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Subject: Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI
From: Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20180114201306.3554-1-namit@vmware.com>
Date: Mon, 15 Jan 2018 09:20:33 -0800
Cc: linux-kernel@vger.kernel.org, dave.hansen@linux.intel.com,
        luto@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        hpa@zytor.com, x86@kernel.org, nadav.amit@gmail.com, w@1wt.eu
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Message-Id: <C6FE7867-EA7D-495F-9A27-D598EAFBD280@amacapital.net>
References: <20180114201306.3554-1-namit@vmware.com>
To: Nadav Amit <namit@vmware.com>
Sender: linux-kernel-owner@vger.kernel.org


> On Jan 14, 2018, at 12:13 PM, Nadav Amit <namit@vmware.com> wrote:
> 
> Currently, when page-table isolation is on to prevent the Meltdown bug
> (CVE-2017-5754), CR3 is always loaded on system-call and interrupt.
> 
> However, it appears that this is an unnecessary measure when programs
> run in compatibility mode. In this mode only 32-bit registers are
> available, which means that there *should* be no way for the CPU to
> access, even speculatively, memory that belongs to the kernel, which
> sits in high addresses.

You're assuming that TIF_IA32 prevents the execution of 64-bit code.  It doesn't.

I've occasionally considered adding an opt-in hardening mechanism to enforce 32-bit or 64-bit execution, but we don't have this now.

Anything like this would also need to spend on SMEP, I think -- the pseudo-SMEP granted by PTI is too valuable to give up on old boxes, I think.