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@xxxxxxxxxxxxxxx > 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@xxxxxxxxxx> > The fix looks good to me, feel free to use above information for a > better changelog > Acked-by: Michal Hocko <mhocko@xxxxxxxx> > >>> --- >>> 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(¤t->mm->mmap_sem); >>> - vma = NULL; >>> - } >>> - >>> err = 0; >>> if (nmask) { >>> if (mpol_store_user_nodemask(pol)) {