Re: [PATCH] btrfs: Fix UAF

[Anand Jain <anand.jain@oracle.com>]

On 01/26/2018 09:20 PM, Nikolay Borisov wrote:
> Commit 4fde46f0cc71 ("Btrfs: free the stale device") introduced
> btrfs_free_stale_device which iterates the device lists for all
> registered btrfs filesystems and deletes those devices which aren't
> mounted. In a btrfs_devices structure has only 1 device attached to it
> and it is unused then btrfs_free_stale_devices will proceed to also
> free the btrfs_fs_devices struct itself. Currently this leads to a UAF
> since list_for_each_entry will try to perform a check on the already-
> freed memory to see if it has to terminated the loop.
> 
> The fix is to use 'break' when we know we are freeing the current
> fs_devs.

  No break is needed as we need to iterate all stale devices and delete
  the found stale entry, so commit [1] used list_for_each_entry_safe()
  and removed the break,

  [1]
   commit 38cf665d338fca33af4b16f9ec7cad6637fc0fec
   Author: Anand Jain <Anand.Jain@oracle.com>
     btrfs: make btrfs_free_stale_device() to iterate all stales


  I am guessing UAF might be in[2], instead ?

  [2]
     free_fs_devices(fs_devs)
::
         while (!list_empty(&fs_devices->devices)) {
                 device = list_entry(fs_devices->devices.next,
                                     struct btrfs_device, dev_list);

Thanks, Anand

> Fixes: 4fde46f0cc71 ("Btrfs: free the stale device")
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
 >
> ---
>   fs/btrfs/volumes.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index f7147740b68e..c3ab55336ee0 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -645,6 +645,7 @@ static void btrfs_free_stale_devices(const char *path,
>   				btrfs_sysfs_remove_fsid(fs_devs);
>   				list_del(&fs_devs->list);
>   				free_fs_devices(fs_devs);
> +				break;
>   			} else {
>   				fs_devs->num_devices--;
>   				list_del(&dev->dev_list);
>