linux-kernel - Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog

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Message-ID: <CAADnVQKMwKYgthoQV4RmGpZm9Hm-=wH3DoaNqs=UZRmJKefwGw@mail.gmail.com>
Date:   Thu, 10 Jun 2021 10:52:37 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Yonghong Song <yhs@...com>, Dmitry Vyukov <dvyukov@...gle.com>,
        Kurt Manucredo <fuzzybritches0@...il.com>,
        syzbot+bed360704c521841c85d@...kaller.appspotmail.com,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        KP Singh <kpsingh@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Song Liu <songliubraving@...com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        nathan@...nel.org, Nick Desaulniers <ndesaulniers@...gle.com>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Shuah Khan <skhan@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

On Thu, Jun 10, 2021 at 10:06 AM Kees Cook <keescook@...omium.org> wrote:
>
> > > I guess the main question: what should happen if a bpf program writer
> > > does _not_ use compiler nor check_shl_overflow()?
>
> I think the BPF runtime needs to make such actions defined, instead of
> doing a blind shift. It needs to check the size of the shift explicitly
> when handling the shift instruction.

Such ideas were brought up in the past and rejected.
We're not going to sacrifice performance to make behavior a bit more
'defined'. CPUs are doing it deterministically. It's the C standard
that needs fixing.

> Sure, but the point of UBSAN is to find and alert about undefined
> behavior, so we still need to fix this.

No. The undefined behavior of C standard doesn't need "fixing" most of the time.