Subject: Re: [PATCH bpf v2 1/2] bpf: mark registers as safe or unknown in all frames

From: Paul Chaignon <[email-masked]>
Date: Wed, 24 Apr 2019 19:13:56 +0200

On Tue, Apr 23, 2019 01:47AM, Daniel Borkmann wrote:
> On 04/22/2019 08:34 PM, Paul Chaignon wrote:
> > In case of a null check on a pointer inside a subprog, we should mark all
> > registers with this pointer as either safe or unknown, in both the current
> > and previous frames.  Currently, only spilled registers and registers in
> > the current frame are marked.  This patch also marks registers in previous
> > frames.
> > 
> > A good reproducer looks as follow:
> > 
> > 1: ptr = bpf_map_lookup_elem(map, &key);
> > 2: ret = subprog(ptr) {
> > 3:   return ptr != NULL;
> > 4: }
> > 5: if (ret)
> > 6:   value = *ptr;
> > 
> > With the above, the verifier will complain on line 6 because it sees ptr
> > as map_value_or_null despite the null check in subprog 1.
> > 
> > Note that this patch fixes another resulting bug when using
> > bpf_sk_release():
> > 
> > 1: sk = bpf_sk_lookup_tcp(...);
> > 2: subprog(sk) {
> > 3:   if (sk)
> > 4:     bpf_sk_release(sk);
> > 5: }
> > 6: if (!sk)
> > 7:   return 0;
> > 8: return 1;
> > 
> > In the above, mark_ptr_or_null_regs will warn on line 6 because it will
> > try to free the reference state, even though it was already freed on
> > line 3.
> > 
> > Fixes: f4d7e40a5b71 ("bpf: introduce function calls (verification)")
> > Signed-off-by: Paul Chaignon <[email-masked]>
> > ---
> >  kernel/bpf/verifier.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index db301e9b5295..777970d8c245 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -4876,11 +4876,11 @@ static void mark_ptr_or_null_regs(struct bpf_verifier_state *vstate, u32 regno,
> >  		 */
> >  		WARN_ON_ONCE(release_reference_state(state, id));
> >  
> > -	for (i = 0; i < MAX_BPF_REG; i++)
> > -		mark_ptr_or_null_reg(state, &regs[i], id, is_null);
> > -
> >  	for (j = 0; j <= vstate->curframe; j++) {
> >  		state = vstate->frame[j];
> > +		for (i = 0; i < MAX_BPF_REG; i++)
> > +			mark_ptr_or_null_reg(state, &state->regs[i], id,
> > +					     is_null);
> 
> Small nit: I think it would be good to follow practice from clear_all_pkt_pointers()
> and release_reg_references() to move handling of a singe frame into its own function.

Will do.

> 
> >  		bpf_for_each_spilled_reg(i, state, reg) {
> >  			if (!reg)
> >  				continue;
> > 
> 
> What about semantics in other, similar functions like find_good_pkt_pointers()?
> Should this also consider all frames instead?

Looks like the same is needed in find_good_pkt_pointers().  It's the only
similar function I could find though.  Did you have others in mind?

I'll add a second test case for the find_good_pkt_pointers() change in v3.

> Thanks,
> Daniel