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

From: Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org>
To: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
Cc: "Theodore Y. Ts'o" <tytso-3s7WtUTddSA@public.gmane.org>,
	Linux Containers
	<containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>,
	linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Seth Forshee
	<seth.forshee-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>,
	linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Christian Brauner
	<christian-STijNZzMWpgWenYVfaLwtA@public.gmane.org>
Subject: Re: [REVIEW][PATCH 0/6] Wrapping up the vfs support for unprivileged mounts
Date: Fri, 25 May 2018 13:57:16 +1000	[thread overview]
Message-ID: <20180525035716.GE10363__30392.0147395276$1527220539$gmane$org@dastard> (raw)
In-Reply-To: <87y3g8y6x9.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>

On Thu, May 24, 2018 at 06:23:30PM -0500, Eric W. Biederman wrote:
> "Theodore Y. Ts'o" <tytso-3s7WtUTddSA@public.gmane.org> 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-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org