[PATCH 3.16 87/99] mm/mempolicy.c: fix out of bounds write in mpol_parse_str()

[Ben Hutchings]

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

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

From: Dan Carpenter <dan.carpenter@xxxxxxxxxx>

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@xxxxxxxxxxxxx
Reported-by: syzbot+e64a13c5369a194d67df@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Cc: Lee Schermerhorn <lee.schermerhorn@xxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 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;