mm: mempolicy: require at least one nodeid for MPOL_PREFERRED

Using an empty (malformed) nodelist that is not caught during mount option
parsing leads to a stack-out-of-bounds access.

The option string that was used was: "mpol=prefer:,".  However,
MPOL_PREFERRED requires a single node number, which is not being provided
here.

Add a check that 'nodes' is not empty after parsing for MPOL_PREFERRED's
nodeid.

Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
Reported-by: Entropy Moe <3ntr0py1337@gmail.com>
Reported-by: syzbot+b055b1a6b2b958707a21@syzkaller.appspotmail.com
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: syzbot+b055b1a6b2b958707a21@syzkaller.appspotmail.com
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Link: http://lkml.kernel.org/r/89526377-7eb6-b662-e1d8-4430928abde9@infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 460683bbe58c84679e70291cf0618af7013b2bcd..5fb427aed61279d066e603fb656b1b7c10e7c30f 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2898,7 +2898,9 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
        switch (mode) {
        case MPOL_PREFERRED:
                /*
-                * Insist on a nodelist of one node only
+                * Insist on a nodelist of one node only, although later
+                * we use first_node(nodes) to grab a single node, so here
+                * nodelist (or nodes) cannot be empty.
                 */
                if (nodelist) {
                        char *rest = nodelist;
@@ -2906,6 +2908,8 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
                                rest++;
                        if (*rest)
                                goto out;
+                       if (nodes_empty(nodes))
+                               goto out;
                }
                break;
        case MPOL_INTERLEAVE: