[ 041/150] tmpfs: fix use-after-free of mempolicy object

Greg Kroah-Hartman
2013-02-26T23:57:31Z

From: Greg Thelen <gthelen@google.com>

commit 5f00110f7273f9ff04ac69a5f85bb535a4fd0987 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/shmem.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2487,6 +2487,7 @@ static int shmem_remount_fs(struct super
unsigned long inodes;
int error = -EINVAL;

+ config.mpol = NULL;
if (shmem_parse_options(data, &config, true))
return error;

@@ -2511,8 +2512,13 @@ static int shmem_remount_fs(struct super
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 */
+ }
out:
spin_unlock(&sbinfo->stat_lock);
return error;