From: Martin KaFai Lau <martin.lau@linux.dev>
To: Paul Chaignon <paul.chaignon@gmail.com>
Cc: bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
Eduard Zingerman <eddyz87@gmail.com>,
John Fastabend <john.fastabend@gmail.com>
Subject: Re: [PATCH bpf-next v2 1/2] bpf: Reject narrower access to pointer ctx fields
Date: Wed, 23 Jul 2025 19:42:41 -0700 [thread overview]
Message-ID: <697adce5-2afe-45bb-9e01-1022efb94e39@linux.dev> (raw)
In-Reply-To: <aICZqWFT77dvmJqc@mail.gmail.com>
On 7/23/25 1:13 AM, Paul Chaignon wrote:
> On Tue, Jul 22, 2025 at 03:28:40PM -0700, Martin KaFai Lau wrote:
>> On 7/22/25 7:32 AM, Paul Chaignon wrote:
>>> The following BPF program, simplified from a syzkaller repro, causes a
>>> kernel warning:
>>>
>>> r0 = *(u8 *)(r1 + 169);
>>> exit;
>>>
>>> With pointer field sk being at offset 168 in __sk_buff. This access is
>>> detected as a narrower read in bpf_skb_is_valid_access because it
>>> doesn't match offsetof(struct __sk_buff, sk). It is therefore allowed
>>> and later proceeds to bpf_convert_ctx_access. At that point,
>>> target_size is null and the verifier errors with a kernel warning and:
>>
>> I think it meant target_size is 0. I suspect !cnt is the condition causing
>> the 'verifier bug: ...'. Please check. No need to resend. The patch lgtm.
>
> I also initially though the error was triggered because cnt was 0, but
> it is not. In case of narrower load, the offset is aligned before
> calling convert_ctx_access, which means we match
> offsetof(struct __sk_buff, sk) in bpf_convert_ctx_access. An
> instruction is added and cnt is thus 1. target_size however stays 0 so
> we hit the verifier bug error.
Got it. I have added this details to the commit message. Applied. Thanks!
next prev parent reply other threads:[~2025-07-24 2:43 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-07-22 14:32 [PATCH bpf-next v2 1/2] bpf: Reject narrower access to pointer ctx fields Paul Chaignon
2025-07-22 14:33 ` [PATCH bpf-next v2 2/2] selftests/bpf: Test invalid narrower ctx load Paul Chaignon
2025-07-22 21:38 ` Eduard Zingerman
2025-07-22 20:40 ` [PATCH bpf-next v2 1/2] bpf: Reject narrower access to pointer ctx fields Eduard Zingerman
2025-07-22 22:28 ` Martin KaFai Lau
2025-07-23 8:13 ` Paul Chaignon
2025-07-24 2:42 ` Martin KaFai Lau [this message]
2025-07-24 2:50 ` patchwork-bot+netdevbpf
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