linux-kernel - Re: [PATCH v2 06/20] x86/alternative: use temporary mm for text poking

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Date:   Tue, 5 Feb 2019 14:29:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, hpa@...or.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Nadav Amit <nadav.amit@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux_dti@...oud.com, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, akpm@...ux-foundation.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        will.deacon@....com, ard.biesheuvel@...aro.org,
        kristen@...ux.intel.com, deneen.t.dock@...el.com,
        Nadav Amit <namit@...are.com>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v2 06/20] x86/alternative: use temporary mm for text
 poking

On Tue, Feb 05, 2019 at 12:31:46PM +0100, Peter Zijlstra wrote:
> In general, text_poke() cannot fail:
> 
>  - suppose changing a single jump label requires poking multiple sites
>    (not uncommon), we fail halfway through and then have to undo the
>    first pokes, but those pokes fail again.
> 
>  - this then leaves us no way forward and no way back, we've got
>    inconsistent text state -> FAIL.

Note that this exact fail scenario still exists in the CPU hotplug code.
See kernel/cpu.c:cpuhp_thread_fun():

		/*
		 * If we fail on a rollback, we're up a creek without no
		 * paddle, no way forward, no way back. We loose, thanks for
		 * playing.
		 */
		WARN_ON_ONCE(st->rollback);