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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Thu, 10 Jun 2021 10:52:37 -0700
Message-ID: <CAADnVQKMwKYgthoQV4RmGpZm9Hm-=wH3DoaNqs=UZRmJKefwGw@mail.gmail.com>
Subject: Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run
To:     Kees Cook <keescook@chromium.org>
Cc:     Yonghong Song <yhs@fb.com>, Dmitry Vyukov <dvyukov@google.com>,
        Kurt Manucredo <fuzzybritches0@gmail.com>,
        syzbot+bed360704c521841c85d@syzkaller.appspotmail.com,
        Andrii Nakryiko <andrii@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "David S. Miller" <davem@davemloft.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Martin KaFai Lau <kafai@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        Song Liu <songliubraving@fb.com>,
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        linux-kernel-mentees@lists.linuxfoundation.org,
        Shuah Khan <skhan@linuxfoundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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On Thu, Jun 10, 2021 at 10:06 AM Kees Cook <keescook@chromium.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.