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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>,
	bpf@vger.kernel.org, live-patching@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	KP Singh <kpsingh@chromium.org>,
	Andy Lutomirski <luto@amacapital.net>,
	kernel list <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Ingo Molnar <mingo@redhat.com>
Subject: Re: BPF tracing trampoline synchronization between update/freeing and execution?
Date: Mon, 6 Jan 2020 14:29:08 -0800
Message-ID: <20200106222907.yjoranutzjdersty@ast-mbp> (raw)
In-Reply-To: <20200106165654.GP2844@hirez.programming.kicks-ass.net>

On Mon, Jan 06, 2020 at 05:56:54PM +0100, 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.

good catch and good suggestion. synchronize_rcu_tasks() is needed here too.

  reply index

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

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