| Message ID | 20211001024506.3762647-1-tkjos@google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Paul Moore |
| Headers | show |
| Series | binder: use cred instead of task for selinux checks | expand |
On Thu, Sep 30, 2021 at 10:45 PM Todd Kjos <tkjos@google.com> wrote: > > Save the struct cred associated with a binder process > at initial open to avoid potential race conditions > when converting to a security ID. > > Since binder was integrated with selinux, it has passed > 'struct task_struct' associated with the binder_proc > to represent the source and target of transactions. > The conversion of task to SID was then done in the hook > implementations. It turns out that there are race conditions > which can result in an incorrect security context being used. > > Fix by saving the 'struct cred' during binder_open and pass > it to the selinux subsystem. > > Fixes: 79af73079d75 ("Add security hooks to binder and implement the > hooks for SELinux.") > Signed-off-by: Todd Kjos <tkjos@google.com> > Cc: stable@vger.kernel.org # 5.14 (need backport for earlier stables) > --- > drivers/android/binder.c | 14 +++++---- > drivers/android/binder_internal.h | 3 ++ > include/linux/lsm_hook_defs.h | 14 ++++----- > include/linux/security.h | 28 +++++++++--------- > security/security.c | 14 ++++----- > security/selinux/hooks.c | 48 +++++++++---------------------- > 6 files changed, 52 insertions(+), 69 deletions(-) Thanks Todd, I'm happy to see someone with a better understanding of binder than me pitch in to clean this up :) A couple of quick comments/questions below ... > diff --git a/drivers/android/binder.c b/drivers/android/binder.c > index 9edacc8b9768..ca599ebdea4a 100644 > --- a/drivers/android/binder.c > +++ b/drivers/android/binder.c > @@ -5055,6 +5056,7 @@ static int binder_open(struct inode *nodp, struct file *filp) > spin_lock_init(&proc->outer_lock); > get_task_struct(current->group_leader); > proc->tsk = current->group_leader; > + proc->cred = get_cred(filp->f_cred); Is it *always* true that filp->f_cred is going to be the same as current->group_leader->cred? Or rather does this help resolve the issue of wanting the subjective creds but not being able to access them mentioned in the task_sid_binder() comment? If the latter, it might be nice to add something to the related comment in struct binder_ref (below). > INIT_LIST_HEAD(&proc->todo); > init_waitqueue_head(&proc->freeze_wait); > proc->default_priority = task_nice(current); > diff --git a/drivers/android/binder_internal.h b/drivers/android/binder_internal.h > index 402c4d4362a8..886fc327a534 100644 > --- a/drivers/android/binder_internal.h > +++ b/drivers/android/binder_internal.h > @@ -364,6 +364,8 @@ struct binder_ref { > * (invariant after initialized) > * @tsk task_struct for group_leader of process > * (invariant after initialized) > + * @cred struct cred for group_leader of process > + * (invariant after initialized) Related to the question above. At the very least the comment should probably be changed to indicate to make it clear the creds are coming directly from the binder file/device and not always the group_leader. > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index e7ebd45ca345..c8bf3db90c8b 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -255,29 +255,6 @@ static inline u32 task_sid_obj(const struct task_struct *task) > return sid; > } > > -/* > - * get the security ID of a task for use with binder > - */ > -static inline u32 task_sid_binder(const struct task_struct *task) > -{ > - /* > - * In many case where this function is used we should be using the > - * task's subjective SID, but we can't reliably access the subjective > - * creds of a task other than our own so we must use the objective > - * creds/SID, which are safe to access. The downside is that if a task > - * is temporarily overriding it's creds it will not be reflected here; > - * however, it isn't clear that binder would handle that case well > - * anyway. > - * > - * If this ever changes and we can safely reference the subjective > - * creds/SID of another task, this function will make it easier to > - * identify the various places where we make use of the task SIDs in > - * the binder code. It is also likely that we will need to adjust > - * the main drivers/android binder code as well. > - */ > - return task_sid_obj(task); > -}
On Fri, Oct 1, 2021 at 10:38 AM Paul Moore <paul@paul-moore.com> wrote: > On Thu, Sep 30, 2021 at 10:45 PM Todd Kjos <tkjos@google.com> wrote: > > > > Save the struct cred associated with a binder process > > at initial open to avoid potential race conditions > > when converting to a security ID. > > > > Since binder was integrated with selinux, it has passed > > 'struct task_struct' associated with the binder_proc > > to represent the source and target of transactions. > > The conversion of task to SID was then done in the hook > > implementations. It turns out that there are race conditions > > which can result in an incorrect security context being used. > > > > Fix by saving the 'struct cred' during binder_open and pass > > it to the selinux subsystem. > > > > Fixes: 79af73079d75 ("Add security hooks to binder and implement the > > hooks for SELinux.") > > Signed-off-by: Todd Kjos <tkjos@google.com> > > Cc: stable@vger.kernel.org # 5.14 (need backport for earlier stables) > > --- > > drivers/android/binder.c | 14 +++++---- > > drivers/android/binder_internal.h | 3 ++ > > include/linux/lsm_hook_defs.h | 14 ++++----- > > include/linux/security.h | 28 +++++++++--------- > > security/security.c | 14 ++++----- > > security/selinux/hooks.c | 48 +++++++++---------------------- > > 6 files changed, 52 insertions(+), 69 deletions(-) > > Thanks Todd, I'm happy to see someone with a better understanding of > binder than me pitch in to clean this up :) A couple of quick > comments/questions below ... Ooops, I was a little over zealous when trimming my response and I accidentally cut off my comment that the associated comment blocks in include/linux/lsm_hooks.h should also be updated to reflect the binder LSM hook changes.
