[PATCH] kernel: fix data race in put_pid

[Dmitry Vyukov <dvyukov@google.com>]

put_pid checks whether the current thread has the only reference
to the pid with atomic_read() which does not have any memory
barriers, and if so proceeds directly to kmem_cache_free().
As the result memory accesses to the object in kmem_cache_free()
or user accesses to the object after reallocation (again without
any memory barriers on fast path) can hoist above the atomic_read()
check and conflict with memory accesses to the pid object in other
threads before they released their references.

There is a control dependency between the atomic_read() check and
kmem_cache_free(), but control dependencies are disregarded by some
architectures. Documentation/memory-barriers.txt explicitly states:
"A load-load control dependency requires a full read memory barrier.
... please note that READ_ONCE_CTRL() is not optional! [even for stores]"
in the CONTROL DEPENDENCIES section.

For example, if store to the first word of the object to build a freelist
in kmem_cache_free() hoists above the check, stores to the first word
in other threads can corrupt the memory allocator freelist.

Use atomic_read_acquire() for the fast path check to hand off properly
acquired object to memory allocator.

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
---
 kernel/pid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index ca36879..3b0b13d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -242,7 +242,7 @@ void put_pid(struct pid *pid)
 		return;
 
 	ns = pid->numbers[pid->level].ns;
-	if ((atomic_read(&pid->count) == 1) ||
+	if ((atomic_read_acquire(&pid->count) == 1) ||
 	     atomic_dec_and_test(&pid->count)) {
 		kmem_cache_free(ns->pid_cachep, pid);
 		put_pid_ns(ns);
-- 
2.6.0.rc0.131.gf624c3d