linux-kernel - Re: [PATCH bpf-next v2] net: Fix RCU usage in task_cls

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Message-ID: <CAADnVQJu3fYTfdRTWxeB5hraqe3_Esm7cgKfO38nxodknABeHg@mail.gmail.com>
Date: Wed, 11 Jun 2025 08:58:35 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Charalampos Mitrodimas <charmitro@...teo.net>
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Feng Yang <yangfeng@...inos.cn>, Tejun Heo <tj@...nel.org>, 
	Network Development <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	bpf <bpf@...r.kernel.org>, syzbot+b4169a1cfb945d2ed0ec@...kaller.appspotmail.com
Subject: Re: [PATCH bpf-next v2] net: Fix RCU usage in task_cls_state() for
 BPF programs

On Wed, Jun 11, 2025 at 2:04 AM Charalampos Mitrodimas
<charmitro@...teo.net> wrote:
>
> The commit ee971630f20f ("bpf: Allow some trace helpers for all prog
> types") made bpf_get_cgroup_classid_curr helper available to all BPF
> program types, not just networking programs.
>
> This helper calls __task_get_classid() which internally calls
> task_cls_state() requiring rcu_read_lock_bh_held(). This works in
> networking/tc context where RCU BH is held, but triggers an RCU
> warning when called from other contexts like BPF syscall programs that
> run under rcu_read_lock_trace():
>
>   WARNING: suspicious RCU usage
>   6.15.0-rc4-syzkaller-g079e5c56a5c4 #0 Not tainted
>   -----------------------------
>   net/core/netclassid_cgroup.c:24 suspicious rcu_dereference_check() usage!
>
> Fix this by also accepting rcu_read_lock_trace_held() as a valid RCU
> context in the task_cls_state() function. This is safe because BPF
> programs are non-sleepable and task_cls_state() is only doing an RCU
> dereference to get the classid.
>
> Reported-by: syzbot+b4169a1cfb945d2ed0ec@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b4169a1cfb945d2ed0ec
> Fixes: ee971630f20f ("bpf: Allow some trace helpers for all prog types")
> Signed-off-by: Charalampos Mitrodimas <charmitro@...teo.net>
> ---
> Changes in v2:
> - Fix RCU usage in task_cls_state() instead of BPF helper
> - Add rcu_read_lock_trace_held() check to accept trace RCU as valdi
>   context
> - Drop the approach of using task_cls_classid() which has in_interrupt()
>   check
> - Link to v1: https://lore.kernel.org/r/20250608-rcu-fix-task_cls_state-v1-1-2a2025b4603b@posteo.net
> ---
>  net/core/netclassid_cgroup.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/netclassid_cgroup.c b/net/core/netclassid_cgroup.c
> index d22f0919821e931fbdedf5a8a7a2998d59d73978..df86f82d747ac40e99597d6f2d921e8cc2834e64 100644
> --- a/net/core/netclassid_cgroup.c
> +++ b/net/core/netclassid_cgroup.c
> @@ -21,7 +21,8 @@ static inline struct cgroup_cls_state *css_cls_state(struct cgroup_subsys_state
>  struct cgroup_cls_state *task_cls_state(struct task_struct *p)
>  {
>         return css_cls_state(task_css_check(p, net_cls_cgrp_id,
> -                                           rcu_read_lock_bh_held()));
> +                                           rcu_read_lock_bh_held() ||
> +                                           rcu_read_lock_trace_held()));

This is incomplete. It only addresses one particular syzbot report.
It needs to include rcu_read_lock_held() as well.

pw-bot: cr