linux-kernel - Re: [PATCH RFC v2 0/6] x86: Disabling PTI in compatibility mode

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Date:   Thu, 15 Feb 2018 16:25:32 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Willy Tarreau <w@....eu>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 0/6] x86: Disabling PTI in compatibility mode

Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> On 02/15/2018 08:35 AM, Nadav Amit wrote:
>> I removed the PTI disabling while SMEP is unsupported, although I
>> must admit I did not fully understand why it is required.
> 
> Do you mean you don't fully understand how PTI gives SMEP-like behavior
> on non-SMEP hardware?

No. I understand how it provide SMEP-like behavior, and I understand the value
of SMEP by itself.

However, I do not understand why SMEP-like protection is required to protect
processes that run in compatibility-mode from Meltdown/Spectre attacks. As
far as I understand, the process should not be able to manipulate the kernel
to execute code in the low 4GB.