Re: [PATCH 1/1] simplified security.nscapability xattr

From: Serge E. Hallyn
Date: Sun May 01 2016 - 23:55:17 EST

On Tue, Apr 26, 2016 at 03:39:54PM -0700, Kees Cook wrote:
> On Tue, Apr 26, 2016 at 3:26 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> > Quoting Kees Cook (keescook@xxxxxxxxxxxx):
> >> On Fri, Apr 22, 2016 at 10:26 AM, <serge.hallyn@xxxxxxxxxx> wrote:
> >> > From: Serge Hallyn <serge.hallyn@xxxxxxxxxx>
> >> This looks like userspace must knowingly be aware that it is in a
> >> namespace and to DTRT instead of it being translated by the kernel
> >> when setxattr is called under !init_user_ns?
> >
> > Yes - my libcap2 patch checks /proc/self/uid_map to decide that. If that
> > shows you are in init_user_ns then it uses security.capability, otherwise
> > it uses security.nscapability.
> >
> > I've occasionally considered having the xattr code do the quiet
> > substitution if need be.
> >
> > In fact, much of this structure comes from when I was still trying to
> > do multiple values per xattr. Given what we're doing here, we could
> > keep the xattr contents exactly the same, just changing the name.
> > So userspace could just get and set security.capability; if you are
> > in a non-init user_ns, if security.capability is set then you cannot
> > set it; if security.capability is not set, then the kernel writes
> > security.nscapability instead and returns success.
> >
> > I don't like magic, but this might be just straightforward enough
> > to not be offensive. Thoughts?
> Yeah, I think it might be better to have the magic in this case, since
> it seems weird to just reject setxattr if a tool didn't realize it was
> in a namespace. I'm not sure -- it is also nice to have an explicit
> API here.
> I would defer to Eric or Michael on that. I keep going back and forth,
> though I suspect it's probably best to do what you already have
> (explicit API).

Michael, Eric, what do you think? The choice we're making here is
whether we should

1. Keep a nice simple separate pair of xattrs, the pre-existing
security.capability which can only be written from init_user_ns,
and the new (in this patch) security.nscapability which you can
write to any file where you are privileged wrt the file.

2. Make security.capability somewhat 'magic' - if someone in a
non-initial user ns tries to write it and has privilege wrt the
file, then the kernel silently writes security.nscapability instead.

The biggest drawback of (1) would be any tar-like program trying
to restore a file which had security.capability, needing to know
to detect its userns and write the security.nscapability instead.
The drawback of (2) is ~\o/~ magic.