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authorPetr Pavlu <petr.pavlu@suse.com>2023-12-05 17:17:36 +0100
committerSteven Rostedt (Google) <rostedt@goodmis.org>2023-12-05 17:17:00 -0500
commitc0591b1cccf708a47bc465c62436d669a4213323 (patch)
tree7d007684ad9f7aefbbb7831d8fc873338d9fc940 /kernel
parent34209fe83ef8404353f91ab4ea4035dbc9922d04 (diff)
tracing: Fix a possible race when disabling buffered events
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>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/trace/trace.c12
1 files 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_rcu();
- /* 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);
}