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

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Date:   Fri, 11 Jan 2019 17:46:36 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.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 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.

alex.