linux-kernel - Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

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Message-ID: <1515170506.29312.149.camel@amazon.co.uk>
Date:   Fri, 5 Jan 2018 16:41:46 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bpetkov@...e.de>
CC:     <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        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>,
        "Rik van Riel" <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Andy Lutomirski" <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

On Fri, 2018-01-05 at 13:56 +0000, Woodhouse, David wrote:
> 
> At some point during this whole painful mess, I had come to the
> conclusion that having relocations in altinstr didn't work, and that's
> why I had X86_xx_NO_RETPOLINE instead of X86_xx_RETPOLINE. I now think
> that something else was wrong when I was testing that, and relocs in
> altinstr do work. So sure, X86_FEATURE_RETPOLINE ought to work. I can
> change that round, and it's simpler for the IBRS patch set to take it
> into account and set it when appropriate.

+bpetkov

Nope, alternatives are broken. Only a jmp as the *first* opcode of
altinstr gets handled by recompute_jump(), while any subsequent insn is
just copied untouched.

To fix that and handle every instruction, the alternative code would
need to know about instruction lengths. I think we need to stick with
the inverted X86_FEATURE_NO_RETPOLINE flag for the moment, and not tie
it to a complex bugfix there.

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