Re: [PATCH v4 0/2] NFS: Fix interaction between fs_context and user namespaces

[Sargun Dhillon <sargun@sargun.me>]

On Thu, Nov 12, 2020 at 12:30:56AM +0000, Sargun Dhillon wrote:
> On Wed, Nov 11, 2020 at 08:03:18PM +0000, Trond Myklebust wrote:
> > On Wed, 2020-11-11 at 18:57 +0000, Sargun Dhillon wrote:
> > > On Wed, Nov 11, 2020 at 02:38:11PM +0000, Trond Myklebust wrote:
> > > > On Wed, 2020-11-11 at 11:12 +0000, Sargun Dhillon wrote:
> > > > 
> > > > The current code for setting server->cred was developed
> > > > independently
> > > > of fsopen() (and predates it actually). I'm fine with the change to
> > > > have server->cred be the cred of the user that called fsopen().
> > > > That's
> > > > in line with what we used to do for sys_mount().
> > > > 
> > > Just curious, without FS_USERNS, how were you mounting NFSv4 in an
> > > unprivileged user ns?
> > 
> > The code was originally developed on a 5.1 kernel. So all my testing
> > has been with ordinary sys_mount() calls in a container that had
> > CAP_SYS_ADMIN privileges.
> > 
> > > > However all the other stuff to throw errors when the user namespace
> > > > is
> > > > not init_user_ns introduces massive regressions.
> > > > 
> > > 
> > > I can remove that and respin the patch. How do you feel about that? 
> > > I would 
> > > still like to keep the log lines though because it is a uapi change.
> > > I am 
> > > worried that someone might exercise this path with GSS and allow for
> > > upcalls 
> > > into the main namespaces by accident -- or be confused of why they're
> > > seeing 
> > > upcalls "in a different namespace".
> > > 
> > > Are you okay with picking up ("NFS: NFSv2/NFSv3: Use cred from
> > > fs_context during 
> > > mount") without any changes?
> > 
> > Why do we need the dprintk()s? It seems to me that either they should
> > be reporting something that the user needs to know (in which case they
> > should be real printk()s) or they are telling us something that we
> > should already know. To me they seem to fit more in the latter
> > category.
> > 
> > > 
> > > I can respin ("NFSv4: Refactor NFS to use user namespaces") without:
> > > /*
> > >  * nfs4idmap is not fully isolated by user namespaces. It is
> > > currently
> > >  * only network namespace aware. If upcalls never happen, we do not
> > >  * need to worry as nfs_client instances aren't shared between
> > >  * user namespaces.
> > >  */
> > > if (idmap_userns(server->nfs_client->cl_idmap) != &init_user_ns && 
> > >         !(server->caps & NFS_CAP_UIDGID_NOMAP)) {
> > >         error = -EINVAL;
> > >         errorf(fc, "Mount credentials are from non init user
> > > namespace and ID mapping is enabled. This is not allowed.");
> > >         goto error;
> > > }
> > > 
> > > (and making it so we can call idmap_userns)
> > > 
> > 
> > Yes. That would be acceptable. Again, though, I'd like to see the
> > dprintk()s gone.
> > 
> 
> I can drop the dprintks, but given this is a uapi change, does it make sense to 
> pr_info_once? Especially, because this can have security impact?

Spending 5 minutes thinking about this, I think that best go out in another patch
that I can spin, and we can discuss there.