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From: Jann Horn <>
	Alexei Starovoitov <>,
	Daniel Borkmann <>,
	Steven Rostedt <>
Cc: KP Singh <>,
	Andy Lutomirski <>,
	kernel list <>,
	"the arch/x86 maintainers" <>,
	Josh Poimboeuf <>,
	Ingo Molnar <>
Subject: BPF tracing trampoline synchronization between update/freeing and execution?
Date: Mon, 6 Jan 2020 17:39:30 +0100
Message-ID: <> (raw)


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

[Trampoline *updates* could probably be handled more easily if a
trampoline consisted of an immutable header that increments something
like a percpu refcount followed by a mutable body, but given that that
doesn't solve freeing trampolines, I'm not sure if it'd be worth the
effort. Unless you just never free trampoline memory, but that's
probably not a great idea.]

             reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-06 16:39 Jann Horn [this message]
2020-01-06 16:56 ` Peter Zijlstra
2020-01-06 22:29   ` Alexei Starovoitov
2020-01-07  8:28   ` Petr Mladek
2020-01-07  9:03     ` Petr Mladek

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