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

[Peter Zijlstra]

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.