linux-kernel - Re: [RFC][PATCH] x86: Add straight-line-speculation mitigation

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Message-ID: <20211028213058.GQ174703@worktop.programming.kicks-ass.net>
Date:   Thu, 28 Oct 2021 23:30:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>, hjl.tools@...il.com
Subject: Re: [RFC][PATCH] x86: Add straight-line-speculation mitigation

On Thu, Oct 28, 2021 at 09:06:30PM +0100, Andrew Cooper wrote:
> It would be remiss of me not to say...  In Xen, I've currently gone with:
> 
> #ifdef CONFIG_SPECULATIVE_HARDEN_STRAIGHT_LINE
> # define ret  ret;  int3
> # define retq retq; int3

#define retq ret

Surely :-)

> #endif
> 
> 
> which avoids needing to patch any asm files, and therefore is a far less
> invasive patch.

All the asm stuff was more or less:

find arch/x86/ -name \*.s | while read file;
do
	sed -i $file -e 's/\<retq?\>/RET/'
done

or thereabout. The pesky bits were the inline asm and realizing that
RETPOLINE_SAFE also means we really don't care about SLS.

> I know those defines probably deserve a WTF, and will confuse anyone who
> doesn't understand the phrase "painting blue", but they have the
> advantage that casual contributions don't need to remember to avoid
> naked `ret` instructions to maintain safety, making it a rather more
> robust option.

objtool while whinge if someone forgets