path: root/ipc/shm.c
diff options
authorDavidlohr Bueso <davidlohr@hp.com>2013-09-23 17:04:45 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-09-24 09:36:53 -0700
commit53dad6d3a8e5ac1af8bacc6ac2134ae1a8b085f1 (patch)
treefc9349452c9bae7e86dbbbeed99c07bde4bad0c4 /ipc/shm.c
parent4a10c2ac2f368583138b774ca41fac4207911983 (diff)
ipc: fix race with LSMs
Currently, IPC mechanisms do security and auditing related checks under RCU. However, since security modules can free the security structure, for example, through selinux_[sem,msg_queue,shm]_free_security(), we can race if the structure is freed before other tasks are done with it, creating a use-after-free condition. Manfred illustrates this nicely, for instance with shared mem and selinux: -> do_shmat calls rcu_read_lock() -> do_shmat calls shm_object_check(). Checks that the object is still valid - but doesn't acquire any locks. Then it returns. -> do_shmat calls security_shm_shmat (e.g. selinux_shm_shmat) -> selinux_shm_shmat calls ipc_has_perm() -> ipc_has_perm accesses ipc_perms->security shm_close() -> shm_close acquires rw_mutex & shm_lock -> shm_close calls shm_destroy -> shm_destroy calls security_shm_free (e.g. selinux_shm_free_security) -> selinux_shm_free_security calls ipc_free_security(&shp->shm_perm) -> ipc_free_security calls kfree(ipc_perms->security) This patch delays the freeing of the security structures after all RCU readers are done. Furthermore it aligns the security life cycle with that of the rest of IPC - freeing them based on the reference counter. For situations where we need not free security, the current behavior is kept. Linus states: "... the old behavior was suspect for another reason too: having the security blob go away from under a user sounds like it could cause various other problems anyway, so I think the old code was at least _prone_ to bugs even if it didn't have catastrophic behavior." I have tested this patch with IPC testcases from LTP on both my quad-core laptop and on a 64 core NUMA server. In both cases selinux is enabled, and tests pass for both voluntary and forced preemption models. While the mentioned races are theoretical (at least no one as reported them), I wanted to make sure that this new logic doesn't break anything we weren't aware of. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> Acked-by: Manfred Spraul <manfred@colorfullife.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'ipc/shm.c')
1 files changed, 12 insertions, 5 deletions
diff --git a/ipc/shm.c b/ipc/shm.c
index 2821cdf93adb..d69739610fd4 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -167,6 +167,15 @@ static inline void shm_lock_by_ptr(struct shmid_kernel *ipcp)
+static void shm_rcu_free(struct rcu_head *head)
+ struct ipc_rcu *p = container_of(head, struct ipc_rcu, rcu);
+ struct shmid_kernel *shp = ipc_rcu_to_struct(p);
+ security_shm_free(shp);
+ ipc_rcu_free(head);
static inline void shm_rmid(struct ipc_namespace *ns, struct shmid_kernel *s)
ipc_rmid(&shm_ids(ns), &s->shm_perm);
@@ -208,8 +217,7 @@ static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp)
fput (shp->shm_file);
- security_shm_free(shp);
- ipc_rcu_putref(shp);
+ ipc_rcu_putref(shp, shm_rcu_free);
@@ -497,7 +505,7 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
shp->shm_perm.security = NULL;
error = security_shm_alloc(shp);
if (error) {
- ipc_rcu_putref(shp);
+ ipc_rcu_putref(shp, ipc_rcu_free);
return error;
@@ -566,8 +574,7 @@ no_id:
user_shm_unlock(size, shp->mlock_user);
- security_shm_free(shp);
- ipc_rcu_putref(shp);
+ ipc_rcu_putref(shp, shm_rcu_free);
return error;