From: "Paul E. McKenney" paulmck@kernel.org

The rcu_tasks_trace_postgp() function uses for_each_process_thread() to scan the task list without the benefit of RCU read-side protection, which can result in use-after-free errors on task_struct structures. This error was missed because the TRACE01 rcutorture scenario enables lockdep, but also builds with CONFIG_PREEMPT_NONE=y. In this situation, preemption is disabled everywhere, so lockdep thinks everywhere can be a legitimate RCU reader. This commit therefore adds the needed rcu_read_lock() and rcu_read_unlock().

Note that this bug can occur only after an RCU Tasks Trace CPU stall warning, which by default only happens after a grace period has extended for ten minutes (yes, not a typo, minutes).

Fixes: 4593e772b502 ("rcu-tasks: Add stall warnings for RCU Tasks Trace") Cc: Alexei Starovoitov alexei.starovoitov@gmail.com Cc: Daniel Borkmann daniel@iogearbox.net Cc: Jiri Olsa jolsa@redhat.com Cc: bpf@vger.kernel.org Cc: stable@vger.kernel.org # 5.7.x Signed-off-by: Paul E. McKenney paulmck@kernel.org --- kernel/rcu/tasks.h | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index fcd9c25..d5d9f2d 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -1088,9 +1088,11 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp) if (ret) break; // Count reached zero. // Stall warning time, so make a list of the offenders. + rcu_read_lock(); for_each_process_thread(g, t) if (READ_ONCE(t->trc_reader_special.b.need_qs)) trc_add_holdout(t, &holdouts); + rcu_read_unlock(); firstreport = true; list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list) { if (READ_ONCE(t->trc_reader_special.b.need_qs))