linux-kernel - Re: [PATCH 3/3] kprobes/x86: Simplify indirect-jump check in retpoline

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Date:   Thu, 1 Nov 2018 17:21:38 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux-Kernel <linux-kernel@...r.kernel.org>, mingo@...hat.com,
        konrad.wilk@...cle.com, dwmw@...zon.co.uk, tglx@...utronix.de,
        Srinivas REDDY Eeda <srinivas.eeda@...cle.com>, bp@...e.de,
        hpa@...or.com
Subject: Re: [PATCH 3/3] kprobes/x86: Simplify indirect-jump check in
 retpoline

On 2018/11/1 16:56, Peter Zijlstra wrote:
> On Thu, Nov 01, 2018 at 10:02:14AM +0800, Zhenzhong Duan wrote:
>>> Hmm, what about the case where we have RETPOLINE runtime disabled? Then
>>> the CALL_NOSPEC alternative patches in an indirect call again, and the
>>> retpolines are gone.
>>
>> Is RETPOLINE runtime toggle supported in upstream? I don't see such code.
> 
> arch/x86/kernel/cpu/bugs.c look for the "nospectre_v2" and related
> options. That will avoid X86_FEATURE_RETPOLINE from being set, and thus
> the JMP_NOSPEC and CALL_NOSPEC alternatives will not patch out the
> indirect jump / call.
> 
Yes, in this case there are also indirect branches. I'll drop 3rd patch 
in v2.

Thanks
Zhenzhong