Re: [REVIEW][PATCH 0/6] Wrapping up the vfs support for unprivileged mounts

[Darrick J. Wong]

On Fri, May 25, 2018 at 01:57:16PM +1000, Dave Chinner wrote:
> On Thu, May 24, 2018 at 06:23:30PM -0500, Eric W. Biederman wrote:
> > "Theodore Y. Ts'o" <tytso@xxxxxxx> writes:
> > 
> > > On Wed, May 23, 2018 at 06:22:56PM -0500, Eric W. Biederman wrote:
> > >> 
> > >> Very slowly the work has been progressing to ensure the vfs has the
> > >> necessary support for mounting filesystems without privilege.
> > >
> > > What's the thinking behind how system administrators and/or file
> > > systems would configure whether or not a particular file system type
> > > will be allowed to be mounted w/o privilege?
> > 
> > The mechanism is .fs_flags in file_system_type.   If the FS_USERNS_MOUNT
> > flag is set then root in a user namespace (AKA an unprivileged user)
> > will be allowed to mount to mount the filesystem.
> > 
> > There are very real concerns about attacking a filesystem with an
> > invalid filesystem image, or by a malicious protocol speaker.  So I
> > don't want to enable anything without the file system maintainers
> > consent and without a reasonable expecation that neither a system wide
> > denial of service attack nor a privilege escalation attack is possible
> > from if the filesystem is enabled.
> > 
> > So at a practical level what we have in the vfs is the non-fuse specific
> > bits that enable unprivileged mounts of fuse.  Things like handling
> > of unmapped uid and gids, how normally trusted xattrs are dealt with,
> > etc.
> > 
> > A big practical one for me is that if either the uid or gid is not
> > mapped the vfs avoids writing to the inode.
> > 
> > Right now my practical goal is to be able to say: "Go run your
> > filesystem in userspace with fuse if you want stronger security
> > guarantees."  I think that will be enough to make removable media
> > reasonably safe from privilege escalation attacks.
> > 
> > There is enough code in most filesystems that I don't know what our
> > chances of locking down very many of them are.  But I figure a few more
> > of them are possible.
> 
> I'm not sure we need to - fusefs-lkl gives users the ability to
> mount any of the kernel filesystems via fuse without us needing to
> support unprivileged kernel mounts for those filesystems.

/me wonders, is there a fusefs-lkl package for Linux?

(He says, knowing that freebsd has one... :))

--D

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx