| Message ID | 1461339521-123191-15-git-send-email-seth.forshee@canonical.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
Quoting Seth Forshee (seth.forshee@canonical.com): > In a userns mount some on-disk inodes may have ids which do not > map into s_user_ns, in which case the in-kernel inodes are owned > by invalid users. The superblock owner should be able to change > attributes of these inodes but cannot. However it is unsafe to > grant the superblock owner privileged access to all inodes in the > superblock since proc, sysfs, etc. use DAC to protect files which > may not belong to s_user_ns. The problem is restricted to only > inodes where the owner or group is an invalid user. > > We can work around this by allowing users with CAP_CHOWN in > s_user_ns to change an invalid owner or group id, so long as the > other id is either invalid or mappable in s_user_ns. After > changing ownership the user will be privileged towards the inode > and thus able to change other attributes. > > As an precaution, checks for invalid ids are added to the proc > and kernfs setattr interfaces. These filesystems are not expected > to have inodes with invalid ids, but if it does happen any > setattr operations will return -EPERM. > > Signed-off-by: Seth Forshee <seth.forshee@canonical.com> Acked-by: Serge Hallyn <serge.hallyn@canonical.com> bug a request below, > --- > fs/attr.c | 47 +++++++++++++++++++++++++++++++++++++++-------- > fs/kernfs/inode.c | 2 ++ > fs/proc/base.c | 2 ++ > fs/proc/generic.c | 3 +++ > fs/proc/proc_sysctl.c | 2 ++ > 5 files changed, 48 insertions(+), 8 deletions(-) > > diff --git a/fs/attr.c b/fs/attr.c > index 3cfaaac4a18e..a8b0931654a5 100644 > --- a/fs/attr.c > +++ b/fs/attr.c > @@ -16,6 +16,43 @@ > #include <linux/evm.h> > #include <linux/ima.h> > > +static bool chown_ok(const struct inode *inode, kuid_t uid) > +{ > + struct user_namespace *user_ns; > + > + if (uid_eq(current_fsuid(), inode->i_uid) && uid_eq(uid, inode->i_uid)) > + return true; > + if (capable_wrt_inode_uidgid(inode, CAP_CHOWN)) > + return true; > + > + user_ns = inode->i_sb->s_user_ns; > + if (!uid_valid(inode->i_uid) && > + (!gid_valid(inode->i_gid) || kgid_has_mapping(user_ns, inode->i_gid)) && This confused me to no end :) Perhaps a "is_unmapped_valid_gid()" helper would make it clearer what this is meant to do? Or else maybe a comment above chown_ok(), explaining that 1. for a blockdev, the uid is converted at inode read so that it is either mapped or invalid 2. for sysfs / etc, uid can be valid but not mapped into the userns > + ns_capable(user_ns, CAP_CHOWN)) > + return true; > + > + return false; > +} > + > +static bool chgrp_ok(const struct inode *inode, kgid_t gid) > +{ > + struct user_namespace *user_ns; > + > + if (uid_eq(current_fsuid(), inode->i_uid) && > + (in_group_p(gid) || gid_eq(gid, inode->i_gid))) > + return true; > + if (capable_wrt_inode_uidgid(inode, CAP_CHOWN)) > + return true; > + > + user_ns = inode->i_sb->s_user_ns; > + if (!gid_valid(inode->i_gid) && > + (!uid_valid(inode->i_uid) || kuid_has_mapping(user_ns, inode->i_uid)) && > + ns_capable(user_ns, CAP_CHOWN)) > + return true; > + > + return false; > +} > + > /** > * inode_change_ok - check if attribute changes to an inode are allowed > * @inode: inode to check > @@ -58,17 +95,11 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr) > return 0; > > /* Make sure a caller can chown. */ > - if ((ia_valid & ATTR_UID) && > - (!uid_eq(current_fsuid(), inode->i_uid) || > - !uid_eq(attr->ia_uid, inode->i_uid)) && > - !capable_wrt_inode_uidgid(inode, CAP_CHOWN)) > + if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid)) > return -EPERM; > > /* Make sure caller can chgrp. */ > - if ((ia_valid & ATTR_GID) && > - (!uid_eq(current_fsuid(), inode->i_uid) || > - (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) && > - !capable_wrt_inode_uidgid(inode, CAP_CHOWN)) > + if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid)) > return -EPERM; > > /* Make sure a caller can chmod. */ > diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c > index 16405ae88d2d..2e97a337ee5f 100644 > --- a/fs/kernfs/inode.c > +++ b/fs/kernfs/inode.c > @@ -117,6 +117,8 @@ int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr) > > if (!kn) > return -EINVAL; > + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) > + return -EPERM; > > mutex_lock(&kernfs_mutex); > error = inode_change_ok(inode, iattr); > diff --git a/fs/proc/base.c b/fs/proc/base.c > index