Re: [PATCH for-5.1 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Jul 22, 2020 at 06:03:28PM +0100, Daniel P. Berrangé wrote:
> On Wed, Jul 22, 2020 at 02:02:06PM +0100, Stefan Hajnoczi wrote:
> > An assertion failure is raised during request processing if
> > unshare(CLONE_FS) fails. Implement a probe at startup so the problem can
> > be detected right away.
> > 
> > Unfortunately Docker/Moby does not include unshare in the seccomp.json
> > list unless CAP_SYS_ADMIN is given. Other seccomp.json lists always
> > include unshare (e.g. podman is unaffected):
> > https://raw.githubusercontent.com/seccomp/containers-golang/master/seccomp.json
> > 
> > Use "docker run --security-opt seccomp=path/to/seccomp.json ..." if the
> > default seccomp.json is missing unshare.
> > 
> > Cc: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  tools/virtiofsd/fuse_virtio.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c
> > index 3b6d16a041..ebeb352514 100644
> > --- a/tools/virtiofsd/fuse_virtio.c
> > +++ b/tools/virtiofsd/fuse_virtio.c
> > @@ -949,6 +949,19 @@ int virtio_session_mount(struct fuse_session *se)
> >  {
> >      int ret;
> >  
> > +    /*
> > +     * Test that unshare(CLONE_FS) works. fv_queue_worker() will need it. It's
> > +     * an unprivileged system call but some Docker/Moby versions are known to
> > +     * reject it via seccomp when CAP_SYS_ADMIN is not given.
> > +     */
> > +    ret = unshare(CLONE_FS);
> > +    if (ret == -1 && errno == EPERM) {
> > +        fuse_log(FUSE_LOG_ERR, "unshare(CLONE_FS) failed with EPERM. If "
> > +                "running in a container please check that the container "
> > +                "runtime seccomp policy allows unshare.\n");
> > +        return -1;
> > +    }
> > +
> 
> This describes the unshare() call as a "probe" and a "test", but that's
> misleading IMHO. A "probe" / "test" implies that after it has completed,
> there's no lingering side-effect, which isn't the case here.
> 
> This is actively changing the process' namespace environment in the
> success case, and not putting it back how it was originally.
> 
> May be this is in fact OK, but if so I think the commit message and
> comment should explain/justify what its fine to have this lingering
> side-effect.
> 
> If we want to avoid the side-effect then we need to fork() and run
> unshare() in the child, and use a check of exit status of the child
> to determine the result.

Thanks for pointing this out. I'll add a comment explaining that
virtiofsd is single-threaded at this point. No other threads share the
file system attributes so the call has no observable side-effects.

Stefan

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