tmpfs: fix oops on mounts with mpol=default

Fix an 'oops' when a tmpfs mount point is mounted with the mpol=default
mempolicy.

Upon remounting a tmpfs mount point with 'mpol=default' option, the mount
code crashed with a null pointer dereference.  The initial problem report
was on 2.6.27, but the problem exists in mainline 2.6.34-rc as well.  On
examining the code, we see that mpol_new returns NULL if default mempolicy
was requested.  This 'NULL' mempolicy is accessed to store the node mask
resulting in oops.

The following patch fixes it.

Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Acked-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 643f66e10187..745ce90308a6 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2215,10 +2215,15 @@ int mpol_parse_str(char *str, struct mempolicy **mpol, int no_context)
                         goto out;
                 mode = MPOL_PREFERRED;
                 break;
-
+        case MPOL_DEFAULT:
+                /*
+                 * Insist on a empty nodelist
+                 */
+                if (!nodelist)
+                        err = 0;
+                goto out;
         /*
          * case MPOL_BIND:    mpol_new() enforces non-empty nodemask.
-         * case MPOL_DEFAULT: mpol_new() enforces empty nodemask, ignores flags.
          */
         }