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

Dave Chinner
2018-05-25T03:57:21Z

On Thu, May 24, 2018 at 06:23:30PM -0500, Eric W. Biederman wrote:
> "Theodore Y. Ts'o" <tytso@mit.edu> 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.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com