Re: [fuse-devel] [PATCH v4 4/5] fuse: Support privileged xattrs only with a mount option

[Seth Forshee <seth.forshee@canonical.com>]

On Tue, Oct 14, 2014 at 01:01:02PM -0700, Eric W. Biederman wrote:
> Michael j Theall <mtheall@us.ibm.com> writes:
> 
> > Seth Forshee <seth.forshee@canonical.com> wrote on 10/14/2014 09:25:55 AM:
> >
> >> From: Seth Forshee <seth.forshee@canonical.com>
> >> To: Miklos Szeredi <miklos@szeredi.hu>
> >> Cc: fuse-devel@lists.sourceforge.net, "Serge H. Hallyn" 
> >> <serge.hallyn@ubuntu.com>, linux-kernel@vger.kernel.org, Seth 
> >> Forshee <seth.forshee@canonical.com>, "Eric W. Biederman" 
> >> <ebiederm@xmission.com>, linux-fsdevel@vger.kernel.org
> >> Date: 10/14/2014 09:27 AM
> >> Subject: [fuse-devel] [PATCH v4 4/5] fuse: Support privileged xattrs
> >> only with a mount option
> >> 
> >> Allowing unprivileged users to provide arbitrary xattrs via fuse
> >> mounts bypasses the normal restrictions on setting xattrs. Such
> >> mounts should be restricted to reading and writing xattrs in the
> >> user.* namespace.
> >> 
> >
> > Can you explain how the normal restrictions on setting xattrs are 
> > bypassed?
> 
> If the fuse server is not run by root.  Which is a large part of the
> point of fuse.

So the server could for example return trusted.* xattrs which were not
set by a privileged user.

> > My filesystem still needs security.* and system.*, and it looks like
> > xattr_permission already prevents non-privileged users from accessing
> > trusted.*
> 
> If the filesystem is mounted with nosuid (typical of a non-privileged
> mount of fuse) then the security.* attributes are ignored.

That I wasn't aware of. In fact I still haven't found where this
restriction is implemented.

Nonetheless, a userns mount could be done without nosuid (though that
mount will also be unaccessible outside of that namespace).

> >> It's difficult though to tell whether a mount is being performed
> >> on behalf of an unprivileged user since fuse mounts are ususally
> >> done via a suid root helper. Thus a new mount option,
> >> privileged_xattrs, is added to indicated that xattrs from other
> >> namespaces are allowed. This option can only be supplied by
> >> system-wide root; supplying the option as an unprivileged user
> >> will cause the mount to fail.
> >
> > I can't say I'm convinced that this is the right direction to head.
> 
> With respect to defaults we could keep the current default if you
> have the global CAP_SYS_ADMIN privilege when the mount takes place
> and then avoid breaking anything.

Except that unprivileged mounts are normally done by a suid root helper,
which is why I've required both global CAP_SYS_ADMIN and a mount option
to get the current default behavior.

Thanks,
Seth