+ userfaultfd-fix-a-race-between-writeprotect-and-exit_mmap.patch added to -mm tree

The patch titled
     Subject: userfaultfd: fix a race between writeprotect and exit_mmap()
has been added to the -mm tree.  Its filename is
     userfaultfd-fix-a-race-between-writeprotect-and-exit_mmap.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/userfaultfd-fix-a-race-between-writeprotect-and-exit_mmap.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/userfaultfd-fix-a-race-between-writeprotect-and-exit_mmap.patch

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------------------------------------------------------
From: Nadav Amit <redacted>
Subject: userfaultfd: fix a race between writeprotect and exit_mmap()

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.

Link: https://lkml.kernel.org/r/20210921200247.25749-1-namit@vmware.com
Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Nadav Amit <redacted>
Tested-by: Li  Wang <redacted>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <redacted>
Cc: <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/fs/userfaultfd.c~userfaultfd-fix-a-race-between-writeprotect-and-exit_mmap
+++ a/fs/userfaultfd.c

@@ -1827,9 +1827,15 @@ static int userfaultfd_writeprotect(stru
 	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;
 

_

Patches currently in -mm which might be from namit@vmware.com are

userfaultfd-fix-a-race-between-writeprotect-and-exit_mmap.patch