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

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Message-Id: <20250611-rcu-fix-task_cls_state-v2-1-1a7fc248232a@posteo.net>
Date: Wed, 11 Jun 2025 09:04:06 +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 v2] net: Fix RCU usage in task_cls_state() for BPF
 programs

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()));
 }
 EXPORT_SYMBOL_GPL(task_cls_state);

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

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