mm: fix double mmap_sem unlock on MMF_UNSTABLE enforced SIGBUS

commit 5b53a6e 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.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 3f70dc3 ("mm: make sure that kthreads will not refault oom reaped memory")
Reported-by: Tetsuo Handa <[email protected]>
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: Andrea Argangeli <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Wenwei Tao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Michal Hocko

mm/memory.c

@@ -3882,8 +3882,18 @@ int handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 	 * 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;
 }