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

From: Steven Rostedt
Date: Mon Jul 23 2018 - 22:10:22 EST

On Sat, 14 Jul 2018 01:27:47 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> 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@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> 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;
}

/**