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

[Christian Brauner]

On Mon, Jan 08, 2024 at 03:58:47PM -0800, Andrii Nakryiko wrote:
> On Mon, Jan 8, 2024 at 4:02 AM Christian Brauner <brauner at kernel.org> wrote:
> >
> > On Fri, Jan 05, 2024 at 02:18:40PM -0800, Andrii Nakryiko wrote:
> > > On Fri, Jan 5, 2024 at 1:45 PM Linus Torvalds
> > > <torvalds at linuxfoundation.org> wrote:
> > > >
> > > > Ok, I've gone through the whole series now, and I don't find anything
> > > > objectionable.
> > >
> > > That's great, thanks for reviewing!
> > >
> > > >
> > > > Which may only mean that I didn't notice something, of course, but at
> > > > least there's nothing I'd consider obvious.
> > > >
> > > > I keep coming back to this 03/29 patch, because it's kind of the heart
> > > > of it, and I have one more small nit, but it's also purely stylistic:
> > > >
> > > > On Wed, 3 Jan 2024 at 14:21, Andrii Nakryiko <andrii at kernel.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.
> > > >
> > > > So the end result would be something like
> > > >
> > > >     bool bpf_token_capable(const struct bpf_token *token, int cap)
> > > >     {
> > > >         struct user_namespace *ns = &init_ns;
> > > >
> > > >         if (token && current_user_ns() == token->userns) {
> > > >                 if (security_bpf_token_capable(token, cap) < 0)
> > > >                         return false;
> > > >                 ns = token->userns;
> > > >         }
> > > >         return ns_capable(ns, cap) ||
> > > >                 (cap != CAP_SYS_ADMIN && capable(CAP_SYS_ADMIN));
> > > >     }
> > >
> > > Yep, it makes sense to use ns_capable with init_ns. I'll change those
> > > two patches to end up with something like what you suggested here.
> > >
> > > >
> > > > although I feel that with that LSM hook, maybe this all should return
> > > > the error code (zero or negative), not a bool for success?
> > > >
> > > > Also, should "current_user_ns() != token->userns" perhaps be an error
> > > > condition, rather than a "fall back to init_ns" condition?
> > > >
> > > > Again, none of this is a big deal. I do think you're dropping the LSM
> > > > error code on the floor, and are duplicating the "ns_capable()" vs
> > > > "capable()" logic as-is, but none of this is a deal breaker, just more
> > > > of my commentary on the patch and about the logic here.
> > > >
> > > > And yeah, I don't exactly love how you say "ok, if there's a token and
> > > > it doesn't match, I'll not use it" rather than "if the token namespace
> > > > doesn't match, it's an error", but maybe there's some usability issue
> > > > here?
> > >
> > > Yes, usability was the primary concern. The overall idea with BPF
> >
> > NAK on not restricting this to not erroring out on current_user_ns()
> > != token->user_ns. I've said this multiple times before.
> 
> I do restrict token usage to *exact* userns in which the token was
> created. See bpf_token_capable()'s
> 
> if (token && current_user_ns() == token->userns) { ... }
> 
> and in bpf_token_allow_cmd():
> 
> if (!token || current_user_ns() != token->userns)
>     return false;
> 
> So I followed what you asked in [1] (just like I said I will in [2]),
> unless I made some stupid mistake which I cannot even see.
> 
> 
> What we are discussing here is a different question. It's the
> difference between erroring out (that is, failing whatever BPF
> operation was attempted with such token, i.e., program loading or map
> creation) vs ignoring the token altogether and just using
> init_ns-based capable() checks. And the latter is vastly more user

Look at this:

+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));
+}

How on earth is it possible that the calling task is in a user namespace
aka current_user_ns() == token->userns while at the same time being
capable in the initial user namespace? When you enter an
unprivileged user namespace you lose all capabilities against your
ancestor user namespace and you can't reenter your ancestor user
namespace.

IOW, if current_user_ns() == token->userns and token->userns !=
init_user_ns, then current_user_ns() != init_user_ns. And therefore that
thing is essentially always false for all interesting cases, no?

Aside from that it would be semantically completely unclean. The user
has specified a token and permission checking should be based on that
token and not magically fallback to a capable check in the inital user
namespace even if that worked.

Because the only scenario where that is maybe useful is if an
unprivileged container has dropped _both_ CAP_BPF and CAP_SYS_ADMIN from
the user namespace of the container.

First of, why? What thread model do you have then? Second, if you do
stupid stuff like that then you don't get bpf in the container via bpf
tokens. Period.

Restrict the meaning and validity of a bpf token to the user namespace
and do not include escape hatches such as this. Especially not in this
initial version, please.

I'm not trying to be difficult but it's clear that the implications of
user namespaces aren't well understood here. And historicaly they are
exploit facilitators as much as exploit preventers.