From: Martin KaFai Lau <martin.lau@linux.dev>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Shankari Anand <shankari.ak0208@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
Martin KaFai Lau <martin.lau@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
John Fastabend <john.fastabend@gmail.com>,
Andrii Nakryiko <andrii@kernel.org>,
Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
KP Singh <kpsingh@kernel.org>,
Stanislav Fomichev <sdf@fomichev.me>, Hao Luo <haoluo@google.com>,
Jiri Olsa <jolsa@kernel.org>,
syzbot+ad4661d6ca888ce7fe11@syzkaller.appspotmail.com
Subject: Re: [PATCH] bpf: restrict verifier access to bpf_lru_node.ref
Date: Tue, 15 Jul 2025 14:26:56 -0700 [thread overview]
Message-ID: <27c7c76c-becf-47b1-812b-05f260a8cd85@linux.dev> (raw)
In-Reply-To: <CAADnVQJ6_pB8ZU2Cw5S6nB4J-6s7bw5Fp-Hst9M_EE9=HxN8+g@mail.gmail.com>
On 7/15/25 7:49 AM, Alexei Starovoitov wrote:
> Also you misread the kcsan report.
>
> It says that 'read' comes from:
>
> read to 0xffff888118f3d568 of 4 bytes by task 4719 on cpu 1:
> lookup_nulls_elem_raw kernel/bpf/hashtab.c:643 [inline]
>
> which is reading hash and key of htab_elem while
> write side actually writes hash too:
> *(u32 *)((void *)node + lru->hash_offset) = hash;
>
> Martin,
> is it really possible for these read/write to race ?
I think it is possible. The elem in the lru's freelist currently does not wait
for a rcu gp before reuse. There is a chance that the rcu reader is still
reading the hash value that was put in the freelist, while the writer is reusing
and updating it.
I think the percpu_freelist used in the regular hashmap should have similar
behavior, so may be worth finding a common solution, such as waiting for a rcu
gp before reusing it.
next prev parent reply other threads:[~2025-07-15 21:27 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-07-15 7:57 [PATCH] bpf: restrict verifier access to bpf_lru_node.ref Shankari Anand
2025-07-15 14:49 ` Alexei Starovoitov
2025-07-15 21:26 ` Martin KaFai Lau [this message]
2025-07-16 6:32 ` Shankari Anand
2025-07-16 20:02 ` Martin KaFai Lau
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