userfaultfd: check VM_MAYWRITE was set after verifying the uffd is registered

Calling UFFDIO_UNREGISTER on virtual ranges not yet registered in uffd
could trigger an harmless false positive WARN_ON.  Check the vma is
already registered before checking VM_MAYWRITE to shut off the false
positive warning.

Link: http://lkml.kernel.org/r/[email protected]
Cc: <[email protected]>
Fixes: 29ec906 ("userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas")
Signed-off-by: Andrea Arcangeli <[email protected]>
Reported-by: [email protected]
Acked-by: Mike Rapoport <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Acked-by: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
(cherry picked from commit 01e881f)

Orabug: 29163742
CVE: CVE-2018-18397

Signed-off-by: Shan Hai <[email protected]>
Reviewed-by: Jack Vogel <[email protected]>

Author: aagit

fs/userfaultfd.c

@@ -1567,7 +1567,6 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		cond_resched();
 
 		BUG_ON(!vma_can_userfault(vma));
-		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
 
 		/*
 		 * Nothing to do: this vma is already registered into this
@@ -1576,6 +1575,8 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		if (!vma->vm_userfaultfd_ctx.ctx)
 			goto skip;
 
+		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
+
 		if (vma->vm_start > start)
 			start = vma->vm_start;
 		vma_end = min(end, vma->vm_end);