fs/hugetlbfs/inode.c: change put_page/unlock_page order in hugetlbfs_fallocate()
authorNadav Amit <namit@vmware.com>
Thu, 30 Nov 2017 00:11:33 +0000 (16:11 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Thu, 30 Nov 2017 02:40:43 +0000 (18:40 -0800)
commit72639e6df4128dfde8fee7638a35be7db8114000
treeaf7edb7d7d722fbc0f3b4c5ea7ca5c6fb4c101e8
parentf4f0a3d85b50a65a348e2b8635041d6b30f01deb
fs/hugetlbfs/inode.c: change put_page/unlock_page order in hugetlbfs_fallocate()

hugetlfs_fallocate() currently performs put_page() before unlock_page().
This scenario opens a small time window, from the time the page is added
to the page cache, until it is unlocked, in which the page might be
removed from the page-cache by another core.  If the page is removed
during this time windows, it might cause a memory corruption, as the
wrong page will be unlocked.

It is arguable whether this scenario can happen in a real system, and
there are several mitigating factors.  The issue was found by code
inspection (actually grep), and not by actually triggering the flow.
Yet, since putting the page before unlocking is incorrect it should be
fixed, if only to prevent future breakage or someone copy-pasting this
code.

Mike said:
 "I am of the opinion that this does not need to be sent to stable.
  Although the ordering is current code is incorrect, there is no way
  for this to be a problem with current locking. In addition, I verified
  that the perhaps bigger issue with sys_fadvise64(POSIX_FADV_DONTNEED)
  for hugetlbfs and other filesystems is addressed in 3a77d214807c ("mm:
  fadvise: avoid fadvise for fs without backing device")"

Link: http://lkml.kernel.org/r/20170826191124.51642-1-namit@vmware.com
Fixes: 70c3547e36f5c ("hugetlbfs: add hugetlbfs_fallocate()")
Signed-off-by: Nadav Amit <namit@vmware.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
fs/hugetlbfs/inode.c