From: Yonghong Song <yonghong.song@linux.dev>
To: Sahil Chandna <chandna.sahil@gmail.com>,
ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
martin.lau@linux.dev, eddyz87@gmail.com, song@kernel.org,
john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me,
haoluo@google.com, jolsa@kernel.org, bigeasy@linutronix.de,
bpf@vger.kernel.org
Cc: syzbot+b0cff308140f79a9c4cb@syzkaller.appspotmail.com
Subject: Re: [PATCH bpf-next v2] bpf: use preempt_disable/enable() to protect bpf_bprintf_buffers nesting
Date: Tue, 11 Nov 2025 12:22:51 -0800 [thread overview]
Message-ID: <fce1c70c-c848-415d-8be4-9fba21c70fee@linux.dev> (raw)
In-Reply-To: <20251111170628.410641-1-chandna.sahil@gmail.com>
On 11/11/25 9:06 AM, Sahil Chandna wrote:
> The bpf_bprintf_prepare() and related helpers (bpf_try_get_buffers() /
> bpf_put_buffers()) rely on a per-CPU counter bpf_bprintf_nest_level to
> manage nested buffer usage. However, when invoked from different contexts
> (process, softirq, NMI), the nesting counter can become inconsistent if
> task migration occurs between CPUs during these operations. This can
> result in warnings such as:
>
> WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_try_get_buffers kernel/bpf/helpers.c:781 [inline]
> WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_bprintf_prepare+0x12cf/0x13a0 kernel/bpf/helpers.c:834
>
> Having only migrate_disable is insufficient here to prevent nesting,
> hence add preempt_disable()/enable() around buffer acquisition and release.
>
> Reported-by: syzbot+b0cff308140f79a9c4cb@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68f6a4c8.050a0220.1be48.0011.GAE@google.com/
> Fixes: 4223bf833c849 ("bpf: Remove preempt_disable in bpf_try_get_buffers")
> Suggested-by: Yonghong Song <yonghong.song@linux.dev>
> Signed-off-by: Sahil Chandna <chandna.sahil@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
next prev parent reply other threads:[~2025-11-11 20:23 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-11-11 17:06 [PATCH bpf-next v2] bpf: use preempt_disable/enable() to protect bpf_bprintf_buffers nesting Sahil Chandna
2025-11-11 20:22 ` Yonghong Song [this message]
2025-11-12 8:23 ` Sebastian Andrzej Siewior
2025-11-14 21:10 ` patchwork-bot+netdevbpf
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