linux-kernel - [PATCH] userfaultfd: fix a race between writeprotect and exit

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Date:   Tue, 21 Sep 2021 13:02:47 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Nadav Amit <namit@...are.com>,
        Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>, stable@...r.kernel.org
Subject: [PATCH] userfaultfd: fix a race between writeprotect and exit_mmap()

From: Nadav Amit <namit@...are.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@...hat.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: stable@...r.kernel.org
Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Nadav Amit <namit@...are.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