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SubjectRe: [PATCH 1/3] [BUGFIX] tracing: Fix double free of event_trigger_data
On Mon, 23 Jul 2018 22:10:06 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> 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?

Hmm, your patch seems to leak a memory since event_trigger_init() will
be called twice on same trigger_data (Note that event_trigger_init()
does not init ref counter, but increment it.) So we should decrement
it when we find it is succeeded. Moreover, if register_trigger()
fails before calling data->ops->init() (see -EEXIST case), the ref
counter will be 0 (-1 +1). But if it fails after data->ops->init(),
the ref counter will be 1 (-1 +1 +1). It still be unstable.
(Ah, that means we may have another trouble...)

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

Hmm it seems we should review the register_trigger() implementation.
It should return the return value of trace_event_trigger_enable_disable(),
shouldn't it?

Thank you,

>
> -- 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;
> }
>
> /**


--
Masami Hiramatsu <mhiramat@kernel.org>

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