linux-kernel - Re: [PATCH 04/29] x86/livepatch: Validate __fentry_

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Date:   Wed, 23 Feb 2022 16:54:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, Miroslav Benes <mbenes@...e.cz>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH 04/29] x86/livepatch: Validate __fentry__ location

On Wed, Feb 23, 2022 at 03:49:41PM +0100, Peter Zijlstra wrote:

> > Since we would want rec->ip if the pointer is before the ftrace
> > instruction. But we would need to audit all use cases and make sure this is
> > not called from any hot paths (in a callback).
> > 
> > This will affect kprobes and BPF as they both use ftrace_location() as well.
> 
> Yes, I already fixed kprobes, still trying to (re)discover how to run
> the bpf-selftests, that stuff is too painful :-(

Ok, I think I managed... I'm obviously hitting the WARN_ON_ONCE() in
is_ftrace_location(). Funnily, no dead kernel, so that's something I
suppose.

Now, I'm trying to make sense of that code, but all that !ftrace_managed
code scares me to death.

At the very least __bpf_arch_text_poke() needs a bunch of help. Let me
go prod it with something sharp to see what falls out ...