On Fri, Oct 1, 2021 at 7:38 AM Paul Moore <paul@paul-moore.com> wrote: > > On Thu, Sep 30, 2021 at 10:45 PM Todd Kjos <tkjos@google.com> wrote: > > > > Save the struct cred associated with a binder process > > at initial open to avoid potential race conditions > > when converting to a security ID. > > > > Since binder was integrated with selinux, it has passed > > 'struct task_struct' associated with the binder_proc > > to represent the source and target of transactions. > > The conversion of task to SID was then done in the hook > > implementations. It turns out that there are race conditions > > which can result in an incorrect security context being used. > > > > Fix by saving the 'struct cred' during binder_open and pass > > it to the selinux subsystem. > > > > Fixes: 79af73079d75 ("Add security hooks to binder and implement the > > hooks for SELinux.") > > Signed-off-by: Todd Kjos <tkjos@google.com> > > Cc: stable@vger.kernel.org # 5.14 (need backport for earlier stables) > > --- > > drivers/android/binder.c | 14 +++++---- > > drivers/android/binder_internal.h | 3 ++ > > include/linux/lsm_hook_defs.h | 14 ++++----- > > include/linux/security.h | 28 +++++++++--------- > > security/security.c | 14 ++++----- > > security/selinux/hooks.c | 48 +++++++++---------------------- > > 6 files changed, 52 insertions(+), 69 deletions(-) > > Thanks Todd, I'm happy to see someone with a better understanding of > binder than me pitch in to clean this up :) A couple of quick > comments/questions below ... > > > diff --git a/drivers/android/binder.c b/drivers/android/binder.c > > index 9edacc8b9768..ca599ebdea4a 100644 > > --- a/drivers/android/binder.c > > +++ b/drivers/android/binder.c > > @@ -5055,6 +5056,7 @@ static int binder_open(struct inode *nodp, struct file *filp) > > spin_lock_init(&proc->outer_lock); > > get_task_struct(current->group_leader); > > proc->tsk = current->group_leader; > > + proc->cred = get_cred(filp->f_cred); > > Is it *always* true that filp->f_cred is going to be the same as > current->group_leader->cred? Not necessarily -- it is current->cred of the task in binder_open() (not group_leader). This is fine. We used to set proc->tsk to current, but switched to group_leader a few years ago to make it easier to detect the same process with multiple opens during mmap (to solve some unrelated issues). We still use group_leader for that purpose, but for the cred, the current cred in binder_open() is sufficient. > Or rather does this help resolve the > issue of wanting the subjective creds but not being able to access > them mentioned in the task_sid_binder() comment? If the latter, it > might be nice to add something to the related comment in struct > binder_ref (below). Yes, we want the subjective cred so that is part of the point. I started with "proc->cred = get_task_cred(current->group_leader)" and got feedback that the "subjective" cred is preferred to avoid some subtle races that could be introduced, for example, if /dev/binder is opened through io_uring. > > > INIT_LIST_HEAD(&proc->todo); > > init_waitqueue_head(&proc->freeze_wait); > > proc->default_priority = task_nice(current); > > diff --git a/drivers/android/binder_internal.h b/drivers/android/binder_internal.h > > index 402c4d4362a8..886fc327a534 100644 > > --- a/drivers/android/binder_internal.h > > +++ b/drivers/android/binder_internal.h > > @@ -364,6 +364,8 @@ struct binder_ref { > > * (invariant after initialized) > > * @tsk task_struct for group_leader of process > > * (invariant after initialized) > > + * @cred struct cred for group_leader of process > > + * (invariant after initialized) > > Related to the question above. At the very least the comment should > probably be changed to indicate to make it clear the creds are coming > directly from the binder file/device and not always the group_leader. Good catch. Will update the comment (it's actually struct binder_proc). > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > > index e7ebd45ca345..c8bf3db90c8b 100644 > > --- a/security/selinux/hooks.c > > +++ b/security/selinux/hooks.c > > @@ -255,29 +255,6 @@ static inline u32 task_sid_obj(const struct task_struct *task) > > return sid; > > } > > > > -/* > > - * get the security ID of a task for use with binder > > - */ > > -static inline u32 task_sid_binder(const struct task_struct *task) > > -{ > > - /* > > - * In many case where this function is used we should be using the > > - * task's subjective SID, but we can't reliably access the subjective > > - * creds of a task other than our own so we must use the objective > > - * creds/SID, which are safe to access. The downside is that if a task > > - * is temporarily overriding it's creds it will not be reflected here; > > - * however, it isn't clear that binder would handle that case well > > - * anyway. > > - * > > - * If this ever changes and we can safely reference the subjective > > - * creds/SID of another task, this function will make it easier to > > - * identify the various places where we make use of the task SIDs in > > - * the binder code. It is also likely that we will need to adjust > > - * the main drivers/android binder code as well. > > - */ > > - return task_sid_obj(task); > > -} > > -- > paul moore > www.paul-moore.com and from your next response: > Ooops, I was a little over zealous when trimming my response and I > accidentally cut off my comment that the associated comment blocks in > include/linux/lsm_hooks.h should also be updated to reflect the binder > LSM hook changes. Thanks for pointing this out! I didn't notice these comment blocks. -Todd
diff --git a/drivers/android/binder.c b/drivers/android/binder.c index 9edacc8b9768..ca599ebdea4a 100644 --- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -2056,7 +2056,7 @@ static int binder_translate_binder(struct flat_binder_object *fp, ret = -EINVAL; goto done; } - if (security_binder_transfer_binder(proc->tsk, target_proc->tsk)) { + if (security_binder_transfer_binder(proc->cred, target_proc->cred)) { ret = -EPERM; goto done; } @@ -2102,7 +2102,7 @@ static int binder_translate_handle(struct flat_binder_object *fp, proc->pid, thread->pid, fp->handle); return -EINVAL; } - if (security_binder_transfer_binder(proc->tsk, target_proc->tsk)) { + if (security_binder_transfer_binder(proc->cred, target_proc->cred)) { ret = -EPERM; goto done; } @@ -2190,7 +2190,7 @@ static int binder_translate_fd(u32 fd, binder_size_t fd_offset, ret = -EBADF; goto err_fget; } - ret = security_binder_transfer_file(proc->tsk, target_proc->tsk, file); + ret = security_binder_transfer_file(proc->cred, target_proc->cred, file); if (ret < 0) { ret = -EPERM; goto err_security; @@ -2595,8 +2595,8 @@ static void binder_transaction(struct binder_proc *proc, return_error_line = __LINE__; goto err_invalid_target_handle; } - if (security_binder_transaction(proc->tsk, - target_proc->tsk) < 0) { + if (security_binder_transaction(proc->cred, + target_proc->cred) < 0) { return_error = BR_FAILED_REPLY; return_error_param = -EPERM; return_error_line = __LINE__; @@ -4353,6 +4353,7 @@ static void binder_free_proc(struct binder_proc *proc) } binder_alloc_deferred_release(&proc->alloc); put_task_struct(proc->tsk); + put_cred(proc->cred); binder_stats_deleted(BINDER_STAT_PROC); kfree(proc); } @@ -4564,7 +4565,7 @@ static int binder_ioctl_set_ctx_mgr(struct file *filp, ret = -EBUSY; goto out; } - ret = security_binder_set_context_mgr(proc->tsk); + ret = security_binder_set_context_mgr(proc->cred); if (ret < 0) goto out; if (uid_valid(context->binder_context_mgr_uid)) { @@ -5055,6 +5056,7 @@ static int binder_open(struct inode *nodp, struct file *filp) spin_lock_init(&proc->outer_lock); get_task_struct(current->group_leader); proc->tsk = current->group_leader; + proc->cred = get_cred(filp->f_cred); INIT_LIST_HEAD(&proc->todo); init_waitqueue_head(&proc->freeze_wait); proc->default_priority = task_nice(current); diff --git a/drivers/android/binder_internal.h b/drivers/android/binder_internal.h index 402c4d4362a8..886fc327a534 100644 --- a/drivers/android/binder_internal.h +++ b/drivers/android/binder_internal.h @@ -364,6 +364,8 @@ struct binder_ref { * (invariant after initialized) * @tsk task_struct for group_leader of process * (invariant after initialized) + * @cred struct cred for group_leader of process + * (invariant after initialized) * @deferred_work_node: element for binder_deferred_list * (protected by binder_deferred_lock) * @deferred_work: bitmap of deferred work to perform @@ -426,6 +428,7 @@ struct binder_proc { struct list_head