[PATCH 5.14 081/169] userfaultfd: fix a race between writeprotect and exit_mmap()

[Greg Kroah-Hartman]

From: Nadav Amit <namit@xxxxxxxxxx>

commit cb185d5f1ebf900f4ae3bf84cee212e6dd035aca upstream.

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@xxxxxxxxxx
Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Tested-by: Li  Wang <liwang@xxxxxxxxxx>
Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/userfaultfd.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1826,9 +1826,15 @@ 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;