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Re: [PATCH for-5.1 3/3] virtiofsd: probe unshare(CLONE_FS) and print an
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH for-5.1 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error |
Date: |
Thu, 23 Jul 2020 13:46:11 +0100 |
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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[PATCH for-5.1 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error, Stefan Hajnoczi, 2020/07/22
Re: [Virtio-fs] [PATCH for-5.1 0/3] virtiofsd: allow virtiofsd to run in a container, Vivek Goyal, 2020/07/22