Code: static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
{
	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
	struct shmem_sb_info config = *sbinfo;
 	unsigned long inodes;
 	int error = -EINVAL;
 
 	if (shmem_parse_options(data, &config, true))
 		return error;
 
	spin_lock(&sbinfo->stat_lock);
	inodes = sbinfo->max_inodes - sbinfo->free_inodes;
	if (percpu_counter_compare(&sbinfo->used_blocks, config.max_blocks) > 0)
		goto out;
	if (config.max_inodes < inodes)
		goto out;
	/*
	 * Those tests disallow limited->unlimited while any are in use;
	 * but we must separately disallow unlimited->limited, because
	 * in that case we have no record of how much is already in use.
	 */
	if (config.max_blocks && !sbinfo->max_blocks)
		goto out;
	if (config.max_inodes && !sbinfo->max_inodes)
		goto out;

	error = 0;
	sbinfo->max_blocks  = config.max_blocks;
 	sbinfo->max_inodes  = config.max_inodes;
 	sbinfo->free_inodes = config.max_inodes - inodes;
 
	mpol_put(sbinfo->mpol);
	sbinfo->mpol        = config.mpol;	/* transfers initial ref */
 out:
 	spin_unlock(&sbinfo->stat_lock);
 	return error;
}

Vulnerability Type: DoS +Priv 
CWE ID: CWE-399
Summary: Use-after-free vulnerability in the shmem_remount_fs function in mm/shmem.c in the Linux kernel before 3.7.10 allows local users to gain privileges or cause a denial of service (system crash) by remounting a tmpfs filesystem without specifying a required mpol (aka mempolicy) mount option.
Commit Message: tmpfs: fix use-after-free of mempolicy object

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

Panic:
    BUG: unable to handle kernel NULL pointer dereference at           (null)
    IP: [<          (null)>]           (null)
    [...]
    Oops: 0010 [#1] SMP DEBUG_PAGEALLOC
    Call Trace:
      mpol_shared_policy_init+0xa5/0x160
      shmem_get_inode+0x209/0x270
      shmem_mknod+0x3e/0xf0
      shmem_create+0x18/0x20
      vfs_create+0xb5/0x130
      do_last+0x9a1/0xea0
      path_openat+0xb3/0x4d0
      do_filp_open+0x42/0xa0
      do_sys_open+0xfe/0x1e0
      compat_sys_open+0x1b/0x20
      cstar_dispatch+0x7/0x1f

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
behavior.

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)
    mpol_put(sbinfo->mpol)
    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@google.com>
Acked-by: 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>