr(dentry, name); @@ -3282,6 +3285,9 @@ static int selinux_inode_listxattr(struct dentry *dentry) static int selinux_inode_removexattr(struct dentry *dentry, const char *name) { + if (strcmp(name, XATTR_NAME_CAPS) == 0) + return cap_inode_removexattr(dentry, name); + if (strcmp(name, XATTR_NAME_SELINUX)) return selinux_inode_setotherxattr(dentry, name);
It looks like once upon a time a long time ago selinux copied code from cap_inode_removexattr and cap_inode_setxattr into selinux_inode_setotherxattr. However the code has now diverged and selinux is implementing a policy that is quite different than cap_inode_setxattr and cap_inode_removexattr especially when it comes to the security.capable xattr. To keep things working and to make the comments in security/security.c correct when the xattr is securit.capable, call cap_inode_setxattr or cap_inode_removexattr as appropriate. I suspect there is a larger conversation to be had here but this is enough to keep selinux from implementing a non-sense hard coded policy that breaks other parts of the kernel. Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> --- security/selinux/hooks.c | 6 ++++++ 1 file changed, 6 insertions(+)