Re: BPF tracing trampoline synchronization between update/freeing and execution?

[Petr Mladek]

On Mon 2020-01-06 17:56:54, Peter Zijlstra wrote:
> On Mon, Jan 06, 2020 at 05:39:30PM +0100, Jann Horn wrote:
> > Hi!
> > 
> > I was chatting with kpsingh about BPF trampolines, and I noticed that
> > it looks like BPF trampolines (as of current bpf-next/master) seem to
> > be missing synchronization between trampoline code updates and
> > trampoline execution. Or maybe I'm missing something?
> > 
> > If I understand correctly, trampolines are executed directly from the
> > fentry placeholders at the start of arbitrary kernel functions, so
> > they can run without any locks held. So for example, if task A starts
> > executing a trampoline on entry to sys_open(), then gets preempted in
> > the middle of the trampoline, and then task B quickly calls
> > BPF_RAW_TRACEPOINT_OPEN twice, and then task A continues execution,
> > task A will end up executing the middle of newly-written machine code,
> > which can probably end up crashing the kernel somehow?
> > 
> > I think that at least to synchronize trampoline text freeing with
> > concurrent trampoline execution, it is necessary to do something
> > similar to what the livepatching code does with klp_check_stack(), and
> > then either use a callback from the scheduler to periodically re-check
> > tasks that were in the trampoline or let the trampoline tail-call into
> > a cleanup helper that is part of normal kernel text. And you'd
> > probably have to gate BPF trampolines on
> > CONFIG_HAVE_RELIABLE_STACKTRACE.
> 
> ftrace uses synchronize_rcu_tasks() to flip between trampolines iirc.

ftrace calls also schedule_on_each_cpu(ftrace_sync) to handle
situations where RCU is not watching, see rcu_is_watching().

The following is called in ftrace_shutdown():

	schedule_on_each_cpu(ftrace_sync);

	if (IS_ENABLED(CONFIG_PREEMPTION))
		synchronize_rcu_tasks();

	arch_ftrace_trampoline_free(ops);

Best Regards,
Petr