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

[Namhyung Kim <namhyung@kernel.org>]

On Tue, Mar 28, 2017 at 10:08:41PM -0400, Steven Rostedt wrote:
> 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.

Ah, missed this part.  Ok, I'll make it a separate function.

Thanks,
Namhyung