Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI

From: Andy Lutomirski <luto@amacapital.net>
To: Nadav Amit <namit@vmware.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	the arch/x86 maintainers <x86@kernel.org>, "w@1wt.eu" <w@1wt.eu>
Subject: Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI
Date: Mon, 15 Jan 2018 09:45:04 -0800	[thread overview]
Message-ID: <3A5624D6-3D7B-4C32-BF97-27A78CBEE14E@amacapital.net> (raw)
In-Reply-To: <451AD6CB-74AA-4869-8CB1-8175126C3F0D@vmware.com>

> On Jan 15, 2018, at 9:42 AM, Nadav Amit <namit@vmware.com> wrote:
> 
> Andy Lutomirski <luto@amacapital.net> wrote:
> 
>> 
>>> 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.
> 
> I noticed it doesn’t. I thought the removing/restoring the __USER_CS
> descriptor on context switch, based on TIF_IA32, would be enough.
> modify_ldt() always keeps the descriptor l-bit clear. I will review the
> other GDT descriptors, and if needed, create two GDTs. Let me know if I
> missed anything else.

There world need to be some opt-in control, I think, for CRIU if nothing else.

Also, on Xen PV, it's a complete nonstarter.  We don't have enough control over the GDT unless someone knows otherwise.  But there's no PTI on Xen PV either.

> 
>> 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.
> 
> If SMEP is not supported, compatibility mode would still require page-table
> isolation.
> 
> Thanks for the feedback. I still look for an ack for the basic idea of
> disabling page-table isolation on compatibility mode.
> 

I'm still not really convinced this is worth it.  It will send a bad message and get people to run critical stuff compiled for 32-bit, which has its own downsides.

> Regards,
> Nadav