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

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Date:   Thu, 10 Jan 2019 17:34:21 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     hpa@...or.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 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.