[02/19] userfaultfd: fix a race between writeprotect and exit_mmap()

Commit Message

Andrew Morton Oct. 18, 2021, 10:15 p.m. UTC

From: Nadav Amit <[e-mail]>
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/[e-mail]
Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Nadav Amit <[e-mail]>
Tested-by: Li  Wang <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

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

Patch

--- 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;