linux-kernel - Re: [PATCH 2/3] objtool: Ignore retpoline alternatives

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Date:   Thu, 11 Jan 2018 16:55:18 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/3] objtool: Ignore retpoline alternatives

On Thu, 2018-01-11 at 10:48 -0600, Josh Poimboeuf wrote:
> 
> The above macro is protected by '#ifdef RETPOLINE', and I seriously
> doubt 0-day is testing with an unreleased version of GCC.  So you
> shouldn't see a 0-day warning.

It's actually #ifdef CONFIG_RETPOLINE isn't it? 

If you enable CONFIG_RETPOLINE but don't have a new compiler, you still
get all the asm thunks (which are the easy-to-attack targets). Only if
you have a new compiler is RETPOLINE also set.

Also, the RSB stuffing we're looking at here is also needed for the
IBRS-based mitigation, so won't even be under CONFIG_RETPOLINE by the
time the IBRS patch set is beaten into shape on top. It'll probably be
unconditional unless we get a CONFIG_IBRS_SUPPORT (which hasn't been
suggested so far).

> I think I heard that retpolines won't be ported to anything older than
> GCC 4.9, so maybe it's safe to use '%='.  I don't remember when it was
> introduced into GCC though.

Hm. Peter? This is all your fault, right? Did you know you were making
us ditch compatibility for older GCC?

Precisely when *did* %= get added to GCC?

Note that we can also just resort to using .macro even from inline asm.
It just takes a rather icky asm(".include ..."). :)

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