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

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Date:   Fri, 11 Jan 2019 10:57:52 -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 05:46:36PM +0100, Alexandre Chartre wrote:
> 
> 
> On 01/11/2019 04:28 PM, Josh Poimboeuf wrote:
> > 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
> > 
> 
> If the function1 call is active then it won't be changed, you will change
> function2. However, I presume you can still have a race but if the function
> is changed twice before calling function1:
> 
> CPU1						CPU2
> static key is false, doesn't jump
> task gets preempted before calling function1
>                                                 -- first function change --
>                                                 patch "call function2"
>                                                 change static key to true
>                                                 -- second function change --
>                                                 start patching "call function1"
> task resumes, sees inconsistent call instruction
> 
> So right, that's a problem.

Right, that's what I meant to say :-)

-- 
Josh