[PATCH 1/2] tmpfs: fix use-after-free of mempolicy object

From: Greg Thelen
Date: Wed Feb 20 2013 - 02:12:12 EST

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)
Call Trace:
[<ffffffff81186ead>] ? mpol_set_nodemask+0x8d/0x100
[<ffffffff811895ef>] ? mpol_shared_policy_init+0x8f/0x160
[<ffffffff81189605>] mpol_shared_policy_init+0xa5/0x160
[<ffffffff811580e1>] ? shmem_get_inode+0x1e1/0x270
[<ffffffff811580e1>] ? shmem_get_inode+0x1e1/0x270
[<ffffffff810db15d>] ? trace_hardirqs_on+0xd/0x10
[<ffffffff81158109>] shmem_get_inode+0x209/0x270
[<ffffffff811581ae>] shmem_mknod+0x3e/0xf0
[<ffffffff811582b8>] shmem_create+0x18/0x20
[<ffffffff811af5d5>] vfs_create+0xb5/0x130
[<ffffffff811afff1>] do_last+0x9a1/0xea0
[<ffffffff811ac77a>] ? link_path_walk+0x7a/0x930
[<ffffffff811b05a3>] path_openat+0xb3/0x4d0
[<ffffffff811be831>] ? __alloc_fd+0x31/0x160
[<ffffffff811b0de2>] do_filp_open+0x42/0xa0
[<ffffffff811be8e0>] ? __alloc_fd+0xe0/0x160
[<ffffffff811a066e>] do_sys_open+0xfe/0x1e0
[<ffffffff811f0aeb>] compat_sys_open+0x1b/0x20
[<ffffffff815d6055>] 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

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

diff --git a/mm/shmem.c b/mm/shmem.c
index 5dd56f6..efd0b3a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2487,6 +2487,7 @@ 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;

@@ -2511,8 +2512,13 @@ 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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