mm/mempolicy.c: fix out of bounds write in mpol_parse_str()

stable inclusion
from stable-v4.19.102
commit 732ecd4aad51d336b49b9be431219d173ac826c8
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6L0EC
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=732ecd4aad51d336b49b9be431219d173ac826c8

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

commit c7a91bc7 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@syzkaller.appspotmail.com
Fixes: 095f1fc4 ("mempolicy: rework shmem mpol parsing and display")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NVlastimil Babka <vbabka@suse.cz>
Dmitry Vyukov <dvyukov@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Ntong tiangen <tongtiangen@huawei.com>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

mm/mempolicy.c

@@ -2886,6 +2886,9 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
 	char *flags = strchr(str, '=');
 	int err = 1;
 
+	if (flags)
+		*flags++ = '\0';	/* terminate mode string */
+
 	if (nodelist) {
 		/* NUL-terminate mode or flags string */
 		*nodelist++ = '\0';
@@ -2896,9 +2899,6 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
 	} 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;