Re: [PATCH bpf-next v6 1/3] bpf: Allow trusted pointers to be passed to KF_TRUSTED_ARGS kfuncs

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Nov 1, 2022 at 11:11 AM David Vernet <void@manifault.com> wrote:
>
> > What kind of bpf prog will be able to pass 'struct nf_conn___init *' into these bpf_ct_* ?
> > We've introduced / vs nf_conf specifically to express the relationship
> > between allocated nf_conn and other nf_conn-s via different types.
> > Why is this not enough?
>
> Kumar should have more context here (he originally suggested this in
> [0]),

Quoting:
"
Unfortunately a side effect of this change is that now since
PTR_TO_BTF_ID without ref_obj_id is considered trusted, the bpf_ct_*
functions would begin working with tp_btf args.
"
I couldn't find any tracepoint that has nf_conn___init as an argument.
The whole point of that new type was to return it to bpf prog,
so the verifier type matches it when it's passed into bpf_ct_*
in turn.
So I don't see a need for a new OWNED flag still.
If nf_conn___init is passed into tracepoint it's a bug and
we gotta fix it.

> but AFAICT you're correct that this should be sufficient. I added
> a negative test case that correctly fails if a BPF program tries to call
> these helpers with a struct nf_conn* rather than a struct
> nf_conn__init*.
>
> [0]: https://lore.kernel.org/all/CAP01T77PTK+bD2mBrxJShKNPhEypT2+nSHcr3=uuJbrghv_wFg@mail.gmail.com/
>
> > I prefer to keep only one flag KF_TRUSTED_ARGS that kfunc-s need to use
> > and eventually make all kfuncs KF_TRUSTED_ARGS by default and remove that flag.
>
> Yes, KF_TRUSTED_ARGS really should be the default. As Kumar describes in
> [1], we'll have to figure out how to avoid trace progs with unsafe args
> from calling these kfuncs. Maybe the right thing to do is allow-listing
> rather than deny-listing, as you pointed out.
>
> [1]: https://lore.kernel.org/bpf/CAP01T77goGbF3GVithEuJ7yMQR9PxHNA9GXFODq_nfA66G=F9g@mail.gmail.com/

That is still the plan. more or less.

> > Separately...
> > I think there was a plan to strengthen PTR_TO_BTF_ID and apply PTR_UNTRUSTED.
>
> That would be nice if we could do it. I assume that the issue is we're
> breaking backwards compat if we do, so I'd be curious to hear what the
> plan was if you're aware. The only plan that I've seen so far is what
> Kumar spelled out above in [1] above.

Right. Backward compat with existing usage of PTR_TO_BTF_ID
is the main issue.

>
> > This PTR_WALKED looks like new thing.
> > If we really need it PTR_TO_BTF_ID should be allowlisted instead of denylisted
> > as PTR_WALKED is doing.
> > I mean we can introduce PTR_TRUSTED and add this flag to return value
> > of bpf_get_current_task_btf() and arguments of tracepoints.
> > As soon as any ptr walking is done we can clear PTR_TRUSTED to keep
> > backward compat behavior of PTR_TO_BTF_ID.
> > PTR_WALKED is sort-of doing the same, but not conservative enough.
> > Too many things produce PTR_TO_BTF_ID. Auditing it all is challenging.
>
> I very much prefer the idea of allowlisting instead of denylisting,
> though I wish we'd taken that approach from the start rather than going
> with PTR_UNTRUSTED. It feels wrong to have both PTR_UNTRUSTED and
> PTR_TRUSTED as type modifiers, as the absence of PTR_UNTRUSTED should
> (and currently does) imply PTR_TRUSTED.

I kind agree, but we gotta have both because of backward compat.
We cannot change PTR_TO_BTF_ID as a whole right now.

Note PTR_TO_BTF_ID appears in kfuncs too.
I'm proposing to use PTR_TO_BTF_ID | PTR_TRUSTED
only in tracepoint args and as return value from
certain helpers like bpf_get_current_task_btf().
afaik it's all safe. There is no uaf here.
uaf is for kfunc. Especially fexit.
Those will stay PTR_TO_BTF_ID. Without PTR_TRUSTED.

>
> If we're going to go with an allowlist approach, then I think we should
> just get rid of PTR_UNTRUSTED altogether. Is that what you're
> suggesting? Otherwise, if we don't get rid of PTR_UNTRUSTED, then
> PTR_WALKED seems like a more natural type modifier addition.

Eventually either PTR_TRUSTED or PTR_UNTRUSTED will be removed.

> > I might have missed earlier discussions on this patch set. Apologies if so.
>
> Just FYI, the main initial thread where this was all discussed was [2].
>
> [2]: https://lore.kernel.org/all/CAP01T76OR3J_P8YMq4ZgKHBpuZyA0zgsPy+tq9htbX=j6AVyOg@mail.gmail.com/