waiting_threads; int pid; struct task_struct *tsk; + const struct cred *cred; struct hlist_node deferred_work_node; int deferred_work; int outstanding_txns; diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h index 2adeea44c0d5..61590c1f2d33 100644 --- a/include/linux/lsm_hook_defs.h +++ b/include/linux/lsm_hook_defs.h @@ -26,13 +26,13 @@ * #undef LSM_HOOK * }; */ -LSM_HOOK(int, 0, binder_set_context_mgr, struct task_struct *mgr) -LSM_HOOK(int, 0, binder_transaction, struct task_struct *from, - struct task_struct *to) -LSM_HOOK(int, 0, binder_transfer_binder, struct task_struct *from, - struct task_struct *to) -LSM_HOOK(int, 0, binder_transfer_file, struct task_struct *from, - struct task_struct *to, struct file *file) +LSM_HOOK(int, 0, binder_set_context_mgr, const struct cred *mgr) +LSM_HOOK(int, 0, binder_transaction, const struct cred *from, + const struct cred *to) +LSM_HOOK(int, 0, binder_transfer_binder, const struct cred *from, + const struct cred *to) +LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from, + const struct cred *to, struct file *file) LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child, unsigned int mode) LSM_HOOK(int, 0, ptrace_traceme, struct task_struct *parent) diff --git a/include/linux/security.h b/include/linux/security.h index 5b7288521300..6344d3362df7 100644 --- a/include/linux/security.h +++ b/include/linux/security.h @@ -258,13 +258,13 @@ extern int security_init(void); extern int early_security_init(void); /* Security operations */ -int security_binder_set_context_mgr(struct task_struct *mgr); -int security_binder_transaction(struct task_struct *from, - struct task_struct *to); -int security_binder_transfer_binder(struct task_struct *from, - struct task_struct *to); -int security_binder_transfer_file(struct task_struct *from, - struct task_struct *to, struct file *file); +int security_binder_set_context_mgr(const struct cred *mgr); +int security_binder_transaction(const struct cred *from, + const struct cred *to); +int security_binder_transfer_binder(const struct cred *from, + const struct cred *to); +int security_binder_transfer_file(const struct cred *from, + const struct cred *to, struct file *file); int security_ptrace_access_check(struct task_struct *child, unsigned int mode); int security_ptrace_traceme(struct task_struct *parent); int security_capget(struct task_struct *target, @@ -508,25 +508,25 @@ static inline int early_security_init(void) return 0; } -static inline int security_binder_set_context_mgr(struct task_struct *mgr) +static inline int security_binder_set_context_mgr(const struct cred *mgr) { return 0; } -static inline int security_binder_transaction(struct task_struct *from, - struct task_struct *to) +static inline int security_binder_transaction(const struct cred *from, + const struct cred *to) { return 0; } -static inline int security_binder_transfer_binder(struct task_struct *from, - struct task_struct *to) +static inline int security_binder_transfer_binder(const struct cred *from, + const struct cred *to) { return 0; } -static inline int security_binder_transfer_file(struct task_struct *from, - struct task_struct *to, +static inline int security_binder_transfer_file(const struct cred *from, + const struct cred *to, struct file *file) { return 0; diff --git a/security/security.c b/security/security.c index 9ffa9e9c5c55..67264cb08fb3 100644 --- a/security/security.c +++ b/security/security.c @@ -747,25 +747,25 @@ static int lsm_superblock_alloc(struct super_block *sb) /* Security operations */ -int security_binder_set_context_mgr(struct task_struct *mgr) +int security_binder_set_context_mgr(const struct cred *mgr) { return call_int_hook(binder_set_context_mgr, 0, mgr); } -int security_binder_transaction(struct task_struct *from, - struct task_struct *to) +int security_binder_transaction(const struct cred *from, + const struct cred *to) { return call_int_hook(binder_transaction, 0, from, to); } -int security_binder_transfer_binder(struct task_struct *from, - struct task_struct *to) +int security_binder_transfer_binder(const struct cred *from, + const struct cred *to) { return call_int_hook(binder_transfer_binder, 0, from, to); } -int security_binder_transfer_file(struct task_struct *from, - struct task_struct *to, struct file *file) +int