Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 11 Jan 2019 00:13:00 -0800
User-Agent: K-9 Mail for Android
In-Reply-To: <20190111013421.GD2365@linux.intel.com>
References: <cover.1547073843.git.jpoimboe@redhat.com> <279b8003f7f0a6831d090ab822d37bc958f974de.1547073843.git.jpoimboe@redhat.com> <8138A1EE-359D-4CD2-8E96-5BF00313AB3B@vmware.com> <20190110172004.wuh45xoafynfm2df@treble> <20190110123243.3b9e0856@gandalf.local.home> <20190110174257.GE16556@linux.intel.com> <1955E727-BF68-4C00-A8C2-54FA9487A5A6@zytor.com> <20190111013421.GD2365@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain;
 charset=utf-8
Content-Transfer-Encoding: 8BIT
Subject: Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible
To:     Sean Christopherson <sean.j.christopherson@intel.com>
CC:     Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nadav Amit <namit@vmware.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@ACULAB.COM>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
From:   hpa@zytor.com
Message-ID: <5DD129EB-3EEF-479E-98BB-DFCAD999DC2D@zytor.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On January 10, 2019 5:34:21 PM PST, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>On Thu, Jan 10, 2019 at 04:59:55PM -0800, hpa@zytor.com wrote:
>> On January 10, 2019 9:42:57 AM PST, Sean Christopherson
><sean.j.christopherson@intel.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@redhat.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.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.