[PATCH bpf-next v3] net: Fix RCU usage in task_cls_state() for BPF programs

[Charalampos Mitrodimas]

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_held() and
rcu_read_lock_trace_held() as valid RCU contexts in the
task_cls_state() function. This ensures the helper works correctly in
all RCU contexts where it might be called, regular RCU, RCU BH (for
networking), and RCU trace (for BPF syscall programs).

Reported-by: syzbot+b4169a1cfb945d2ed0ec@xxxxxxxxxxxxxxxxxxxxxxxxx
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@xxxxxxxxxx>
---
Changes in v3:
- Add rcu_read_lock_held() check as well 
- Link to v2: https://lore.kernel.org/r/20250611-rcu-fix-task_cls_state-v2-1-1a7fc248232a@xxxxxxxxxx

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 valid
  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@xxxxxxxxxx
---
 net/core/netclassid_cgroup.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/netclassid_cgroup.c b/net/core/netclassid_cgroup.c
index d22f0919821e931fbdedf5a8a7a2998d59d73978..dff66d8fb325d28bb15f42641b9ec738b0022353 100644
--- a/net/core/netclassid_cgroup.c
+++ b/net/core/netclassid_cgroup.c
@@ -21,7 +21,9 @@ 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_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@xxxxxxxxxx>