From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx114.postini.com [74.125.245.114]) by kanga.kvack.org (Postfix) with SMTP id 2446B6B0002 for ; Wed, 20 Feb 2013 02:11:52 -0500 (EST) Received: by mail-qc0-f201.google.com with SMTP id o22so751872qcr.2 for ; Tue, 19 Feb 2013 23:11:51 -0800 (PST) From: Greg Thelen Subject: [PATCH 1/2] tmpfs: fix use-after-free of mempolicy object Date: Tue, 19 Feb 2013 23:11:41 -0800 Message-Id: <1361344302-26565-1-git-send-email-gthelen@google.com> Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Greg Thelen 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_set_nodemask+0x8d/0x100 [] ? mpol_shared_policy_init+0x8f/0x160 [] mpol_shared_policy_init+0xa5/0x160 [] ? shmem_get_inode+0x1e1/0x270 [] ? shmem_get_inode+0x1e1/0x270 [] ? trace_hardirqs_on+0xd/0x10 [] shmem_get_inode+0x209/0x270 [] shmem_mknod+0x3e/0xf0 [] shmem_create+0x18/0x20 [] vfs_create+0xb5/0x130 [] do_last+0x9a1/0xea0 [] ? link_path_walk+0x7a/0x930 [] path_openat+0xb3/0x4d0 [] ? __alloc_fd+0x31/0x160 [] do_filp_open+0x42/0xa0 [] ? __alloc_fd+0xe0/0x160 [] 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 --- 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org