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

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

-------- Original Message --------
Subject: Re: [PATCH] btrfs: Make btrfs handle security mount options 
internally to avoid losing security label.
From: Eric Sandeen <sandeen@redhat.com>
To: Chris Mason <clm@fb.com>, Qu Wenruo <quwenruo@cn.fujitsu.com>, 
<linux-btrfs@vger.kernel.org>
Date: 2014年09月24日 02:51
> On 9/23/14 7:49 AM, Chris Mason wrote:
>> 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.
> My ideas didn't get very far.  ;)
>
> What I was after was a way for multiple subvolumes to have unique contexts.
> It looks like this might do the trick, as long as they are mounted on a unique
> mount point.
>
> Would this allow "subvolume create" to take a context, so that everything
> under /mnt/btrfs/subvol/ has a unique subvol-wide context?
>
> thanks,
> -Eric
Did you mean the following situation?
/dev/sdb default subvol(FS_TREE) mounted on /mnt/default with context A
/dev/sdb subvol=subvol mounted on /mnt/subvol with context B

If that's your goal, I am afraid that my patch can't achieve it and even 
worse, will even forbid it. :(

SELinux doesn't allow same superblock mounted with different context, 
and the patch follows it.
If SELinux is modified to allow same superblock different context, then 
my patch also needs to be modified.

Thanks,
Qu