[RFC PATCH] tracing: Fix syscall tracepoint use-after-free

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

The grace period used internally within tracepoint.c:release_probes()
uses call_rcu() to batch waiting for quiescence of old probe arrays,
rather than using the tracepoint_synchronize_unregister() which blocks
while waiting for quiescence.

This causes use-after-free issues reproduced with syzkaller.

Fix this by introducing the following call_rcu chaining:

   call_rcu() -> rcu_free_old_probes -> call_rcu_tasks_trace() -> rcu_tasks_trace_free_old_probes.

Just like it was done when SRCU was used.

Reported-by: syzbot+b390c8062d8387b6272a@syzkaller.appspotmail.com
Fixes: a363d27cdbc2 ("tracing: Allow system call tracepoints to handle page faults")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michael Jeanson <mjeanson@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Yonghong Song <yhs@fb.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf@vger.kernel.org
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Jordan Rife <jrife@google.com>
---
 kernel/tracepoint.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 6474e2cf22c9..33f6fa94d383 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -101,11 +101,16 @@ static inline void *allocate_probes(int count)
 	return p == NULL ? NULL : p->probes;
 }
 
-static void rcu_free_old_probes(struct rcu_head *head)
+static void rcu_tasks_trace_free_old_probes(struct rcu_head *head)
 {
 	kfree(container_of(head, struct tp_probes, rcu));
 }
 
+static void rcu_free_old_probes(struct rcu_head *head)
+{
+	call_rcu_tasks_trace(head, rcu_tasks_trace_free_old_probes);
+}
+
 static inline void release_probes(struct tracepoint_func *old)
 {
 	if (old) {
-- 
2.39.5