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

[Alexei Starovoitov]

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