From: Nadav Amit <namit@vmware.com>

A race is possible when a process exits, its VMAs are removed
by exit_mmap() and at the same time userfaultfd_writeprotect() is
called.

The race was detected by KASAN on a development kernel, but it appears
to be possible on vanilla kernels as well.

Use mmget_not_zero() to prevent the race as done in other userfaultfd
operations.

Cc: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Nadav Amit <namit@vmware.com>

---
 fs/userfaultfd.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 003f0d31743e..22bf14ab2d16 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1827,9 +1827,15 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
        if (mode_wp && mode_dontwake)
                return -EINVAL;

-       ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
-                                 uffdio_wp.range.len, mode_wp,
-                                 &ctx->mmap_changing);
+       if (mmget_not_zero(ctx->mm)) {
+               ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
+                                         uffdio_wp.range.len, mode_wp,
+                                         &ctx->mmap_changing);
+               mmput(ctx->mm);
+       } else {
+               return -ESRCH;
+       }
+
        if (ret)
                return ret;

--
2.25.1