netdev - Re: [PATCH bpf-next 03/29] bpf: introduce BPF token object

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Date: Mon, 8 Jan 2024 11:45:17 -0500
From: Paul Moore <paul@...l-moore.com>
To: Linus Torvalds <torvalds@...uxfoundation.org>
Cc: Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, netdev@...r.kernel.org, 
	brauner@...nel.org, linux-fsdevel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH bpf-next 03/29] bpf: introduce BPF token object

On Fri, Jan 5, 2024 at 4:45 PM Linus Torvalds
<torvalds@...uxfoundation.org> wrote:
> On Wed, 3 Jan 2024 at 14:21, Andrii Nakryiko <andrii@...nel.org> wrote:
> >
> > +bool bpf_token_capable(const struct bpf_token *token, int cap)
> > +{
> > +       /* BPF token allows ns_capable() level of capabilities, but only if
> > +        * token's userns is *exactly* the same as current user's userns
> > +        */
> > +       if (token && current_user_ns() == token->userns) {
> > +               if (ns_capable(token->userns, cap))
> > +                       return true;
> > +               if (cap != CAP_SYS_ADMIN && ns_capable(token->userns, CAP_SYS_ADMIN))
> > +                       return true;
> > +       }
> > +       /* otherwise fallback to capable() checks */
> > +       return capable(cap) || (cap != CAP_SYS_ADMIN && capable(CAP_SYS_ADMIN));
> > +}
>
> This *feels* like it should be written as
>
>     bool bpf_token_capable(const struct bpf_token *token, int cap)
>     {
>         struct user_namespace *ns = &init_ns;
>
>         /* BPF token allows ns_capable() level of capabilities, but only if
>          * token's userns is *exactly* the same as current user's userns
>          */
>         if (token && current_user_ns() == token->userns)
>                 ns = token->userns;
>         return ns_capable(ns, cap) ||
>                 (cap != CAP_SYS_ADMIN && capable(CAP_SYS_ADMIN));
>     }
>
> And yes, I realize that the function will end up later growing a
>
>         security_bpf_token_capable(token, cap)
>
> test inside that 'if (token ..)' statement, and this would change the
> order of that test so that the LSM hook would now be done before the
> capability checks are done, but that all still seems just more of an
> argument for the simplification.

I have no problem with rewriting things, my only ask is that we stick
with the idea of doing the capability checks before the LSM hook.  The
DAC-before-MAC (capability-before-LSM) pattern is one we try to stick
to most everywhere in the kernel and deviating from it here could
potentially result in some odd/unexpected behavior from a user
perspective.

-- 
paul-moore.com