[PATCH AUTOSEL 4.14 74/95] btrfs: harden agaist duplicate fsid on scanned devices

Sasha Levin
2019-05-07T05:48:46Z

From: Anand Jain <anand.jain@oracle.com>

[ Upstream commit a9261d4125c97ce8624e9941b75dee1b43ad5df9 ]

It's not that impossible to imagine that a device OR a btrfs image is
copied just by using the dd or the cp command. Which in case both the
copies of the btrfs will have the same fsid. If on the system with
automount enabled, the copied FS gets scanned.

We have a known bug in btrfs, that we let the device path be changed
after the device has been mounted. So using this loop hole the new
copied device would appears as if its mounted immediately after it's
been copied.

For example:

Initially.. /dev/mmcblk0p4 is mounted as /

$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
mmcblk0 179:0 0 29.2G 0 disk
|-mmcblk0p4 179:4 0 4G 0 part /
|-mmcblk0p2 179:2 0 500M 0 part /boot
|-mmcblk0p3 179:3 0 256M 0 part [SWAP]
`-mmcblk0p1 179:1 0 256M 0 part /boot/efi

$ btrfs fi show
Label: none uuid: 07892354-ddaa-4443-90ea-f76a06accaba
Total devices 1 FS bytes used 1.40GiB
devid 1 size 4.00GiB used 3.00GiB path /dev/mmcblk0p4

Copy mmcblk0 to sda

$ dd if=/dev/mmcblk0 of=/dev/sda

And immediately after the copy completes the change in the device
superblock is notified which the automount scans using btrfs device scan
and the new device sda becomes the mounted root device.

$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 1 14.9G 0 disk
|-sda4 8:4 1 4G 0 part /
|-sda2 8:2 1 500M 0 part
|-sda3 8:3 1 256M 0 part
`-sda1 8:1 1 256M 0 part
mmcblk0 179:0 0 29.2G 0 disk
|-mmcblk0p4 179:4 0 4G 0 part
|-mmcblk0p2 179:2 0 500M 0 part /boot
|-mmcblk0p3 179:3 0 256M 0 part [SWAP]
`-mmcblk0p1 179:1 0 256M 0 part /boot/efi

$ btrfs fi show /
Label: none uuid: 07892354-ddaa-4443-90ea-f76a06accaba
Total devices 1 FS bytes used 1.40GiB
devid 1 size 4.00GiB used 3.00GiB path /dev/sda4

The bug is quite nasty that you can't either unmount /dev/sda4 or
/dev/mmcblk0p4. And the problem does not get solved until you take sda
out of the system on to another system to change its fsid using the
'btrfstune -u' command.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
fs/btrfs/volumes.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 38ed8e259e00..bd1117720fc1 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -696,6 +696,35 @@ static noinline int device_list_add(const char *path,
return -EEXIST;
}

+ /*
+ * We are going to replace the device path for a given devid,
+ * make sure it's the same device if the device is mounted
+ */
+ if (device->bdev) {
+ struct block_device *path_bdev;
+
+ path_bdev = lookup_bdev(path);
+ if (IS_ERR(path_bdev)) {
+ mutex_unlock(&fs_devices->device_list_mutex);
+ return ERR_CAST(path_bdev);
+ }
+
+ if (device->bdev != path_bdev) {
+ bdput(path_bdev);
+ mutex_unlock(&fs_devices->device_list_mutex);
+ btrfs_warn_in_rcu(device->fs_info,
+ "duplicate device fsid:devid for %pU:%llu old:%s new:%s",
+ disk_super->fsid, devid,
+ rcu_str_deref(device->name), path);
+ return ERR_PTR(-EEXIST);
+ }
+ bdput(path_bdev);
+ btrfs_info_in_rcu(device->fs_info,
+ "device fsid %pU devid %llu moved old:%s new:%s",
+ disk_super->fsid, devid,
+ rcu_str_deref(device->name), path);
+ }
+
name = rcu_string_strdup(path, GFP_NOFS);
if (!name)
return -ENOMEM;
--
2.20.1