Patch "mm: fix double mmap_sem unlock on MMF_UNSTABLE enforced SIGBUS" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

mm: fix double mmap_sem unlock on MMF_UNSTABLE enforced SIGBUS

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-fix-double-mmap_sem-unlock-on-mmf_unstable-enforced-sigbus.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Michal Hocko <[email protected]>
Date: Fri, 18 Aug 2017 15:16:12 -0700
Subject: mm: fix double mmap_sem unlock on MMF_UNSTABLE enforced SIGBUS

From: Michal Hocko <[email protected]>

commit 5b53a6ea886700a128b697a6fe8375340dea2c30 upstream.

Tetsuo Handa has noticed that MMF_UNSTABLE SIGBUS path in handle_mm_fault causes a lockdep splat

Out of memory: Kill process 1056 (a.out) score 603 or sacrifice child
Killed process 1056 (a.out) total-vm:4268108kB, anon-rss:2246048kB, file-rss:0kB, shmem-rss:0kB a.out (1169) used greatest stack depth: 11664 bytes left DEBUG_LOCKS_WARN_ON(depth <= 0)

  ------------[ cut here ]------------
  WARNING: CPU: 6 PID: 1339 at kernel/locking/lockdep.c:3617 lock_release+0x172/0x1e0
  CPU: 6 PID: 1339 Comm: a.out Not tainted 4.13.0-rc3-next-20170803+ #142
  Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/02/2015
  RIP: 0010:lock_release+0x172/0x1e0
  Call Trace:
     up_read+0x1a/0x40
     __do_page_fault+0x28e/0x4c0
     do_page_fault+0x30/0x80
     page_fault+0x28/0x30

The reason is that the page fault path might have dropped the mmap_sem and returned with VM_FAULT_RETRY. MMF_UNSTABLE check however rewrites the error path to VM_FAULT_SIGBUS and we always expect mmap_sem taken in that path. Fix this by taking mmap_sem when VM_FAULT_RETRY is held in the MMF_UNSTABLE path.

We cannot simply add VM_FAULT_SIGBUS to the existing error code because all arch specific page fault handlers and g-u-p would have to learn a new error code combination.

---
 mm/memory.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

@@ -3635,8 +3635,18 @@ int handle_mm_fault(struct vm_area_struc
 	 * further.
 	 */
 	if (unlikely((current->flags & PF_KTHREAD) && !(ret & VM_FAULT_ERROR)
-				&& test_bit(MMF_UNSTABLE, &vma->vm_mm->flags)))
+				&& test_bit(MMF_UNSTABLE, &vma->vm_mm->flags))) {
+
+		/*
+		 * We are going to enforce SIGBUS but the PF path might have
+		 * dropped the mmap_sem already so take it again so that
+		 * we do not break expectations of all arch specific PF paths
+		 * and g-u-p
+		 */
+		if (ret & VM_FAULT_RETRY)
+			down_read(&vma->vm_mm->mmap_sem);
 		ret = VM_FAULT_SIGBUS;
+	}
 
 	return ret;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mm-discard-memblock-data-later.patch queue-4.9/mm-fix-double-mmap_sem-unlock-on-mmf_unstable-enforced-sigbus.patch queue-4.9/mm-mempolicy-fix-use-after-free-when-calling-get_mempolicy.patch