Setting FS_USERNS_MOUNT in btrfs_fs_type.fs_flags

Setting FS_USERNS_MOUNT in btrfs_fs_type.fs_flags

[Shea Levy]

Hi all,

What work would be required to mark btrfs_fs_type with FS_USERNS_MOUNT
so that btrfs images can be mounted by unprivileged users within a user
namespace (along with something like [1])? I'd like to be able to create
disk images without having to start a VM (and --rootdir isn't flexible
enough because I want to make subvolumes).

Cheers,
Shea Levy

P.S. I am not subscribed to either list, please CC me in replies

[1] https://lkml.org/lkml/2014/5/27/690

Re: Setting FS_USERNS_MOUNT in btrfs_fs_type.fs_flags

[Al Viro]

On Tue, Sep 16, 2014 at 11:05:00PM -0400, Shea Levy wrote:
> Hi all,
> 
> What work would be required to mark btrfs_fs_type with FS_USERNS_MOUNT
> so that btrfs images can be mounted by unprivileged users within a user
> namespace (along with something like [1])? I'd like to be able to create
> disk images without having to start a VM (and --rootdir isn't flexible
> enough because I want to make subvolumes).

Er...  Which is to say, you have an audit of btrfs code making sure that
it can cope with arbitrary image hand-crafted by potential attacker?
Because without that FS_USERNS_MOUNT could open one hell of security hole;
things like user being able to execute an arbitrary code in kernel mode,
etc.

Re: Setting FS_USERNS_MOUNT in btrfs_fs_type.fs_flags

[Zach Brown]

On Wed, Sep 17, 2014 at 04:54:48AM +0100, Al Viro wrote:
> On Tue, Sep 16, 2014 at 11:05:00PM -0400, Shea Levy wrote:
> > Hi all,
> > 
> > What work would be required to mark btrfs_fs_type with FS_USERNS_MOUNT
> > so that btrfs images can be mounted by unprivileged users within a user
> > namespace (along with something like [1])? I'd like to be able to create
> > disk images without having to start a VM (and --rootdir isn't flexible
> > enough because I want to make subvolumes).
> 
> Er...  Which is to say, you have an audit of btrfs code making sure that
> it can cope with arbitrary image hand-crafted by potential attacker?

It definitely can't cope. The easiest places to find bugs are the
hundreds of BUG_ON() sites, many can be triggered by on-disk structures.
The sheer volume of those makes me trust that you could find much worse
if you did a thorough audit.

- z
(fun related fact: distros automount btrfs images)

Re: Setting FS_USERNS_MOUNT in btrfs_fs_type.fs_flags

[Shea Levy]

On Wed, Sep 17, 2014 at 09:12:14AM -0700, Zach Brown wrote:
> On Wed, Sep 17, 2014 at 04:54:48AM +0100, Al Viro wrote:
> > On Tue, Sep 16, 2014 at 11:05:00PM -0400, Shea Levy wrote:
> > > Hi all,
> > > 
> > > What work would be required to mark btrfs_fs_type with FS_USERNS_MOUNT
> > > so that btrfs images can be mounted by unprivileged users within a user
> > > namespace (along with something like [1])? I'd like to be able to create
> > > disk images without having to start a VM (and --rootdir isn't flexible
> > > enough because I want to make subvolumes).
> > 
> > Er...  Which is to say, you have an audit of btrfs code making sure that
> > it can cope with arbitrary image hand-crafted by potential attacker?
> 
> It definitely can't cope. The easiest places to find bugs are the
> hundreds of BUG_ON() sites, many can be triggered by on-disk structures.
> The sheer volume of those makes me trust that you could find much worse
> if you did a thorough audit.
> 
> - z
> (fun related fact: distros automount btrfs images)

OK, so it seems like the answer to my question is "a helluva lot". Guess
I won't count on seeing it any time soon :)

Thanks,
Shea