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>

1 files changed, 12 insertions, 20 deletions

diff --git a/ipc/util.c b/ipc/util.c
index e829da9ed01..fdb8ae74077 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -474,11 +474,6 @@ void ipc_free(void* ptr, int size)
 		kfree(ptr);
 }
 
-struct ipc_rcu {
-	struct rcu_head rcu;
-	atomic_t refcount;
-} ____cacheline_aligned_in_smp;
-
 /**
  *	ipc_rcu_alloc	-	allocate ipc and rcu space 
  *	@size: size desired
@@ -505,27 +500,24 @@ int ipc_rcu_getref(void *ptr)
 	return atomic_inc_not_zero(&p->refcount);
 }
 
-/**
- * ipc_schedule_free - free ipc + rcu space
- * @head: RCU callback structure for queued work
- */
-static void ipc_schedule_free(struct rcu_head *head)
-{
-	vfree(container_of(head, struct ipc_rcu, rcu));
-}
-
-void ipc_rcu_putref(void *ptr)
+void ipc_rcu_putref(void *ptr, void (*func)(struct rcu_head *head))
 {
 	struct ipc_rcu *p = ((struct ipc_rcu *)ptr) - 1;
 
 	if (!atomic_dec_and_test(&p->refcount))
 		return;
 
-	if (is_vmalloc_addr(ptr)) {
-		call_rcu(&p->rcu, ipc_schedule_free);
-	} else {
-		kfree_rcu(p, rcu);
-	}
+	call_rcu(&p->rcu, func);
+}
+
+void ipc_rcu_free(struct rcu_head *head)
+{
+	struct ipc_rcu *p = container_of(head, struct ipc_rcu, rcu);
+
+	if (is_vmalloc_addr(p))
+		vfree(p);
+	else
+		kfree(p);
 }
 
 /**