Re: [PATCH 1/4] ftrace: Fix function pid filter on instances

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 29 Mar 2017 10:46:22 +0900
Namhyung Kim <namhyung@kernel.org> wrote:

> When function tracer has a pid filter, it adds a probe to sched_switch
> to track if current task can be ignored.  The probe checks the
> ftrace_ignore_pid from current tr to filter tasks.  But it misses to
> delete the probe when removing an instance so that it can cause a crash
> due to the invalid tr pointer (use-after-free).
> 
> This is easily reproducible with the following:
> 
>   # cd /sys/kernel/debug/tracing
>   # mkdir instances/buggy
>   # echo $$ > instances/buggy/set_ftrace_pid
>   # rmdir instances/buggy
> 
>   ============================================================================
>   BUG: KASAN: use-after-free in ftrace_filter_pid_sched_switch_probe+0x3d/0x90
>   Read of size 8 by task kworker/0:1/17
>   CPU: 0 PID: 17 Comm: kworker/0:1 Tainted: G    B           4.11.0-rc3  #198
>   Call Trace:
>    dump_stack+0x68/0x9f
>    kasan_object_err+0x21/0x70
>    kasan_report.part.1+0x22b/0x500
>    ? ftrace_filter_pid_sched_switch_probe+0x3d/0x90
>    kasan_report+0x25/0x30
>    __asan_load8+0x5e/0x70
>    ftrace_filter_pid_sched_switch_probe+0x3d/0x90
>    ? fpid_start+0x130/0x130
>    __schedule+0x571/0xce0
>    ...
> 
> To fix it, use ftrace_pid_reset() to unregister the probe.  As
> instance_rmdir() already updated ftrace codes, it can just free the
> filter safely.
> 
> Fixes: 0c8916c34203 ("tracing: Add rmdir to remove multibuffer instances")
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  kernel/trace/ftrace.c | 10 ++++++----
>  kernel/trace/trace.c  |  1 +
>  kernel/trace/trace.h  |  2 ++
>  3 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 0556a202c055..b451a860e885 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5598,13 +5598,15 @@ static void clear_ftrace_pids(struct trace_array *tr)
>  	trace_free_pid_list(pid_list);
>  }
>  
> -static void ftrace_pid_reset(struct trace_array *tr)
> +void ftrace_pid_reset(struct trace_array *tr, bool update)
>  {
>  	mutex_lock(&ftrace_lock);
>  	clear_ftrace_pids(tr);
>  
> -	ftrace_update_pid_func();
> -	ftrace_startup_all(0);
> +	if (update) {
> +		ftrace_update_pid_func();
> +		ftrace_startup_all(0);
> +	}
>  
>  	mutex_unlock(&ftrace_lock);
>  }

I think it is better to create a new function here. I mean, you just
added a bool, that removes 2 thirds of the code when false.


> @@ -5676,7 +5678,7 @@ ftrace_pid_open(struct inode *inode, struct file *file)
>  
>  	if ((file->f_mode & FMODE_WRITE) &&
>  	    (file->f_flags & O_TRUNC))
> -		ftrace_pid_reset(tr);
> +		ftrace_pid_reset(tr, true);
>  
>  	ret = seq_open(file, &ftrace_pid_sops);
>  	if (ret < 0) {
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index b5d4b80f2d45..b92489dfa829 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -7485,6 +7485,7 @@ static int instance_rmdir(const char *name)
>  
>  	tracing_set_nop(tr);
>  	event_trace_del_tracer(tr);
> +	ftrace_pid_reset(tr, false);

Actually, if this is called after event_trace_del_tracer(), the tr is
already invisible and nothing new should change.

Make a wrapper around clear_ftrace_pids() and call that instead. We
don't even need to take a lock, but as I see there's a lockdep test for
ftrace_lock, we should still do so just to be safe.

void ftrace_clear_pids(struct trace_array *tr)
{
	mutex_lock(&ftrace_lock);

	clear_ftrace_pids(tr);

	mutex_unlock(&ftrace_lock);
}

-- Steve


>  	ftrace_destroy_function_files(tr);
>  	tracefs_remove_recursive(tr->dir);
>  	free_trace_buffers(tr);
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 571acee52a32..4d9804fd9a2d 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -897,6 +897,7 @@ void ftrace_init_tracefs(struct trace_array *tr, struct dentry *d_tracer);
>  void ftrace_init_tracefs_toplevel(struct trace_array *tr,
>  				  struct dentry *d_tracer);
>  int init_function_trace(void);
> +void ftrace_pid_reset(struct trace_array *tr, bool update);
>  #else
>  static inline int ftrace_trace_task(struct trace_array *tr)
>  {
> @@ -916,6 +917,7 @@ static inline void ftrace_reset_array_ops(struct trace_array *tr) { }
>  static inline void ftrace_init_tracefs(struct trace_array *tr, struct dentry *d) { }
>  static inline void ftrace_init_tracefs_toplevel(struct trace_array *tr, struct dentry *d) { }
>  static inline int init_function_trace(void) { return 0; }
> +static inline void ftrace_pid_reset(struct trace_array *tr, bool update) { }
>  /* ftace_func_t type is not defined, use macro instead of static inline */
>  #define ftrace_init_array_ops(tr, func) do { } while (0)
>  #endif /* CONFIG_FUNCTION_TRACER */