[PATCH 1/1] userfaultfd: check VM_MAYWRITE was set after verifying the uffd is registered

[Andrea Arcangeli]

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.

Cc: <sta...@vger.kernel.org>
Fixes: 29ec90660d68 ("userfaultfd: shmem/hugetlbfs: only allow to register 
VM_MAYWRITE vmas")
Reported-by: syzbot+06c7092e7d71218a2...@syzkaller.appspotmail.com
Signed-off-by: Andrea Arcangeli <aarca...@redhat.com>
---
 fs/userfaultfd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index cd58939dc977..7a85e609fc27 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1566,7 +1566,6 @@ 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
@@ -1575,6 +1574,8 @@ 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);