netdev - [PATCH bpf-next] bpf: Fix RCU usage in bpf_get_cgroup_classid

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Message-Id: <20250608-rcu-fix-task_cls_state-v1-1-2a2025b4603b@posteo.net>
Date: Sun, 08 Jun 2025 15:34:55 +0000
From: Charalampos Mitrodimas <charmitro@...teo.net>
To: "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>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 bpf@...r.kernel.org, syzbot+b4169a1cfb945d2ed0ec@...kaller.appspotmail.com, 
 Charalampos Mitrodimas <charmitro@...teo.net>
Subject: [PATCH bpf-next] bpf: Fix RCU usage in bpf_get_cgroup_classid_curr
 helper

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);
 }

 const struct bpf_func_proto bpf_get_cgroup_classid_curr_proto = {

---
base-commit: 079e5c56a5c41d285068939ff7b0041ab10386fa
change-id: 20250608-rcu-fix-task_cls_state-0ed73f437d1e

Best regards,
-- 
Charalampos Mitrodimas <charmitro@...teo.net>