linux-kernel - Re: [PATCH bpf RESEND 2/4] bpf: Remove size check for sk in bpf_skb_is_valid

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Message-ID: <CAADnVQ+gX8Xc57K2hSG5ZNfU1RtKBFgEp2yOWq08X68bWjMqsg@mail.gmail.com>
Date:   Thu, 3 Nov 2022 11:15:00 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Yang Jihong <yangjihong1@...wei.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Shubham Bansal <illusionist.neo@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Delyan Kratunov <delyank@...com>,
        Artem Savkov <asavkov@...hat.com>, bpf <bpf@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf RESEND 2/4] bpf: Remove size check for sk in
 bpf_skb_is_valid_access for 32-bit architecture

On Thu, Nov 3, 2022 at 4:23 AM Russell King (Oracle)
<linux@...linux.org.uk> wrote:
>
> On Thu, Nov 03, 2022 at 05:21:16PM +0800, Yang Jihong wrote:
> > The error code -EACCES is returned when bpf prog is tested in 32-bit environment,
> > This is because bpf_object__relocate modifies the instruction to change memory
> > size to 4 bytes, as shown in the following messages:
> >
> > libbpf: prog 'kfunc_call_test1': relo #2: matching candidate #0 <byte_off> [18342] struct __sk_buff.sk (0:30:0 @ offset 168)
> > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) off 168 -> 168
> > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) mem_sz 8 -> 4
> >
> > As a result, the bpf_skb_is_valid_access check fails. For 32-bit architecture,
> > unnecessary checks need to be deleted.
>
> Isn't the purpose of this check to ensure that the entire pointer is
> written, and BPF can't write half of it?
>
>
> >       case offsetof(struct __sk_buff, sk):
> > -             if (type == BPF_WRITE || size != sizeof(__u64))
> > -                     return false;
>
> Wouldn't "(size != sizeof(struct bpf_sock *) && size != sizeof(__u64))"
> be more appropriate here, so 32-bit can only write the 32-bit pointer
> or the full 64-bit value, and 64-bit can only write the 64-bit pointer?
> Or is there a reason not to? bpf folk?

You're correct. The patch is completely wrong.
The bug is elsewhere.