Re: [PATCH 1/2] tmpfs: fix use-after-free of mempolicy object
From: Hugh Dickins <hughd@google.com>
Date: 2013-02-20 20:22:07
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On Tue, 19 Feb 2013, Greg Thelen wrote:
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:
[<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
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.This looks very good to me, thank you Greg. I did start to wonder about the consequence of an error in shmem_parse_options() after allocating mempolicy, but you're ahead of me, and have sensibly deferred leak fixes to 2/2.
How far back does this issue go? I see it in both 2.6.36 and 3.3. I did not look back further.
It probably goes back to 2.6.26, when the refcounting came in at this mount level. But there's been such a sad history of bugs in mempolicy refcounting, occasionally one bug has protected against another. And whether anyone actually uses these tmpfs mpol= options is another question. 2.6.26 claimed to add support for "mpol=local", yet nobody noticed that it was rejected as an invalid option, until an independent bugfix in 2.6.34 rearranged the code and happened to get it working. I only discovered that a month or two ago, when working to fix the corruption which 2.6.35 then immediately bestowed upon "mpol=local".
Signed-off-by: Greg Thelen <redacted>
Acked-by: Hugh Dickins <hughd@google.com> I'm inclined to add Cc: stable@vger.kernel.org to this one, but not to the 2/2 leak one; but I'm perfectly happy if Andrew chooses differently when he picks them up.
quoted hunk ↗ jump to hunk
--- 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 */ + } out: spin_unlock(&sbinfo->stat_lock); return error;-- 1.8.1.3
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