linux-kernel - Re: [PATCH v3 5/6] x86/alternative: Use a single access in text

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Date:   Fri, 11 Jan 2019 09:28:09 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Nadav Amit <namit@...are.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.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>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v3 5/6] x86/alternative: Use a single access in
 text_poke() where possible

On Fri, Jan 11, 2019 at 01:10:52PM +0100, Alexandre Chartre wrote:
> To avoid any issue with live patching the call instruction, what about
> toggling between two call instructions: one would be the currently active
> call, while the other would currently be inactive but to be used after a
> change is made. You can safely patch the inactive call and then toggle
> the call flow (using a jump label) between the active and inactive calls.
> 
> So instead of having a single call instruction:
> 
> 	call function
> 
> You would have:
> 
> 	STATIC_JUMP_IF_TRUE label, key
> 	call function1
> 	jmp done
> label:
> 	call function2
> done:
> 
> If the key is set so that function1 is currently called then you can
> safely update the call instruction for function2. Once this is done,
> just flip the key to make the function2 call active. On a next update,
> you would, of course, have to switch and update the call for function1.

What about the following race?

CPU1						CPU2
static key is false, doesn't jump
task gets preempted before calling function1
						change static key to true
						start patching "call function1"
task resumes, sees inconsistent call instruction

-- 
Josh