From: Petr Pavlu <petr.pavlu@suse.com>
Date: Tue, 5 Dec 2023 17:17:36 +0100
Subject: tracing: Fix a possible race when disabling buffered events
Git-commit: c0591b1cccf708a47bc465c62436d669a4213323
Patch-mainline: v6.7-rc5
References: bsc#1217036
Function trace_buffered_event_disable() is responsible for freeing pages
backing buffered events and this process can run concurrently with
trace_event_buffer_lock_reserve().
The following race is currently possible:
* Function trace_buffered_event_disable() is called on CPU 0. It
increments trace_buffered_event_cnt on each CPU and waits via
synchronize_rcu() for each user of trace_buffered_event to complete.
* After synchronize_rcu() is finished, function
trace_buffered_event_disable() has the exclusive access to
trace_buffered_event. All counters trace_buffered_event_cnt are at 1
and all pointers trace_buffered_event are still valid.
* At this point, on a different CPU 1, the execution reaches
trace_event_buffer_lock_reserve(). The function calls
preempt_disable_notrace() and only now enters an RCU read-side
critical section. The function proceeds and reads a still valid
pointer from trace_buffered_event[CPU1] into the local variable
"entry". However, it doesn't yet read trace_buffered_event_cnt[CPU1]
which happens later.
* Function trace_buffered_event_disable() continues. It frees
trace_buffered_event[CPU1] and decrements
trace_buffered_event_cnt[CPU1] back to 0.
* Function trace_event_buffer_lock_reserve() continues. It reads and
increments trace_buffered_event_cnt[CPU1] from 0 to 1. This makes it
believe that it can use the "entry" that it already obtained but the
pointer is now invalid and any access results in a use-after-free.
Fix the problem by making a second synchronize_rcu() call after all
trace_buffered_event values are set to NULL. This waits on all potential
users in trace_event_buffer_lock_reserve() that still read a previous
pointer from trace_buffered_event.
Link: https://lore.kernel.org/all/20231127151248.7232-2-petr.pavlu@suse.com/
Link: https://lkml.kernel.org/r/20231205161736.19663-4-petr.pavlu@suse.com
Cc: stable@vger.kernel.org
Fixes: 0fc1b09ff1ff ("tracing: Use temp buffer when filtering events")
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
[ppavlu: s/synchronize_rcu()/synchronize_sched()/ because the branch
doesn't have the RCU consolidation changes.]
---
kernel/trace/trace.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ef72354f61ce..fbcd3bafb93e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2791,13 +2791,17 @@ void trace_buffered_event_disable(void)
free_page((unsigned long)per_cpu(trace_buffered_event, cpu));
per_cpu(trace_buffered_event, cpu) = NULL;
}
+
/*
- * Make sure trace_buffered_event is NULL before clearing
- * trace_buffered_event_cnt.
+ * Wait for all CPUs that potentially started checking if they can use
+ * their event buffer only after the previous synchronize_rcu() call and
+ * they still read a valid pointer from trace_buffered_event. It must be
+ * ensured they don't see cleared trace_buffered_event_cnt else they
+ * could wrongly decide to use the pointed-to buffer which is now freed.
*/
- smp_wmb();
+ synchronize_sched();
- /* Do the work on each cpu */
+ /* For each CPU, relinquish the buffer */
on_each_cpu_mask(tracing_buffer_mask, enable_trace_buffered_event, NULL,
true);
}