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.
