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

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Message-ID: <E4AE85BE-FB3B-45BE-8E1E-DD27E2520A4C@zytor.com>
Date:   Fri, 07 Jun 2019 13:22:31 -0700
From:   hpa@...or.com
To:     Andy Lutomirski <luto@...capital.net>,
        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>,
        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 June 7, 2019 11:10:19 AM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>
>
>> 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?

I don't see how it could be, except *perhaps* inside an NMI have, because you could have a preempted or interrupted now having an in-memory IP pointing inside the middle of the region you are intending to patch.


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