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).

Link: http://lkml.kernel.org/r/20200115055426.vdjwvry44nfug7yy@kili.mountain
Reported-by: syzbot+e64a13c5369a194d67df@syzkaller.appspotmail.com
Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Vlastimil 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b2920ae..977c641 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2821,6 +2821,9 @@ 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';
@@ -2831,9 +2834,6 @@ 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;