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

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Message-ID: <20190110180428.GG16556@linux.intel.com>
Date:   Thu, 10 Jan 2019 10:04:28 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        "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 Thu, Jan 10, 2019 at 12:57:57PM -0500, Steven Rostedt wrote:
> On Thu, 10 Jan 2019 09:42:57 -0800
> 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.
> 
> Note, the text_poke_bp() process was this: (nothing to do with atomic
> guarantees)
> 
> add breakpoint (one byte) to instruction.
> 
> Sync all cores (send an IPI to each one).
> 
> change the back half of the instruction (the rest of the instruction
> after the breakpoint).
> 
> Sync all cores
> 
> Remove the breakpoint with the new byte of the new instruction.
> 
> 
> What atomicity guarantee does the above require?

I was asking in the context of static calls.  My understanding is that
the write to change the imm32 of the CALL needs to be atomic from a
code fetch perspective so that we don't jump to a junk address.

Or were you saying that Intel gave an official OK on text_poke_bp()?