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

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Message-ID: <CAADnVQJW3CisB3L2nNOC0aGkPPBTHnyM-ZCXoZJc-KtNNEj+QQ@mail.gmail.com>
Date:   Fri, 4 Nov 2022 16:32:26 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
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>,
        Russell King <linux@...linux.org.uk>,
        "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>, colin.i.king@...il.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 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access
 for 32-bit architecture

On Fri, Nov 4, 2022 at 2:56 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Thu, Nov 3, 2022 at 1:36 AM Yang Jihong <yangjihong1@...wei.com> 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.
> >
> > Signed-off-by: Yang Jihong <yangjihong1@...wei.com>
> > ---
> >  net/core/filter.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index bb0136e7a8e4..eab7ce89740c 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -8269,8 +8269,6 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
> >                         return false;
> >                 break;
> >         case offsetof(struct __sk_buff, sk):
> > -               if (type == BPF_WRITE || size != sizeof(__u64))
> > -                       return false;
>
> this probably should be specific to host architecture bitness? I'd
> imagine that size = 4 should be invalid on 64-bit arches (reading half
> of the pointer is bad)

Not quite.
In __sk_buff the field 'sk' is defined as:
__bpf_md_ptr(struct bpf_sock *, sk);
so it's always 64-bit load when bpf prog reads it.
In this case CO_RE shouldn't have been applied to uapi struct __sk_buff.