linux-kernel - Re: [PATCH 3/5] x86/ibrs: Add direct access support for MSR_IA32_SPEC

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1516024762.22147.173.camel@infradead.org>
Date:   Mon, 15 Jan 2018 14:59:22 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>
Subject: Re: [PATCH 3/5] x86/ibrs: Add direct access support for
 MSR_IA32_SPEC_CTRL

On Mon, 2018-01-15 at 14:45 +0100, Peter Zijlstra wrote:
> On Fri, Jan 12, 2018 at 10:09:08AM +0000, David Woodhouse wrote:
> > static_cpu_has() + asm-goto is NOT SUFFICIENT.
> > 
> > It's still *possible* for a missed optimisation in GCC to still leave
> > us with a conditional branch around the wrmsr, letting the CPU
> > speculate around it too.
> 
> OK, so GCC would have to be bloody retarded to mess this up;

Like *that's* never happened before? In corner cases where it just gets
confused and certain optimisations go out the window?

>  but would something like the below work for you?
> 
> The usage is like:
> 
>   if (static_branch_unlikely(key)) {
>         arch_static_assert();
>         stuff();
>   }
> 
> And then objtool will fail things if the first instruction into that
> branch is not immediately after a NOP/JMP patch site (on either the NOP
> or the JMP+disp side of things).

That seems reasonable; thanks. Bonus points if you can make the
arch_static_assert happen() automatically with vile tricks like

#define IF_FEATURE(ftr) if (static_cpu_has(ftr)) arch_static_assert, 

So then it just becomes

   IF_FEATURE(key) {
       stuff();
   }

There might not be a sane way to do that though. And it's OK to have to
manually annotate the call sites where this is for correctness and not
purely optimisation.
Download attachment "smime.p7s" of type "application/x-pkcs7-signature" (5213 bytes)