Re: [PATCH bpf-next 7/7] bpf: unify PTR_TO_MAP_{KEY,VALUE} with default case in regsafe()

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

On Tue, Jan 03, 2023 at 02:04:44PM -0800, Andrii Nakryiko wrote:
> > It sounds logical, but it can get tricky with ranges and branch taken logic.
> > Consider something like:
> > R1=(min=2,max=8), R2=(min=1, max=10)
> > if (R1 within R2) // bpf prog is doing its own 'within'
> 
> a bit confused what is "R1 within R2" here and what you mean "bpf prog
> is doing its own 'within'"? Any sort of `R1 < R2` checks (and any
> other op: <=, >=, etc) can't really kick in branch elimination because
> R2_min=1 < R1_max=8, so arithmetically speaking we can't conclude that
> "R1 is always smaller than R2", so both branches would have to be
> examined.

Something like that. Details didn't matter to me.
It was hypothetical 'within' operation just to illustrate the point.

> But I probably misunderstood your example, sorry.
> 
> >   // branch taken kicks in
> > else
> >   // issues that were never checked
> >
> > Now new state has:
> > R1=(min=4,max=6), R2=(min=5, max=5)
> >
> > Both R1 and R2 of new state individually range_within of old safe state,
> > but together the prog may go to the unverified path.
> > Not sure whether it's practical today.
> > You asked for hypothetical, so here it goes :)
> 
> No problem with "hypothetical-ness". But my confusion and argument is
> similarly "in principle"-like. Because if such an example above can be
> constructed then this would be an issue for SCALAR as well, right? And
> if you can bypass verifier's safety with SCALAR, you (hypothetically)
> could use that SCALAR to do out-of-bounds memory access by adding this
> SCALAR to some mem-like register.

Correct. The issue would apply to regular scalar if such 'within' operation
was available.

> So that's my point and my source of confusion: if we don't trust
> var_off+range_within() logic to handle *all* situations correctly,
> then we should be worried about SCALARs just as much as anything else
> (unless, as usual, I missed something).

Yes. I personally don't believe that doing range_within for all regtypes
by default is a safer way forward.
The example wasn't real. It was trying to demonstrate a possible issue.
You insist to see a real example with range_within.
I don't have it. It's a gut feel that it could be there because
I could construct it with fake 'within'.

> > More gut feel than real issue.
> >
> > >
> > > >
> > > > SCALARS and PTR_TO_BTF_ID will likely dominate future bpf progs.
> > > > Keeping default as regs_exact (that does ID match) is safer default.
> > >
> > > It's fine, though the point of this patch set was patch #7, enabling
> > > logic similar to PTR_TO_MAP_VALUE for PTR_TO_MEM and PTR_TO_BUF. I can
> > > send specific fixes for that, no problem. But as I said above, I'm
> > > really curious to understand what kind of situations will lead to
> > > unsafety if we do var_off+range_within checks.
> >
> > PTR_TO_MEM and PTR_TO_BUF explicitly are likely ok despite my convoluted
> > example above.
> > I'm less sure about PTR_TO_BTF_ID. It could be ok.
> > Just feels safer to opt-in each type explicitly.
> 
> Sure, I can just do a simple opt-in, no problem. As I said, mostly
> trying to understand the issue overall.
> 
> For PTR_TO_BTF_ID specifically, I can see how we can enable
> var_off+range_within for cases when we access some array, right? But
> then I think we'll be enforcing that we are staying within the
> boundaries of a single array field, never crossing into another field.

Likely yes, but why?
You're trying hard to collapse the switch statement in regsafe()
while claiming it's a safer way. I don't see it this way.
For example the upcoming active_lock_id would need its own check_ids() call.
It will be necessary for PTR_TO_BTF_ID only.
Why collapse the switch into 'default:' just to bring some back?
The default without checking active_lock_id through check_ids
would be wrong, so collapsed switch doesn't make things safer.

> But just to take a step back, from my perspective var_off and
> range_within are complementary and solve slightly different uses, but
> should be both satisfied:
>   - var_off is not precise with range boundaries (due to some bits too
> coarsely marked as unknown), but it's useful to enforce having a value
> being a multiple of some power-of-2 (e.g., knowing for sure that
> lowest 2 bits are zero means that value is multiple of 4; I haven't
> checked, but I assume we check with for various pointer accesses to
> ensure we don't have misaligned reads). They can be only approximately
> used for actual possible range of values.

Right. var_off is used for alignment checking too. grep tnum_is_aligned.
We have bare minimum of testing for that though.
Only few tests in the test_verifier use BPF_F_STRICT_ALIGNMENT

>   - range_within() can and should be used for *precise* range of value
> tracking, but it can't express that alignment restriction.

Right.

> So while I previously thought that we can do away without var_off, I
> now think there are cases when it's necessary. But if we are sure that
> we handle any SCALAR case correctly for any possible var_off +
> range_within situation, it should be fine to do that for any mem-like
> pointer just as much, as var_off+range_within is basically a MEM +
> SCALAR combined case.

Right. Likely true.

> Anyways, I'm not blocked on this, but I think we'll benefit from
> taking this discussion to its logical conclusion.

Not sure what conclusion you're looking for.