linux-kernel - RE: optimized kprobes illegal instructions in v4.19 stable kernels

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Date:   Wed, 6 Feb 2019 11:48:44 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mathieu Desnoyers' <mathieu.desnoyers@...icios.com>,
        Kees Cook <keescook@...omium.org>
CC:     William Cohen <wcohen@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "lttng@...iableembeddedsystems.com" 
        <lttng@...iableembeddedsystems.com>,
        lttng-dev <lttng-dev@...ts.lttng.org>
Subject: RE: optimized kprobes illegal instructions in v4.19 stable kernels

From: Mathieu Desnoyers
> Sent: 04 February 2019 19:15
> I notice this commit as a possible culprit of the illegal instructions my lttng
> users are noticing on arm32 when using kprobes on a v4.19.13 Linux kernel
> in a Yocto environment [1]. They were able to reproduce the issue with perf
> as well.
> 
> commit e46daee53bb50bde38805f1823a182979724c229
> Author: Kees Cook <keescook@...omium.org>
> Date:   Tue Oct 30 22:12:56 2018 +0100
> 
>     ARM: 8806/1: kprobes: Fix false positive with FORTIFY_SOURCE
> 
> I *think* the intent there was to do
> 
> -       memcpy(code, &optprobe_template_entry,
> +       memcpy(code, (unsigned long *)&optprobe_template_entry,
> 
> But if you look at the commit, the "&" seems to have been stripped away,
> which happens to change the behavior significantly.

Which is why you shouldn't add casts to shut up compiler warnings ....

	David

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