Re: [PATCH 1/3] [BUGFIX] tracing: Fix double free of event_trigger_data

[Steven Rostedt <rostedt@goodmis.org>]

On Sat, 14 Jul 2018 01:27:47 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Fix a double free bug of event_trigger_data caused by
> calling unregister_trigger() from register_snapshot_trigger().
> This kicks a kernel BUG if double free checker is enabled
> as below;
> 
>  kernel BUG at /home/mhiramat/ksrc/linux/mm/slub.c:296!
>  invalid opcode: 0000 [#1] SMP PTI
>  CPU: 2 PID: 4312 Comm: ftracetest Not tainted 4.18.0-rc1+ #44
>  Hardware name: ASUS All Series/B85M-G, BIOS 2108 08/11/2014
>  RIP: 0010:set_freepointer.part.37+0x0/0x10
>  Code: 41 b8 01 00 00 00 29 c8 4d 8d 0c 0c b9 10 00 00 00 50 e8 e3 28 23 00 8b 53 08 5e 5f 89 d1 81 e1 00 04 00 00 e9 e9 fe ff ff 90 <0f> 0b 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 48 c7 c6 90 7f 0d
>  RSP: 0018:ffffa799caa3bd90 EFLAGS: 00010246
>  RAX: ffff9b825f8c8e80 RBX: ffff9b825f8c8e80 RCX: ffff9b825f8c8e80
>  RDX: 0000000000021562 RSI: ffff9b830e9e70e0 RDI: 0000000000000202
>  RBP: 0000000000000246 R08: 0000000000000001 R09: 0000000000000000
>  R10: 0000000000000000 R11: 0000000000000000 R12: ffff9b830e0072c0
>  R13: ffffeb8e0d7e3200 R14: ffffffff961db7af R15: 00000000fffffffe
>  FS:  00007f135ba9f700(0000) GS:ffff9b830e800000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000563736b5f3a2 CR3: 0000000295916005 CR4: 00000000001606e0
>  Call Trace:
>   kfree+0x35d/0x380
>   event_trigger_callback+0x13f/0x1c0
>   event_trigger_write+0xf2/0x1a0
>   ? lock_acquire+0x9f/0x200
>   __vfs_write+0x26/0x170
>   ? rcu_read_lock_sched_held+0x6b/0x80
>   ? rcu_sync_lockdep_assert+0x2e/0x60
>   ? __sb_start_write+0x13e/0x1a0
>   ? vfs_write+0x18a/0x1b0
>   vfs_write+0xc1/0x1b0
>   ksys_write+0x45/0xa0
>   do_syscall_64+0x60/0x200
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> unregister_trigger() will free given event_trigger_data
> at last. But that event_trigger_data will be freed again
> in event_trigger_callback() if register_snapshot_trigger()
> is failed, and causes a double free bug.
> 
> Registering the data should be the final operation in the
> register function on normal path, because the trigger
> must be ready for taking action right after it is
> registered.

Nice catch. I can reproduce this. But this patch is fixing the symptom
and not the disease.

> 
> Fixes: commit 93e31ffbf417 ("tracing: Add 'snapshot' event trigger command")
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
> Cc: stable@vger.kernel.org
> ---
>  kernel/trace/trace.c                |    5 +++++
>  kernel/trace/trace.h                |    2 ++
>  kernel/trace/trace_events_trigger.c |   10 ++++++----
>  3 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index f054bd6a1c66..2556d8c097d2 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -980,6 +980,11 @@ static void free_snapshot(struct trace_array *tr)
>  	tr->allocated_snapshot = false;
>  }
>  
> +void tracing_free_snapshot_instance(struct trace_array *tr)
> +{
> +	free_snapshot(tr);
> +}
> +
>  /**
>   * tracing_alloc_snapshot - allocate snapshot buffer.
>   *
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index f8f86231ad90..03468bb8a79a 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1823,12 +1823,14 @@ static inline void trace_event_eval_update(struct trace_eval_map **map, int len)
>  #ifdef CONFIG_TRACER_SNAPSHOT
>  void tracing_snapshot_instance(struct trace_array *tr);
>  int tracing_alloc_snapshot_instance(struct trace_array *tr);
> +void tracing_free_snapshot_instance(struct trace_array *tr);
>  #else
>  static inline void tracing_snapshot_instance(struct trace_array *tr) { }
>  static inline int tracing_alloc_snapshot_instance(struct trace_array *tr)
>  {
>  	return 0;
>  }
> +static inline void tracing_free_snapshot_instance(struct trace_array *tr) { }
>  #endif
>  
>  extern struct trace_iterator *tracepoint_print_iter;
> diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
> index d18249683682..40e2f4406b2c 100644
> --- a/kernel/trace/trace_events_trigger.c
> +++ b/kernel/trace/trace_events_trigger.c
> @@ -1079,11 +1079,13 @@ register_snapshot_trigger(char *glob, struct event_trigger_ops *ops,
>  			  struct event_trigger_data *data,
>  			  struct trace_event_file *file)
>  {
> -	int ret = register_trigger(glob, ops, data, file);
> +	int free_if_fail = !file->tr->allocated_snapshot;
> +	int ret = 0;
>  
> -	if (ret > 0 && tracing_alloc_snapshot_instance(file->tr) != 0) {
> -		unregister_trigger(glob, ops, data, file);
> -		ret = 0;
> +	if (!tracing_alloc_snapshot_instance(file->tr)) {
> +		ret = register_trigger(glob, ops, data, file);
> +		if (ret == 0 && free_if_fail)
> +			tracing_free_snapshot_instance(file->tr);
>  	}
>  
>  	return ret;

Really, when we register_trigger() we should be able to freely call
unresgister_trigger() with no side effects like the above happens.
Instead of doing the above, I believe this is a better approach:

Thoughts?

P.S. This brings up another minor bug. The failure should return ENOMEM
not ENOENT.

-- Steve

diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index d18249683682..d15a746bcdfb 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -679,6 +679,7 @@ event_trigger_callback(struct event_command *cmd_ops,
 		goto out_free;
 
  out_reg:
+	event_trigger_init(trigger_ops, trigger_data);
 	ret = cmd_ops->reg(glob, trigger_ops, trigger_data, file);
 	/*
 	 * The above returns on success the # of functions enabled,
@@ -687,9 +688,9 @@ event_trigger_callback(struct event_command *cmd_ops,
 	 */
 	if (!ret) {
 		ret = -ENOENT;
-		goto out_free;
+		goto out_free_trigger;
 	} else if (ret < 0)
-		goto out_free;
+		goto out_free_trigger;
 	ret = 0;
  out:
 	return ret;
@@ -699,6 +700,10 @@ event_trigger_callback(struct event_command *cmd_ops,
 		cmd_ops->set_filter(NULL, trigger_data, NULL);
 	kfree(trigger_data);
 	goto out;
+
+ out_free_trigger:
+	event_trigger_free(trigger_ops, trigger_data);
+	goto out;
 }
 
 /**