security_binder_transfer_file(const struct cred *from, + const struct cred *to, struct file *file) { return call_int_hook(binder_transfer_file, 0, from, to, file); } diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index e7ebd45ca345..c8bf3db90c8b 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -255,29 +255,6 @@ static inline u32 task_sid_obj(const struct task_struct *task) return sid; } -/* - * get the security ID of a task for use with binder - */ -static inline u32 task_sid_binder(const struct task_struct *task) -{ - /* - * In many case where this function is used we should be using the - * task's subjective SID, but we can't reliably access the subjective - * creds of a task other than our own so we must use the objective - * creds/SID, which are safe to access. The downside is that if a task - * is temporarily overriding it's creds it will not be reflected here; - * however, it isn't clear that binder would handle that case well - * anyway. - * - * If this ever changes and we can safely reference the subjective - * creds/SID of another task, this function will make it easier to - * identify the various places where we make use of the task SIDs in - * the binder code. It is also likely that we will need to adjust - * the main drivers/android binder code as well. - */ - return task_sid_obj(task); -} - static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dentry); /* @@ -2066,18 +2043,19 @@ static inline u32 open_file_to_av(struct file *file) /* Hook functions begin here. */ -static int selinux_binder_set_context_mgr(struct task_struct *mgr) +static int selinux_binder_set_context_mgr(const struct cred *mgr) { return avc_has_perm(&selinux_state, - current_sid(), task_sid_binder(mgr), SECCLASS_BINDER, + current_sid(), cred_sid(mgr), SECCLASS_BINDER, BINDER__SET_CONTEXT_MGR, NULL); } -static int selinux_binder_transaction(struct task_struct *from, - struct task_struct *to) +static int selinux_binder_transaction(const struct cred *from, + const struct cred *to) { u32 mysid = current_sid(); - u32 fromsid = task_sid_binder(from); + u32 fromsid = cred_sid(from); + u32 tosid = cred_sid(to); int rc; if (mysid != fromsid) { @@ -2088,24 +2066,24 @@ static int selinux_binder_transaction(struct task_struct *from, return rc; } - return avc_has_perm(&selinux_state, fromsid, task_sid_binder(to), + return avc_has_perm(&selinux_state, fromsid, tosid, SECCLASS_BINDER, BINDER__CALL, NULL); } -static int selinux_binder_transfer_binder(struct task_struct *from, - struct task_struct *to) +static int selinux_binder_transfer_binder(const struct cred *from, + const struct cred *to) { return avc_has_perm(&selinux_state, - task_sid_binder(from), task_sid_binder(to), + cred_sid(from), cred_sid(to), SECCLASS_BINDER, BINDER__TRANSFER, NULL); } -static int selinux_binder_transfer_file(struct task_struct *from, - struct task_struct *to, +static int selinux_binder_transfer_file(const struct cred *from, + const struct cred *to, struct file *file) { - u32 sid = task_sid_binder(to); + u32 sid = cred_sid(to); struct file_security_struct *fsec = selinux_file(file); struct dentry *dentry = file->f_path.dentry; struct inode_security_struct *isec;
Save the struct cred associated with a binder process at initial open to avoid potential race conditions when converting to a security ID. Since binder was integrated with selinux, it has passed 'struct task_struct' associated with the binder_proc to represent the source and target of transactions. The conversion of task to SID was then done in the hook implementations. It turns out that there are race conditions which can result in an incorrect security context being used. Fix by saving the 'struct cred' during binder_open and pass it to the selinux subsystem. Fixes: 79af73079d75 ("Add security hooks to binder and implement the hooks for SELinux.") Signed-off-by: Todd Kjos <tkjos@google.com> Cc: stable@vger.kernel.org # 5.14 (need backport for earlier stables) --- drivers/android/binder.c | 14 +++++---- drivers/android/binder_internal.h | 3 ++ include/linux/lsm_hook_defs.h | 14 ++++----- include/linux/security.h | 28 +++++++++--------- security/security.c | 14 ++++----- security/selinux/hooks.c | 48 +++++++++---------------------- 6 files changed, 52 insertions(+), 69 deletions(-)