[CVE-2013-1767 Lucid Precise] tmpfs: fix use-after-free of mempolicy object

Luis Henriques luis.henriques at canonical.com
Thu Mar 7 13:35:16 UTC 2013

From: Greg Thelen <gthelen at google.com>


BugLink: http://bugs.launchpad.net/bugs/1143815

The tmpfs remount logic preserves filesystem mempolicy if the mpol=M
option is not specified in the remount request.  A new policy can be
specified if mpol=M is given.

Before this patch remounting an mpol bound tmpfs without specifying
mpol= mount option in the remount request would set the filesystem's
mempolicy object to a freed mempolicy object.

To reproduce the problem boot a DEBUG_PAGEALLOC kernel and run:
    # mkdir /tmp/x

    # mount -t tmpfs -o size=100M,mpol=interleave nodev /tmp/x

    # grep /tmp/x /proc/mounts
    nodev /tmp/x tmpfs rw,relatime,size=102400k,mpol=interleave:0-3 0 0

    # mount -o remount,size=200M nodev /tmp/x

    # grep /tmp/x /proc/mounts
    nodev /tmp/x tmpfs rw,relatime,size=204800k,mpol=??? 0 0
        # note ? garbage in mpol=... output above

    # dd if=/dev/zero of=/tmp/x/f count=1
        # panic here

    BUG: unable to handle kernel NULL pointer dereference at           (null)
    IP: [<          (null)>]           (null)
    Oops: 0010 [#1] SMP DEBUG_PAGEALLOC
    Call Trace:

Non-debug kernels will not crash immediately because referencing the
dangling mpol will not cause a fault.  Instead the filesystem will
reference a freed mempolicy object, which will cause unpredictable

The problem boils down to a dropped mpol reference below if
shmem_parse_options() does not allocate a new mpol:

    config = *sbinfo
    shmem_parse_options(data, &config, true)
    sbinfo->mpol = config.mpol  /* BUG: saves unreferenced mpol */

This patch avoids the crash by not releasing the mempolicy if
shmem_parse_options() doesn't create a new mpol.

How far back does this issue go? I see it in both 2.6.36 and 3.3.  I did
not look back further.

Signed-off-by: Greg Thelen <gthelen at google.com>
Acked-by: Hugh Dickins <hughd at google.com>
Cc: <stable at vger.kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit 5f00110f7273f9ff04ac69a5f85bb535a4fd0987)

Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 mm/shmem.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 12b9e80..a78acf0 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2121,6 +2121,7 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
 	unsigned long inodes;
 	int error = -EINVAL;
+	config.mpol = NULL;
 	if (shmem_parse_options(data, &config, true))
 		return error;
@@ -2145,8 +2146,13 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
 	sbinfo->max_inodes  = config.max_inodes;
 	sbinfo->free_inodes = config.max_inodes - inodes;
-	mpol_put(sbinfo->mpol);
-	sbinfo->mpol        = config.mpol;	/* transfers initial ref */
+	/*
+	 * Preserve previous mempolicy unless mpol remount option was specified.
+	 */
+	if (config.mpol) {
+		mpol_put(sbinfo->mpol);
+		sbinfo->mpol = config.mpol;	/* transfers initial ref */
+	}
 	return error;

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