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:   Thu, 25 Jan 2018 10:56:46 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     David Woodhouse <dwmw2@infradead.org>
Cc:     Borislav Petkov <bp@alien8.de>, tim.c.chen@linux.intel.com,
        pjt@google.com, jikos@kernel.org, gregkh@linux-foundation.org,
        dave.hansen@intel.com, mingo@kernel.org, riel@redhat.com,
        luto@amacapital.net, torvalds@linux-foundation.org,
        ak@linux.intel.com, keescook@google.com, peterz@infradead.org,
        tglx@linutronix.de, hpa@zytor.com, linux-kernel@vger.kernel.org,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on vmexit
Message-ID: <20180125165646.ytc4upthpaqtwi45@treble>
References: <1515755487-8524-1-git-send-email-dwmw@amazon.co.uk>
 <tip-117cc7a908c83697b0b737d15ae1eb5943afe35b@git.kernel.org>
 <20180125120743.ey32gvl5mjam4r2s@pd.tnic>
 <1516882849.30244.94.camel@infradead.org>
 <20180125124554.vdx7rrnfrxrzl2ng@pd.tnic>
 <20180125151024.bidjr26r667vs7h5@treble>
 <20180125155110.mw655b7fwgm7qqc7@pd.tnic>
 <1516896198.30244.133.camel@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <1516896198.30244.133.camel@infradead.org>
User-Agent: Mutt/1.6.0.1 (2016-04-01)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Jan 25, 2018 at 04:03:18PM +0000, David Woodhouse wrote:
> On Thu, 2018-01-25 at 16:51 +0100, Borislav Petkov wrote:
> > 
> > > And the seg fault is objtool's way of telling you you need a
> > > ANNOTATE_NOSPEC_ALTERNATIVE above the alternative ;-)
> > 
> > Except that it blew up when I did this which doesn't have ALTERNATIVE
> > (it's the diff I saved :-))
> 
> Yeah, ANNOTATE_NOSPEC_ALTERNATIVE just tells objtool "don't look at the
> alternative; you're not going to like it".
> 
> If you start putting a __fill_rsb() function out of line somewhere and
> only *calling* it from alternatives, then objtool is going to shit
> itself when it sees that function, regardless.

Right, if you *really* want it always inline, the short term solution is
to just patch it in with X86_FEATURE_ALWAYS.

And don't ask me what the long term solution is, I don't even know what
I'm having for lunch today.

-- 
Josh