linux-kernel - Re: [PATCH] x86/cpu, x86/pti: Do not enable PTI on AMD processors

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Message-ID: <26b626e4-d01b-4a43-1c70-a4db7322dbbe@amd.com>
Date:   Wed, 27 Dec 2017 08:47:18 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH] x86/cpu, x86/pti: Do not enable PTI on AMD processors



On 12/27/2017 2:48 AM, Dave Hansen wrote:
> On 12/26/2017 09:43 PM, Tom Lendacky wrote:
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -923,8 +923,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>>  
>>  	setup_force_cpu_cap(X86_FEATURE_ALWAYS);
>>  
>> -	/* Assume for now that ALL x86 CPUs are insecure */
>> -	setup_force_cpu_bug(X86_BUG_CPU_INSECURE);
>> +	if (c->x86_vendor != X86_VENDOR_AMD)
>> +		setup_force_cpu_bug(X86_BUG_CPU_INSECURE);
> 
> Does this disable it in a way that it can be turned back on via the
> kernel command-line?
> 

Yes, specifying pti=on on the command line will turn kernel page table
isolation on regardless of this setting.

Thanks,
Tom

> This is a rather wide class of issues and I would rather not just
> hard-code it in a way that we say one vendor has never and will never be
> affected.
>