[PATCH 1/1] Fix: trace sched switch start/stop racy updates

From: Mathieu Desnoyers
Date: Fri Aug 16 2019 - 10:26:58 EST


Reading the sched_cmdline_ref and sched_tgid_ref initial state within
tracing_start_sched_switch without holding the sched_register_mutex is
racy against concurrent updates, which can lead to tracepoint probes
being registered more than once (and thus trigger warnings within
tracepoint.c).

Also, write and read to/from those variables should be done with
WRITE_ONCE() and READ_ONCE(), given that those are read within tracing
probes without holding the sched_register_mutex.

[ Compile-tested only. I suspect it might fix the following syzbot
report:

syzbot+774fddf07b7ab29a1e55@xxxxxxxxxxxxxxxxxxxxxxxxx ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
CC: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
kernel/trace/trace_sched_switch.c | 32 ++++++++++++++++++++++----------
1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
index e288168661e1..902e8bf59aeb 100644
--- a/kernel/trace/trace_sched_switch.c
+++ b/kernel/trace/trace_sched_switch.c
@@ -26,8 +26,8 @@ probe_sched_switch(void *ignore, bool preempt,
{
int flags;

- flags = (RECORD_TGID * !!sched_tgid_ref) +
- (RECORD_CMDLINE * !!sched_cmdline_ref);
+ flags = (RECORD_TGID * !!READ_ONCE(sched_tgid_ref)) +
+ (RECORD_CMDLINE * !!READ_ONCE(sched_cmdline_ref));

if (!flags)
return;
@@ -39,8 +39,8 @@ probe_sched_wakeup(void *ignore, struct task_struct *wakee)
{
int flags;

- flags = (RECORD_TGID * !!sched_tgid_ref) +
- (RECORD_CMDLINE * !!sched_cmdline_ref);
+ flags = (RECORD_TGID * !!READ_ONCE(sched_tgid_ref)) +
+ (RECORD_CMDLINE * !!READ_ONCE(sched_cmdline_ref));

if (!flags)
return;
@@ -89,21 +89,28 @@ static void tracing_sched_unregister(void)

static void tracing_start_sched_switch(int ops)
{
- bool sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
+ bool sched_register;
+
mutex_lock(&sched_register_mutex);
+ sched_register = (!sched_cmdline_ref && !sched_tgid_ref);

switch (ops) {
case RECORD_CMDLINE:
- sched_cmdline_ref++;
+ WRITE_ONCE(sched_cmdline_ref, sched_cmdline_ref + 1);
break;

case RECORD_TGID:
- sched_tgid_ref++;
+ WRITE_ONCE(sched_tgid_ref, sched_tgid_ref + 1);
break;
+
+ default:
+ WARN_ONCE(1, "Unsupported tracing op: %d", ops);
+ goto end;
}

- if (sched_register && (sched_cmdline_ref || sched_tgid_ref))
+ if (sched_register)
tracing_sched_register();
+end:
mutex_unlock(&sched_register_mutex);
}

@@ -113,16 +120,21 @@ static void tracing_stop_sched_switch(int ops)

switch (ops) {
case RECORD_CMDLINE:
- sched_cmdline_ref--;
+ WRITE_ONCE(sched_cmdline_ref, sched_cmdline_ref - 1);
break;

case RECORD_TGID:
- sched_tgid_ref--;
+ WRITE_ONCE(sched_tgid_ref, sched_tgid_ref - 1);
break;
+
+ default:
+ WARN_ONCE(1, "Unsupported tracing op: %d", ops);
+ goto end;
}

if (!sched_cmdline_ref && !sched_tgid_ref)
tracing_sched_unregister();
+end:
mutex_unlock(&sched_register_mutex);
}

--
2.11.0