linux-kernel - [PATCH 3.16 87/99] mm/mempolicy.c: fix out of bounds write in mpol_parse

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Message-ID: <lsq.1589984009.838810863@decadent.org.uk>
Date:   Wed, 20 May 2020 15:14:55 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Hugh Dickins" <hughd@...gle.com>,
        "Lee Schermerhorn" <lee.schermerhorn@...com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        syzbot+e64a13c5369a194d67df@...kaller.appspotmail.com,
        "Andrea Arcangeli" <aarcange@...hat.com>,
        "Michal Hocko" <mhocko@...nel.org>,
        "Dan Carpenter" <dan.carpenter@...cle.com>
Subject: [PATCH 3.16 87/99] mm/mempolicy.c: fix out of bounds write in
 mpol_parse_str()

3.16.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Dan Carpenter <dan.carpenter@...cle.com>

commit c7a91bc7c2e17e0a9c8b9745a2cb118891218fd1 upstream.

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).

Link: http://lkml.kernel.org/r/20200115055426.vdjwvry44nfug7yy@kili.mountain
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>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Dmitry Vyukov <dvyukov@...gle.com>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Dan Carpenter <dan.carpenter@...cle.com>
Cc: Lee Schermerhorn <lee.schermerhorn@...com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/mempolicy.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2687,6 +2687,9 @@ int mpol_parse_str(char *str, struct mem
 	char *flags = strchr(str, '=');
 	int err = 1;
 
+	if (flags)
+		*flags++ = '\0';	/* terminate mode string */
+
 	if (nodelist) {
 		/* NUL-terminate mode or flags string */
 		*nodelist++ = '\0';
@@ -2697,9 +2700,6 @@ int mpol_parse_str(char *str, struct mem
 	} else
 		nodes_clear(nodes);
 
-	if (flags)
-		*flags++ = '\0';	/* terminate mode string */
-
 	for (mode = 0; mode < MPOL_MAX; mode++) {
 		if (!strcmp(str, policy_modes[mode])) {
 			break;