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

In the Linux kernel, the following vulnerability has been resolved:

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

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/[email protected]/