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

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Date:   Mon, 12 Feb 2018 10:37:36 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux Kernel Mailing List <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 Mon, Feb 12, 2018 at 10:26 AM, Pavel Machek <pavel@....cz> wrote:
> On Tue 2017-12-26 23:43:54, Tom Lendacky wrote:
>> AMD processors are not subject to the types of attacks that the kernel
>> page table isolation feature protects against.  The AMD microarchitecture
>> does not allow memory references, including speculative references, that
>> access higher privileged data when running in a lesser privileged mode
>> when that access would result in a page fault.
>>
>> Disable page table isolation by default on AMD processors by not setting
>> the X86_BUG_CPU_INSECURE feature, which controls whether X86_FEATURE_PTI
>> is set.
>
> PTI was originally meant to protect KASLR from memory leaks, before
> Spectre was public. I guess that's still valid use on AMD cpus?
>                                                                 Pavel

KASLR leaks are a much lower threat than Meltdown.  Given that no AMD
processor supports PCID, enabling PTI has a much more significant
performance impact for a much smaller benefit.  For the paranoid user
they still have the option to enable PTI at boot, but it should not be
on by default.

--
Brian Gerst