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

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Date:   Fri, 11 Jan 2019 00:13:00 -0800
From:   hpa@...or.com
To:     Sean Christopherson <sean.j.christopherson@...el.com>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        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>,
        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 January 10, 2019 5:34:21 PM PST, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
>On Thu, Jan 10, 2019 at 04:59:55PM -0800, hpa@...or.com wrote:
>> On January 10, 2019 9:42:57 AM PST, Sean Christopherson
><sean.j.christopherson@...el.com> wrote:
>> >On Thu, Jan 10, 2019 at 12:32:43PM -0500, Steven Rostedt wrote:
>> >> On Thu, 10 Jan 2019 11:20:04 -0600
>> >> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>> >> 
>> >> 
>> >> > > While I can't find a reason for hypervisors to emulate this
>> >instruction,
>> >> > > smarter people might find ways to turn it into a security
>> >exploit.  
>> >> > 
>> >> > Interesting point... but I wonder if it's a realistic concern. 
>> >BTW,
>> >> > text_poke_bp() also relies on undocumented behavior.
>> >> 
>> >> But we did get an official OK from Intel that it will work. Took a
>> >bit
>> >> of arm twisting to get them to do so, but they did. And it really
>is
>> >> pretty robust.
>> >
>> >Did we (they?) list any caveats for this behavior?  E.g. I'm fairly
>> >certain atomicity guarantees go out the window if WC memtype is
>used.
>> 
>> If you run code from non-WB memory, all bets are off and you better
>> not be doing cross-modifying code.
>
>I wasn't thinking of running code from non-WB, but rather running code
>in WB while doing a CMC write via WC.

Same thing. Don't do that.
-- 
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