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Subject: Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190607173427.GK3436@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Jun 2019 11:10:19 -0700
Cc:     Masami Hiramatsu <mhiramat@kernel.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Nadav Amit <namit@vmware.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
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References: <20190605130753.327195108@infradead.org> <20190605131945.005681046@infradead.org> <20190608004708.7646b287151cf613838ce05f@kernel.org> <20190607173427.GK3436@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jun 7, 2019, at 10:34 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> On Sat, Jun 08, 2019 at 12:47:08AM +0900, Masami Hiramatsu wrote:
>=20
>>> 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.
>>=20
>> 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.
>=20
> 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.
>=20

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