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

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Date:   Fri, 5 Jan 2018 17:12:33 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Borislav Petkov <bp@...e.de>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

On Fri, 2018-01-05 at 17:45 +0100, Borislav Petkov wrote:
> On Fri, Jan 05, 2018 at 04:41:46PM +0000, Woodhouse, David wrote:
> > 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.
> 
> Not broken - simply no one needed it until now. I'm looking into it.
> Looks like the insn decoder might come in handy finally.#

I typed 'jmp __x86.indirect_thunk' and it actually jumped to an address
which I believe is (__x86.indirect_thunk + &altinstr - &oldinstr).
Which made me sad, and took a while to debug.

Let's call it "not working for this use case", and I'll stick with
X86_FEATURE_NO_RETPOLINE for now, so the jump can go in the oldinstr
case not in altinstr.
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