[RFC PATCH v3 3/3] 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.

With the introduction of faultable syscall tracepoints, this causes
use-after-free issues reproduced with syzkaller.

Fix this by using the appropriate call_rcu() or call_rcu_tasks_trace()
before invoking the rcu_free_old_probes callback. This can be chosen
using the tracepoint_is_syscall() API.

A similar issue exists in bpf use of call_rcu(). Fixing this is left to
a separate change.

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>
---
Changes since v0:
- Introduce tracepoint_call_rcu(),
- Fix bpf_link_free() use of call_rcu as well.

Changes since v1:
- Use tracepoint_call_rcu() for bpf_prog_put as well.

Changes since v2:
- Do not cover bpf changes in the same commit, let bpf developers
  implement it.
---
 kernel/tracepoint.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 5658dc92f5b5..47569fb06596 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -106,13 +106,16 @@ static void rcu_free_old_probes(struct rcu_head *head)
 	kfree(container_of(head, struct tp_probes, rcu));
 }
 
-static inline void release_probes(struct tracepoint_func *old)
+static inline void release_probes(struct tracepoint *tp, struct tracepoint_func *old)
 {
 	if (old) {
 		struct tp_probes *tp_probes = container_of(old,
 			struct tp_probes, probes[0]);
 
-		call_rcu(&tp_probes->rcu, rcu_free_old_probes);
+		if (tracepoint_is_syscall(tp))
+			call_rcu_tasks_trace(&tp_probes->rcu, rcu_free_old_probes);
+		else
+			call_rcu(&tp_probes->rcu, rcu_free_old_probes);
 	}
 }
 
@@ -334,7 +337,7 @@ static int tracepoint_add_func(struct tracepoint *tp,
 		break;
 	}
 
-	release_probes(old);
+	release_probes(tp, old);
 	return 0;
 }
 
@@ -405,7 +408,7 @@ static int tracepoint_remove_func(struct tracepoint *tp,
 		WARN_ON_ONCE(1);
 		break;
 	}
-	release_probes(old);
+	release_probes(tp, old);
 	return 0;
 }
 
-- 
2.39.5