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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Mon, 6 May 2024 16:35:55 -0700
Message-ID: <CAADnVQ+eQ36rc8Ew5a4YfFbB0rgbJhXXwwgPASjFVQ7=QFyM1A@mail.gmail.com>
Subject: Re: [PATCH] fix array-index-out-of-bounds in bpf_prog_select_runtime
To: Camila Alvarez Inostroza <cam.alvarez.i@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, bpf <bpf@vger.kernel.org>, 
	LKML <linux-kernel@vger.kernel.org>, 
	syzbot+d2a2c639d03ac200a4f1@syzkaller.appspotmail.com
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On Sun, May 5, 2024 at 4:18=E2=80=AFPM Camila Alvarez Inostroza
<cam.alvarez.i@gmail.com> wrote:
>
>
>
> On Sun, 5 May 2024, Alexei Starovoitov wrote:
>
> > On Sat, May 4, 2024 at 6:49=E2=80=AFPM Camila Alvarez <cam.alvarez.i@gm=
ail.com> wrote:
> >>
> >> The error indicates that the verifier is letting through a program wit=
h
> >> a stack depth bigger than 512.
> >>
> >> This is due to the verifier not checking the stack depth after
> >> instruction rewrites are perfomed. For example, the MAY_GOTO instructi=
on
> >> adds 8 bytes to the stack, which means that if the stack at the moment
> >> was already 512 bytes it would overflow after rewriting the instructio=
n.
> >
> > This is by design. may_goto and other constructs like bpf_loop
> > inlining can consume a few words above 512 limit.
> >
>
> Is this the only case where the verifier should allow the stack to go ove=
r
> the 512 limit? If that's the case, maybe we could use the extra stack
> depth to store how much the rewrites affect the stack depth? This would
> only be used to obtain the correct interpreter when
> CONFIG_BPF_JIT_ALWAYS_ON is not set.
> That would allow choosing the interpreter by considering the stack depth
> before the rewrites.
>
> >> The fix involves adding a stack depth check after all instruction
> >> rewrites are performed.
> >>
> >> Reported-by: syzbot+d2a2c639d03ac200a4f1@syzkaller.appspotmail.com
> >
> > This syzbot report is likely unrelated.
> > It says that it bisected it to may_goto, but it has this report
> > before may_goto was introduced, so bisection is incorrect.
> >
> > pw-bot: cr
>
> I can see that may_goto was introduced on march 6th, and the first report
> was on march 13th. Is there any report I'm missing?

Could you please craft a selftest for this issue then?
It will be much easier to reason about the fix.
We can either add another interpreter to interpreters_args[]
or just gate may_goto with prog->jit_requested.