linux-kernel - [PATCH] mm/mempolicy.c: Fix out of bounds write in mpol_parse

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Date:   Wed, 15 Jan 2020 08:54:26 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Lee Schermerhorn <lee.schermerhorn@...com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        syzbot <syzbot+e64a13c5369a194d67df@...kaller.appspotmail.com>,
        aarcange@...hat.com, hughd@...gle.com, mhocko@...e.com,
        syzkaller-bugs@...glegroups.com, vbabka@...e.cz,
        viro@...iv.linux.org.uk, yang.shi@...ux.alibaba.com
Subject: [PATCH] mm/mempolicy.c: Fix out of bounds write in mpol_parse_str()

What we are trying to do is change the '=' character to a NUL terminator
and then at the end of the function we restore it back to an '='.  The
problem is there are two error paths where we jump to the end of the
function before we have replaced the '=' with NUL.  We end up putting
the '=' in the wrong place (possibly one element before the start of
the buffer).

Reported-by: syzbot+e64a13c5369a194d67df@...kaller.appspotmail.com
Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
---
 mm/mempolicy.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 067cf7d3daf5..1340c5c496b5 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2817,6 +2817,9 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
 	char *flags = strchr(str, '=');
 	int err = 1, mode;
 
+	if (flags)
+		*flags++ = '\0';	/* terminate mode string */
+
 	if (nodelist) {
 		/* NUL-terminate mode or flags string */
 		*nodelist++ = '\0';
@@ -2827,9 +2830,6 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
 	} else
 		nodes_clear(nodes);
 
-	if (flags)
-		*flags++ = '\0';	/* terminate mode string */
-
 	mode = match_string(policy_modes, MPOL_MAX, str);
 	if (mode < 0)
 		goto out;
-- 
2.11.0