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

[zhong jiang <zhongjiang@huawei.com>]

HI, Michal

Thank you for review and the better changelog. I will  resend it in v2.

Thanks
zhongjiang
On 2017/8/16 22:04, Michal Hocko wrote:
> Hmm, I haven't received the original patch.
> On Wed 16-08-17 21:18:58, zhong jiang wrote:
>> + stable@vger.kernel.org
> Anyway the proper way to route this to the stable tree is to put Cc:
> stable to the sign-off-by area. It is also preferable if you could add
> Fixes: sha1
>
> which has caused the issue so that people know to which kernel versions
> this should be backported.
>
>> On 2017/8/16 10:13, zhong jiang wrote:
>>> 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
>>> when calling get_mempolicy from userspace, it only hold the mmap_sem
>>> and increase the shared pol count, the unshared pol is not be increased.
>>> it premature release the mmap_sem, it will result in the related mempolicy
>>> maybe had freed by mbind. then, the issue will trigger.
> This is really hard to read. I assume you meant to say that !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."). I didn't check since when we have the current
> mempolicy ref count model though but it is really old...
>
>>> The patch fix the issue by removing the premature release. it will safe
>>> access the mempolicy. The issue will leave.
>>>
>>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> The fix looks good to me, feel free to use above information for a
> better changelog
> Acked-by: Michal Hocko <mhocko@suse.com>
>
>>> ---
>>>  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)) {


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