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author | Petr Pavlu <petr.pavlu@suse.com> | 2023-12-05 19:17:36 +0300 |
---|---|---|
committer | Steven Rostedt (Google) <rostedt@goodmis.org> | 2023-12-06 01:17:00 +0300 |
commit | c0591b1cccf708a47bc465c62436d669a4213323 (patch) | |
tree | 7d007684ad9f7aefbbb7831d8fc873338d9fc940 | |
parent | 34209fe83ef8404353f91ab4ea4035dbc9922d04 (diff) | |
download | linux-c0591b1cccf708a47bc465c62436d669a4213323.tar.xz |
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>
-rw-r--r-- | kernel/trace/trace.c | 12 |
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); } |