[PATCH] tracing: Fix use-after-free in print_graph_function_flags during tracer switching

[Tengda Wu <wutengda@huaweicloud.com>]

Kairui reported a UAF issue in print_graph_function_flags() during
ftrace stress testing [1]. This issue can be reproduced if puting a
'mdelay(10)' after 'mutex_unlock(&trace_types_lock)' in s_start(),
and executing the following script:

  $ echo function_graph > current_tracer
  $ cat trace > /dev/null &
  $ sleep 5  # Ensure the 'cat' reaches the 'mdelay(10)' point
  $ echo timerlat > current_tracer

The root cause lies in the two calls to print_graph_function_flags
within print_trace_line during each s_show():

  * One through 'iter->trace->print_line()';
  * Another through 'event->funcs->trace()', which is hidden in
    print_trace_fmt() before print_trace_line returns.

Tracer switching only updates the former, while the latter continues
to use the print_line function of the old tracer, which in the script
above is print_graph_function_flags.

Moreover, when switching from the 'function_graph' tracer to the
'timerlat' tracer, s_start only calls graph_trace_close of the
'function_graph' tracer to free 'iter->private', but does not set
it to NULL. This provides an opportunity for 'event->funcs->trace()'
to use an invalid 'iter->private'.

To fix this issue, set 'iter->private' to NULL immediately after
freeing it in graph_trace_close(), ensuring that an invalid pointer
is not passed to other tracers. Additionally, clean up the unnecessary
'iter->private = NULL' during each 'cat trace' when using wakeup and
irqsoff tracers.

 [1] https://lore.kernel.org/all/20231112150030.84609-1-ryncsn@gmail.com/

Fixes: eecb91b9f98d ("tracing: Fix memleak due to race between current_tracer and trace")
Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
Reported-by: Kairui Song <kasong@tencent.com>
Closes: https://lore.kernel.org/all/CAMgjq7BW79KDSCyp+tZHjShSzHsScSiJxn5ffskp-QzVM06fxw@mail.gmail.com/
Cc: stable@vger.kernel.org # v6.5+
---
 kernel/trace/trace_functions_graph.c | 1 +
 kernel/trace/trace_irqsoff.c         | 2 --
 kernel/trace/trace_sched_wakeup.c    | 2 --
 3 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 71b2fb068b6b..f995bddf2ab0 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -1614,6 +1614,7 @@ void graph_trace_close(struct trace_iterator *iter)
 	if (data) {
 		free_percpu(data->cpu_data);
 		kfree(data);
+		iter->private = NULL;
 	}
 }
 
diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index c8bfa7310a91..40c39e946940 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -250,8 +250,6 @@ static void irqsoff_trace_open(struct trace_iterator *iter)
 {
 	if (is_graph(iter->tr))
 		graph_trace_open(iter);
-	else
-		iter->private = NULL;
 }
 
 static void irqsoff_trace_close(struct trace_iterator *iter)
diff --git a/kernel/trace/trace_sched_wakeup.c b/kernel/trace/trace_sched_wakeup.c
index c9ba4259e03e..a0db3404f7f7 100644
--- a/kernel/trace/trace_sched_wakeup.c
+++ b/kernel/trace/trace_sched_wakeup.c
@@ -188,8 +188,6 @@ static void wakeup_trace_open(struct trace_iterator *iter)
 {
 	if (is_graph(iter->tr))
 		graph_trace_open(iter);
-	else
-		iter->private = NULL;
 }
 
 static void wakeup_trace_close(struct trace_iterator *iter)
-- 
2.34.1