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authorGreg Thelen <gthelen@google.com>2013-02-23 00:36:01 (GMT)
committerLinus Torvalds <torvalds@linux-foundation.org>2013-02-24 01:50:23 (GMT)
commit5f00110f7273f9ff04ac69a5f85bb535a4fd0987 (patch)
tree766ec74663d4e184d824dde7201d9d5265fb943f
parentmm/fadvise.c: drain all pagevecs if POSIX_FADV_DONTNEED fails to discard all pages (diff)
downloadlinux-5f00110f7273f9ff04ac69a5f85bb535a4fd0987.zip
linux-5f00110f7273f9ff04ac69a5f85bb535a4fd0987.tar.xz
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>
-rw-r--r--mm/shmem.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index 7162c58..5e2ff59 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2486,6 +2486,7 @@ 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;
@@ -2510,8 +2511,13 @@ 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;