netdev - Re: [PATCH bpf-next v5 1/8] bpf: Add support for forcing kfunc args to be referenced

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Message-ID: <20220706212903.az2mtqodtzmn2gwq@MacBook-Pro-3.local>
Date:   Wed, 6 Jul 2022 14:29:03 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, Yonghong Song <yhs@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org
Subject: Re: [PATCH bpf-next v5 1/8] bpf: Add support for forcing kfunc args
 to be referenced

On Thu, Jul 07, 2022 at 12:51:15AM +0530, Kumar Kartikeya Dwivedi wrote:
> On Thu, 7 Jul 2022 at 00:14, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Sun, Jul 03, 2022 at 11:04:22AM +0530, Kumar Kartikeya Dwivedi wrote:
> > > On Sun, 3 Jul 2022 at 10:54, Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
> > > >
> > > > On Wed, 29 Jun 2022 at 08:53, Alexei Starovoitov
> > > > <alexei.starovoitov@...il.com> wrote:
> > > > >
> > > > > On Fri, Jun 24, 2022 at 12:56:30AM +0530, Kumar Kartikeya Dwivedi wrote:
> > > > > > Similar to how we detect mem, size pairs in kfunc, teach verifier to
> > > > > > treat __ref suffix on argument name to imply that it must be a
> > > > > > referenced pointer when passed to kfunc. This is required to ensure that
> > > > > > kfunc that operate on some object only work on acquired pointers and not
> > > > > > normal PTR_TO_BTF_ID with same type which can be obtained by pointer
> > > > > > walking. Release functions need not specify such suffix on release
> > > > > > arguments as they are already expected to receive one referenced
> > > > > > argument.
> > > > > >
> > > > > > Note that we use strict type matching when a __ref suffix is present on
> > > > > > the argument.
> > > > > ...
> > > > > > +             /* Check if argument must be a referenced pointer, args + i has
> > > > > > +              * been verified to be a pointer (after skipping modifiers).
> > > > > > +              */
> > > > > > +             arg_ref = is_kfunc_arg_ref(btf, args + i);
> > > > > > +             if (is_kfunc && arg_ref && !reg->ref_obj_id) {
> > > > > > +                     bpf_log(log, "R%d must be referenced\n", regno);
> > > > > > +                     return -EINVAL;
> > > > > > +             }
> > > > > > +
> > > > >
> > > > > imo this suffix will be confusing to use.
> > > > > If I understand the intent the __ref should only be used
> > > > > in acquire (and other) kfuncs that also do release.
> > > > > Adding __ref to actual release kfunc will be a nop.
> > > > > It will be checked, but it's not necessary.
> > > > >
> > > > > At the end
> > > > > +struct nf_conn *bpf_ct_insert_entry(struct nf_conn___init *nfct__ref)
> > > > > will behave like kptr_xchg with exception that kptr_xchg takes any btf_id
> > > > > while here it's fixed.
> > > > >
> > > > > The code:
> > > > >  if (rel && reg->ref_obj_id)
> > > > >         arg_type |= OBJ_RELEASE;
> > > > > should probably be updated with '|| arg_ref'
> > > > > to make sure reg->off == 0 ?
> > > > > That looks like a small bug.
> > > > >
> > > >
> > > > Indeed, I missed that. Thanks for catching it.
> > > >
> > > > > But stepping back... why __ref is needed ?
> > > > > We can add bpf_ct_insert_entry to acq and rel sets and it should work?
> > > > > I'm assuming you're doing the orthogonal cleanup of resolve_btfid,
> > > > > so we will have a single kfunc set where bpf_ct_insert_entry will
> > > > > have both acq and rel flags.
> > > > > I'm surely missing something.
> > > >
> > > > It is needed to prevent the case where someone might do:
> > > > ct = bpf_xdp_ct_alloc(...);
> > > > bpf_ct_set_timeout(ct->master, ...);
> > > >
> > >
> > > A better illustration is probably bpf_xdp_ct_lookup and
> > > bpf_ct_change_timeout, since here the type for ct->master won't match
> > > with bpf_ct_set_timeout, but the point is the same.
> >
> > Sorry, I'm still not following.
> > Didn't we make pointer walking 'untrusted' so ct->master cannot be
> > passed into any kfunc?
> >
> 
> I don't believe that is the case, it is only true for kptrs loaded
> from BPF maps (that too those with BPF_LDX, not the ones with
> kptr_xchg). There we had a chance to do things differently. For normal
> PTR_TO_BTF_ID obtained from kfuncs/BPF helpers, there is no untrusted
> flag set on them, nor is it set when walking them.
> 
> I also think we discussed switching to this mode, by making many cases
> untrusted by default, and using annotation to allow cases, making
> pointers trusted at one level (like args for tracing/lsm progs, but
> next deref becomes untrusted), but admittedly it may not cover enough
> ground, and you didn't like it much either, so I stopped pursuing it.

Ahh. Now I remember. Thanks for reminding :)
Could you please summarize this thread and add all of it as a big comment
in the source code next to __ref handling to explain the motivation
and an example on when and how this __ref suffix should be used.
Otherwise somebody, like me, will forget the context soon.

I was thinking of better name than __ref, but couldn't come up with one.
__ref fits this use case the best.