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Subject: Re: [PATCH RFC v2 0/6] x86: Disabling PTI in compatibility mode
From: Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <27a0082c-fadb-792a-740e-70932d51f1b5@linux.intel.com>
Date: Thu, 15 Feb 2018 16:25:32 -0800
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Willy Tarreau <w@1wt.eu>,
        x86@kernel.org, linux-kernel@vger.kernel.org
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References: <20180215163602.61162-1-namit@vmware.com>
 <27a0082c-fadb-792a-740e-70932d51f1b5@linux.intel.com>
To: Dave Hansen <dave.hansen@linux.intel.com>
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Dave Hansen <dave.hansen@linux.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.