[Virtio-fs] [PATCH for-5.1 2/3] virtiofsd: add container-friendly -o chroot sandboxing option

[Stefan Hajnoczi]

On Wed, Jul 22, 2020 at 08:03:18PM +0100, Dr. David Alan Gilbert wrote:
> * Stefan Hajnoczi (stefanha at redhat.com) wrote:
> > +    /*
> > +     * Make the shared directory the file system root so that FUSE_OPEN
> > +     * (lo_open()) cannot escape the shared directory by opening a symlink.
> > +     *
> > +     * It's still possible to escape the chroot via lo->proc_self_fd but that
> > +     * requires gaining control of the process first.
> > +     */
> > +    if (chroot(lo->source) != 0) {
> > +        fuse_log(FUSE_LOG_ERR, "chroot(\"%s\"): %m\n", lo->source);
> > +        exit(1);
> > +    }
> 
> I'm seeing suggestions that you should drop CAP_SYS_CHROOT after
> chroot'ing to stop an old escape (where you create another jail inside
> the jail and the kernel then lets you walk outside of the old one).

That's already the case:

1. setup_seccomp() blocks further chroot(2) calls.
2. setup_capabilities() drops CAP_SYS_CHROOT.

Stefan
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