linux-kernel - Re: [PATCH 08/15] x86/alternatives: Teach text_poke

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Date:   Fri, 7 Jun 2019 11:10:19 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions



> On Jun 7, 2019, at 10:34 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Sat, Jun 08, 2019 at 12:47:08AM +0900, Masami Hiramatsu wrote:
> 
>>> This fits almost all text_poke_bp() users, except
>>> arch_unoptimize_kprobe() which restores random text, and for that site
>>> we have to build an explicit emulate instruction.
>> 
>> Hm, actually it doesn't restores randome text, since the first byte
>> must always be int3. As the function name means, it just unoptimizes
>> (jump based optprobe -> int3 based kprobe).
>> Anyway, that is not an issue. With this patch, optprobe must still work.
> 
> I thought it basically restored 5 bytes of original text (with no
> guarantee it is a single instruction, or even a complete instruction),
> with the first byte replaced with INT3.
> 

I am surely missing some kprobe context, but is it really safe to use this mechanism to replace more than one instruction?