[PATCH 4.19 23/86] btrfs: fix mount failure caused by race with umount

Greg Kroah-Hartman
2020-07-27T14:39:31Z

From: Boris Burkov <boris@bur.io>

commit 48cfa61b58a1fee0bc49eef04f8ccf31493b7cdd upstream.

It is possible to cause a btrfs mount to fail by racing it with a slow
umount. The crux of the sequence is generic_shutdown_super not yet
calling sop->put_super before btrfs_mount_root calls btrfs_open_devices.
If that occurs, btrfs_open_devices will decide the opened counter is
non-zero, increment it, and skip resetting fs_devices->total_rw_bytes to
0. From here, mount will call sget which will result in grab_super
trying to take the super block umount semaphore. That semaphore will be
held by the slow umount, so mount will block. Before up-ing the
semaphore, umount will delete the super block, resulting in mount's sget
reliably allocating a new one, which causes the mount path to dutifully
fill it out, and increment total_rw_bytes a second time, which causes
the mount to fail, as we see double the expected bytes.

Here is the sequence laid out in greater detail:

CPU0 CPU1
down_write sb->s_umount
btrfs_kill_super
kill_anon_super(sb)
generic_shutdown_super(sb);
shrink_dcache_for_umount(sb);
sync_filesystem(sb);
evict_inodes(sb); // SLOW

btrfs_mount_root
btrfs_scan_one_device
fs_devices = device->fs_devices
fs_info->fs_devices = fs_devices
// fs_devices-opened makes this a no-op
btrfs_open_devices(fs_devices, mode, fs_type)
s = sget(fs_type, test, set, flags, fs_info);
find sb in s_instances
grab_super(sb);
down_write(&s->s_umount); // blocks

sop->put_super(sb)
// sb->fs_devices->opened == 2; no-op
spin_lock(&sb_lock);
hlist_del_init(&sb->s_instances);
spin_unlock(&sb_lock);
up_write(&sb->s_umount);
return 0;
retry lookup
don't find sb in s_instances (deleted by CPU0)
s = alloc_super
return s;
btrfs_fill_super(s, fs_devices, data)
open_ctree // fs_devices total_rw_bytes improperly set!
btrfs_read_chunk_tree
read_one_dev // increment total_rw_bytes again!!
super_total_bytes < fs_devices->total_rw_bytes // ERROR!!!

To fix this, we clear total_rw_bytes from within btrfs_read_chunk_tree
before the calls to read_one_dev, while holding the sb umount semaphore
and the uuid mutex.

To reproduce, it is sufficient to dirty a decent number of inodes, then
quickly umount and mount.

for i in $(seq 0 500)
do
dd if=/dev/zero of="/mnt/foo/$i" bs=1M count=1
done
umount /mnt/foo&
mount /mnt/foo

does the trick for me.

CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/volumes.c | 8 ++++++++
1 file changed, 8 insertions(+)

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6935,6 +6935,14 @@ int btrfs_read_chunk_tree(struct btrfs_f
mutex_lock(&fs_info->chunk_mutex);

/*
+ * It is possible for mount and umount to race in such a way that
+ * we execute this code path, but open_fs_devices failed to clear
+ * total_rw_bytes. We certainly want it cleared before reading the
+ * device items, so clear it here.
+ */
+ fs_info->fs_devices->total_rw_bytes = 0;
+
+ /*
* Read all device items, and then all the chunk items. All
* device items are found before any chunk item (their object id
* is smaller than the lowest possible object id for a chunk