b1755b23893e..648d623e2158 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -711,6 +711,8 @@ int proc_setattr(struct dentry *dentry, struct iattr *attr) > > if (attr->ia_valid & ATTR_MODE) > return -EPERM; > + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) > + return -EPERM; > > error = inode_change_ok(inode, attr); > if (error) > diff --git a/fs/proc/generic.c b/fs/proc/generic.c > index ff3ffc76a937..1461570c552c 100644 > --- a/fs/proc/generic.c > +++ b/fs/proc/generic.c > @@ -105,6 +105,9 @@ static int proc_notify_change(struct dentry *dentry, struct iattr *iattr) > struct proc_dir_entry *de = PDE(inode); > int error; > > + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) > + return -EPERM; > + > error = inode_change_ok(inode, iattr); > if (error) > return error; > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c > index fe5b6e6c4671..f5d575157194 100644 > --- a/fs/proc/proc_sysctl.c > +++ b/fs/proc/proc_sysctl.c > @@ -752,6 +752,8 @@ static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr) > > if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) > return -EPERM; > + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) > + return -EPERM; > > error = inode_change_ok(inode, attr); > if (error) > -- > 1.9.1
On Mon, Apr 25, 2016 at 03:30:47PM -0500, Serge E. Hallyn wrote: > Quoting Seth Forshee (seth.forshee@canonical.com): > > In a userns mount some on-disk inodes may have ids which do not > > map into s_user_ns, in which case the in-kernel inodes are owned > > by invalid users. The superblock owner should be able to change > > attributes of these inodes but cannot. However it is unsafe to > > grant the superblock owner privileged access to all inodes in the > > superblock since proc, sysfs, etc. use DAC to protect files which > > may not belong to s_user_ns. The problem is restricted to only > > inodes where the owner or group is an invalid user. > > > > We can work around this by allowing users with CAP_CHOWN in > > s_user_ns to change an invalid owner or group id, so long as the > > other id is either invalid or mappable in s_user_ns. After > > changing ownership the user will be privileged towards the inode > > and thus able to change other attributes. > > > > As an precaution, checks for invalid ids are added to the proc > > and kernfs setattr interfaces. These filesystems are not expected > > to have inodes with invalid ids, but if it does happen any > > setattr operations will return -EPERM. > > > > Signed-off-by: Seth Forshee <seth.forshee@canonical.com> > > Acked-by: Serge Hallyn <serge.hallyn@canonical.com> > > bug a request below, > > > --- > > fs/attr.c | 47 +++++++++++++++++++++++++++++++++++++++-------- > > fs/kernfs/inode.c | 2 ++ > > fs/proc/base.c | 2 ++ > > fs/proc/generic.c | 3 +++ > > fs/proc/proc_sysctl.c | 2 ++ > > 5 files changed, 48 insertions(+), 8 deletions(-) > > > > diff --git a/fs/attr.c b/fs/attr.c > > index 3cfaaac4a18e..a8b0931654a5 100644 > > --- a/fs/attr.c > > +++ b/fs/attr.c > > @@ -16,6 +16,43 @@ > > #include <linux/evm.h> > > #include <linux/ima.h> > > > > +static bool chown_ok(const struct inode *inode, kuid_t uid) > > +{ > > + struct user_namespace *user_ns; > > + > > + if (uid_eq(current_fsuid(), inode->i_uid) && uid_eq(uid, inode->i_uid)) > > + return true; > > + if (capable_wrt_inode_uidgid(inode, CAP_CHOWN)) > > + return true; > > + > > + user_ns = inode->i_sb->s_user_ns; > > + if (!uid_valid(inode->i_uid) && > > + (!gid_valid(inode->i_gid) || kgid_has_mapping(user_ns, inode->i_gid)) && > > This confused me to no end :) Perhaps a "is_unmapped_valid_gid()" helper > would make it clearer what this is meant to do? Or else maybe a comment > above chown_ok(), explaining that > > 1. for a blockdev, the uid is converted at inode read so that it is > either mapped or invalid > 2. for sysfs / etc, uid can be valid but not mapped into the userns Even with a helper a comment is probably helpful to explain why. I'll do that first, then see if a helper would make things any clearer. Honestly, I had to think about the helper name you proposed for a minute before it made sense even though I already understood the code ;-)
