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

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Message-ID: <CAADnVQ+mzrDH+8S=ddDCtyo6YUO4dUUsAS88Jza93pDQ2K3Bng@mail.gmail.com>
Date: Tue, 10 Jun 2025 08:42:02 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Charalampos Mitrodimas <charmitro@...teo.net>
Cc: Daniel Borkmann <daniel@...earbox.net>, "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 Tue, Jun 10, 2025 at 8:23 AM Charalampos Mitrodimas
<charmitro@...teo.net> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Tue, Jun 10, 2025 at 5:58 AM Daniel Borkmann <daniel@...earbox.net> wrote:
> >>
> >> On 6/9/25 5:51 PM, Alexei Starovoitov wrote:
> >> > 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.
> >>
> >> Yeap, we cannot break tc(x) BPF programs. It probably makes sense to have
> >> a new helper implementation for the more generic, non-networking case which
> >> then internally uses task_cls_classid().
> >
> > Instead of forking the helper I think we can :
> > rcu_read_lock_bh_held() || rcu_read_lock_held()
> > in task_cls_state().
>
> I tested your suggestion with,
>
>   rcu_read_lock_bh_held() || rcu_read_lock_held()
>
> but it still triggers the RCU warning because BPF syscall programs use
> rcu_read_lock_trace().
>
> Adding rcu_read_lock_trace_held() fixes it functionally but triggers a
> checkpatch warning:
>
>   WARNING: use of RCU tasks trace is incorrect outside BPF or core RCU code

It's safe to ignore checkpatch in this case.

> I think the best solution here would be to add
> local_bh_disable()/enable() protection directly in the BPF helper. This
> keeps the fix localized to where the problem exists, and avoids
> modifying core cgroup RCU.

That works, but it will add runtime overhead.
I doubt the helper is in a critical path, so I don't mind.