Patch "tracing: Fix a possible race when disabling buffered events" has been added to the 6.6-stable tree

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This is a note to let you know that I've just added the patch titled

    tracing: Fix a possible race when disabling buffered events

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     tracing-fix-a-possible-race-when-disabling-buffered-events.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From c0591b1cccf708a47bc465c62436d669a4213323 Mon Sep 17 00:00:00 2001
From: Petr Pavlu <petr.pavlu@xxxxxxxx>
Date: Tue, 5 Dec 2023 17:17:36 +0100
Subject: tracing: Fix a possible race when disabling buffered events

From: Petr Pavlu <petr.pavlu@xxxxxxxx>

commit c0591b1cccf708a47bc465c62436d669a4213323 upstream.

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@xxxxxxxx/
Link: https://lkml.kernel.org/r/20231205161736.19663-4-petr.pavlu@xxxxxxxx

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 0fc1b09ff1ff ("tracing: Use temp buffer when filtering events")
Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/trace/trace.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2790,13 +2790,17 @@ 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);
 }


Patches currently in stable-queue which might be from petr.pavlu@xxxxxxxx are

queue-6.6/tracing-fix-a-possible-race-when-disabling-buffered-events.patch
queue-6.6/tracing-fix-incomplete-locking-when-disabling-buffered-events.patch
queue-6.6/tracing-fix-a-warning-when-allocating-buffered-event.patch




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