netdev - Re: [PATCH v2 bpf-next 2/6] bpf: propagate poke descriptors to subprograms

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Date:   Wed, 22 Jul 2020 20:37:49 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     ast@...nel.org, bpf@...r.kernel.org, netdev@...r.kernel.org,
        bjorn.topel@...el.com, magnus.karlsson@...el.com
Subject: Re: [PATCH v2 bpf-next 2/6] bpf: propagate poke descriptors to
 subprograms

On Wed, Jul 22, 2020 at 04:40:42PM +0200, Daniel Borkmann wrote:
> On 7/21/20 1:53 PM, Maciej Fijalkowski wrote:
> > Previously, there was no need for poke descriptors being present in
> > subprogram's bpf_prog_aux struct since tailcalls were simply not allowed
> > in them. Each subprog is JITed independently so in order to enable
> > JITing such subprograms, simply copy poke descriptors from main program
> > to subprogram's poke tab.
> > 
> > Add also subprog's aux struct to the BPF map poke_progs list by calling
> > on it map_poke_track().
> > 
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> > ---
> >   kernel/bpf/verifier.c | 20 ++++++++++++++++++++
> >   1 file changed, 20 insertions(+)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 3c1efc9d08fd..3428edf85220 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -9936,6 +9936,9 @@ static int jit_subprogs(struct bpf_verifier_env *env)
> >   		goto out_undo_insn;
> >   	for (i = 0; i < env->subprog_cnt; i++) {
> > +		struct bpf_map *map_ptr;
> > +		int j;
> > +
> >   		subprog_start = subprog_end;
> >   		subprog_end = env->subprog_info[i + 1].start;
> > @@ -9960,6 +9963,23 @@ static int jit_subprogs(struct bpf_verifier_env *env)
> >   		func[i]->aux->btf = prog->aux->btf;
> >   		func[i]->aux->func_info = prog->aux->func_info;
> > +		for (j = 0; j < prog->aux->size_poke_tab; j++) {
> > +			int ret;
> > +
> > +			ret = bpf_jit_add_poke_descriptor(func[i],
> > +							  &prog->aux->poke_tab[j]);
> > +			if (ret < 0) {
> > +				verbose(env, "adding tail call poke descriptor failed\n");
> > +				goto out_free;
> > +			}
> > +			map_ptr = func[i]->aux->poke_tab[j].tail_call.map;
> > +			ret = map_ptr->ops->map_poke_track(map_ptr, func[i]->aux);
> > +			if (ret < 0) {
> > +				verbose(env, "tracking tail call prog failed\n");
> > +				goto out_free;
> > +			}
> 
> Hmm, I don't think this is correct/complete. If some of these have been registered or
> if later on the JIT'ing fails but the subprog is already exposed to the prog array then
> it's /public/ at this point, so a later bpf_jit_free() in out_free will rip them mem
> while doing live patching on prog updates leading to UAF.

Ugh. So if we would precede the out_free label with map_poke_untrack() on error
path - would that be sufficient?

> 
> > +		}
> > +
> >   		/* Use bpf_prog_F_tag to indicate functions in stack traces.
> >   		 * Long term would need debug info to populate names
> >   		 */
> > 
>