Re: [PATCH] btrfs: Make btrfs handle security mount options internally to avoid losing security label.

[Chris Mason <clm@fb.com>]

On 09/23/2014 01:40 AM, Qu Wenruo wrote:
> [BUG]
> Originally when mount btrfs with "-o subvol=" mount option, btrfs will
> lose all security lable.
> And if the btrfs fs is mounted somewhere else, due to the lost of
> security lable, SELinux will refuse to mount since the same super block
> is being mounted using different security lable.
> 
> [REPRODUCER]
> With SELinux enabled:
>  #mkfs -t btrfs /dev/sda5
>  #mount -o context=system_u:object_r:nfs_t:s0 /dev/sda5 /mnt/btrfs
>  #btrfs subvolume create /mnt/btrfs/subvol
>  #mount -o subvol=subvol,context=system_u:object_r:nfs_t:s0 /dev/sda5
>   /mnt/test
> 
> kernel message:
> SELinux: mount invalid.  Same superblock, different security settings
> for (dev sda5, type btrfs)
> 
> [REASON]
> This happens because btrfs will call vfs_kern_mount() and then
> mount_subtree() to handle subvolume name lookup.
> First mount will cut off all the security lables and when it comes to
> the second vfs_kern_mount(), it has no security label now.
> 
> [FIX]
> This patch will makes btrfs behavior much more like nfs,
> which has the type flag FS_BINARY_MOUNTDATA,
> making btrfs handles the security label internally.
> So security label will be set in the real mount time and won't lose
> label when use with "subvol=" mount option.

Thanks for working on this.  Eric Sandeen (cc'd) was trying out
something similar recently, so I want to make sure this doesn't conflict
with his ideas.

-chris