linux-kernel - Re: [PATCH bpf-next] bpf: Fix RCU usage in bpf_get_cgroup_classid

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Message-ID: <CAADnVQLxaxVpCaK90FfePOKMLpH=axaK3gDwVZLp0L1+fNxgtA@mail.gmail.com>
Date: Mon, 9 Jun 2025 08:51:42 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Charalampos Mitrodimas <charmitro@...teo.net>, Daniel Borkmann <daniel@...earbox.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>, 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] bpf: Fix RCU usage in bpf_get_cgroup_classid_curr
 helper

On Sun, Jun 8, 2025 at 8:35 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.  This helper used __task_get_classid() which calls
> task_cls_state() that requires rcu_read_lock_bh_held().
>
> This triggers an RCU warning when called from BPF syscall programs
> which 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 replacing __task_get_classid() with task_cls_classid()
> which handles RCU locking internally using regular rcu_read_lock() and
> is safe to call from any context.
>
> 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>
> ---
>  net/core/filter.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 30e7d36790883b29174654315738e93237e21dd0..3b3f81cf674dde7d2bd83488450edad4e129bdac 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3083,7 +3083,7 @@ static const struct bpf_func_proto bpf_msg_pop_data_proto = {
>  #ifdef CONFIG_CGROUP_NET_CLASSID
>  BPF_CALL_0(bpf_get_cgroup_classid_curr)
>  {
> -       return __task_get_classid(current);
> +       return task_cls_classid(current);
>  }

Daniel added this helper in
commit 5a52ae4e32a6 ("bpf: Allow to retrieve cgroup v1 classid from v2 hooks")
with intention to use it from networking hooks.

But task_cls_classid() has
        if (in_interrupt())
                return 0;

which will trigger in softirq and tc hooks.
So this might break Daniel's use case.

Daniel,
ptal.