[PATCH v2] mm/mempolicy: fix use after free when calling get_mempolicy

[zhong jiang <zhongjiang@huawei.com>]

I hit an use after free issue by executing trinity. and repoduce it
with KASAN enabled. The related call trace is as follows.

BUG: KASan: use after free in SyS_get_mempolicy+0x3c8/0x960 at addr ffff8801f582d766
Read of size 2 by task syz-executor1/798

INFO: Allocated in mpol_new.part.2+0x74/0x160 age=3 cpu=1 pid=799
__slab_alloc+0x768/0x970
kmem_cache_alloc+0x2e7/0x450
mpol_new.part.2+0x74/0x160
mpol_new+0x66/0x80
SyS_mbind+0x267/0x9f0
system_call_fastpath+0x16/0x1b
INFO: Freed in __mpol_put+0x2b/0x40 age=4 cpu=1 pid=799
__slab_free+0x495/0x8e0
kmem_cache_free+0x2f3/0x4c0
__mpol_put+0x2b/0x40
SyS_mbind+0x383/0x9f0
system_call_fastpath+0x16/0x1b
INFO: Slab 0xffffea0009cb8dc0 objects=23 used=8 fp=0xffff8801f582de40 flags=0x200000000004080
INFO: Object 0xffff8801f582d760 @offset=5984 fp=0xffff8801f582d600

Bytes b4 ffff8801f582d750: ae 01 ff ff 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a  ........ZZZZZZZZ
Object ffff8801f582d760: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
Object ffff8801f582d770: 6b 6b 6b 6b 6b 6b 6b a5                          kkkkkkk.
Redzone ffff8801f582d778: bb bb bb bb bb bb bb bb                          ........
Padding ffff8801f582d8b8: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ
Memory state around the buggy address:
ffff8801f582d600: fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801f582d680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8801f582d700: fc fc fc fc fc fc fc fc fc fc fc fc fb fb fb fc

!shared memory policy is not protected against parallel removal by other
thread which is normally protected by the mmap_sem. do_get_mempolicy, 
however, drops the lock midway while we can still access it later. Early
premature up_read is a historical artifact from times when put_user was
called in this path see https://lwn.net/Articles/124754/ but that is
gone since 8bccd85ffbaf ("[PATCH] Implement sys_* do_* layering in the
memory policy layer."). but when we have the the current mempolicy ref 
count model. The issue was introduced accordingly.

The patch fix the issue by removing the premature release. it will safe
access the mempolicy. The issue will leave.

Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: <stable@vger.kernel.org>		 [2.6+] 
---
v1->v2
  - changelog is modified as Michal suggestion.

 mm/mempolicy.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index d911fa5..618ab12 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -861,11 +861,6 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
 		*policy |= (pol->flags & MPOL_MODE_FLAGS);
 	}
 
-	if (vma) {
-		up_read(&current->mm->mmap_sem);
-		vma = NULL;
-	}
-
 	err = 0;
 	if (nmask) {
 		if (mpol_store_user_nodemask(pol)) {
-- 
1.7.12.4

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