Re: stack access issue. Re: [syzbot] [bpf?] UBSAN: array-index-out-of-bounds in check_stack_range_initialized

[Andrei Matei]

On Sat, Mar 23, 2024 at 8:52 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Sat, Mar 23, 2024 at 5:50 PM Andrei Matei <andreimatei1@xxxxxxxxx> wrote:
> >
> > + Edward
> >
> > On Thu, Mar 21, 2024 at 3:33 AM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > Hi Andrei,
> > >
> > > looks like the refactoring of stack access introduced a bug.
> > > See the reproducer below.
> > > positive offsets are not caught by check_stack_access_within_bounds().
> >
> > check_stack_access_within_bounds() tries to catch positive offsets;
> > It does: [1]
> >
> > err = check_stack_slot_within_bounds(env, min_off, state, type);
> > if (!err && max_off > 0)
> >   err = -EINVAL; /* out of stack access into non-negative offsets */
> >
> > Notice the max_off > 0 in there.
> > And we have various tests that seem to check that positive offsets are
> > rejected. Do you know what the bug is?
> > I'm thinking maybe there's some overflow going on, except that UBSAN
> > reported an index of -1 as being the problem.
> >
> > Edward, I see that you've been tickling the robot trying to narrow the issue;
> > perhaps you've figured it out?
> >
> > If the bug is not immediately apparent to anyone, I would really appreciate a
> > bit of tutoring around how to reproduce and get verifier logs.
>
> The repro is right there in the email I forwarded:
>
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15c38711180000

I understand, but how does one go from this to either BPF assembly,
or to running it in such a way that you also get verifier logs?