diff --git a/fs/attr.c b/fs/attr.c index 3cfaaac4a18e..a8b0931654a5 100644 --- a/fs/attr.c +++ b/fs/attr.c @@ -16,6 +16,43 @@ #include <linux/evm.h> #include <linux/ima.h> +static bool chown_ok(const struct inode *inode, kuid_t uid) +{ + struct user_namespace *user_ns; + + if (uid_eq(current_fsuid(), inode->i_uid) && uid_eq(uid, inode->i_uid)) + return true; + if (capable_wrt_inode_uidgid(inode, CAP_CHOWN)) + return true; + + user_ns = inode->i_sb->s_user_ns; + if (!uid_valid(inode->i_uid) && + (!gid_valid(inode->i_gid) || kgid_has_mapping(user_ns, inode->i_gid)) && + ns_capable(user_ns, CAP_CHOWN)) + return true; + + return false; +} + +static bool chgrp_ok(const struct inode *inode, kgid_t gid) +{ + struct user_namespace *user_ns; + + if (uid_eq(current_fsuid(), inode->i_uid) && + (in_group_p(gid) || gid_eq(gid, inode->i_gid))) + return true; + if (capable_wrt_inode_uidgid(inode, CAP_CHOWN)) + return true; + + user_ns = inode->i_sb->s_user_ns; + if (!gid_valid(inode->i_gid) && + (!uid_valid(inode->i_uid) || kuid_has_mapping(user_ns, inode->i_uid)) && + ns_capable(user_ns, CAP_CHOWN)) + return true; + + return false; +} + /** * inode_change_ok - check if attribute changes to an inode are allowed * @inode: inode to check @@ -58,17 +95,11 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr) return 0; /* Make sure a caller can chown. */ - if ((ia_valid & ATTR_UID) && - (!uid_eq(current_fsuid(), inode->i_uid) || - !uid_eq(attr->ia_uid, inode->i_uid)) && - !capable_wrt_inode_uidgid(inode, CAP_CHOWN)) + if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid)) return -EPERM; /* Make sure caller can chgrp. */ - if ((ia_valid & ATTR_GID) && - (!uid_eq(current_fsuid(), inode->i_uid) || - (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) && - !capable_wrt_inode_uidgid(inode, CAP_CHOWN)) + if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid)) return -EPERM; /* Make sure a caller can chmod. */ diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c index 16405ae88d2d..2e97a337ee5f 100644 --- a/fs/kernfs/inode.c +++ b/fs/kernfs/inode.c @@ -117,6 +117,8 @@ int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr) if (!kn) return -EINVAL; + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) + return -EPERM; mutex_lock(&kernfs_mutex); error = inode_change_ok(inode, iattr); diff --git a/fs/proc/base.c b/fs/proc/base.c index b1755b23893e..648d623e2158 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -711,6 +711,8 @@ int proc_setattr(struct dentry *dentry, struct iattr *attr) if (attr->ia_valid & ATTR_MODE) return -EPERM; + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) + return -EPERM; error = inode_change_ok(inode, attr); if (error) diff --git a/fs/proc/generic.c b/fs/proc/generic.c index ff3ffc76a937..1461570c552c 100644 --- a/fs/proc/generic.c +++ b/fs/proc/generic.c @@ -105,6 +105,9 @@ static int proc_notify_change(struct dentry *dentry, struct iattr *iattr) struct proc_dir_entry *de = PDE(inode); int error; + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) + return -EPERM; + error = inode_change_ok(inode, iattr); if (error) return error; diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c index fe5b6e6c4671..f5d575157194 100644 --- a/fs/proc/proc_sysctl.c +++ b/fs/proc/proc_sysctl.c @@ -752,6 +752,8 @@ static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr) if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) return -EPERM; + if (!uid_valid(inode->i_uid) || !gid_valid(inode->i_gid)) + return -EPERM; error = inode_change_ok(inode, attr); if (error)
In a userns mount some on-disk inodes may have ids which do not map into s_user_ns, in which case the in-kernel inodes are owned by invalid users. The superblock owner should be able to change attributes of these inodes but cannot. However it is unsafe to grant the superblock owner privileged access to all inodes in the superblock since proc, sysfs, etc. use DAC to protect files which may not belong to s_user_ns. The problem is restricted to only inodes where the owner or group is an invalid user. We can work around this by allowing users with CAP_CHOWN in s_user_ns to change an invalid owner or group id, so long as the other id is either invalid or mappable in s_user_ns. After changing ownership the user will be privileged towards the inode and thus able to change other attributes. As an precaution, checks for invalid ids are added to the proc and kernfs setattr interfaces. These filesystems are not expected to have inodes with invalid ids, but if it does happen any setattr operations will return -EPERM. Signed-off-by: Seth Forshee <seth.forshee@canonical.com> --- fs/attr.c | 47 +++++++++++++++++++++++++++++++++++++++-------- fs/kernfs/inode.c | 2 ++ fs/proc/base.c | 2 ++ fs/proc/generic.c | 3 +++ fs/proc/proc_sysctl.c | 2 ++ 5 files changed, 48 insertions(+), 8 deletions(-)