Re: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)

From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <nadav.amit@gmail.com>, Ingo Molnar <mingo@kernel.org>,
	 Khalid Aziz <khalid.aziz@oracle.com>,
	juergh@gmail.com,  Tycho Andersen <tycho@tycho.ws>,
	jsteckli@amazon.de,  Kees Cook <keescook@google.com>,
	 Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	 Juerg Haefliger <juerg.haefliger@canonical.com>,
	 deepa.srinivasan@oracle.com, chris.hyser@oracle.com,
	 Tyler Hicks <tyhicks@canonical.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	 Andrew Cooper <andrew.cooper3@citrix.com>,
	Jon Masters <jcm@redhat.com>,
	 Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	 iommu <iommu@lists.linux-foundation.org>,
	X86 ML <x86@kernel.org>,
	 "linux-alpha@vger.kernel.org"
	<linux-arm-kernel@lists.infradead.org>,
	 "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	 Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	 Linux-MM <linux-mm@kvack.org>,
	 LSM List <linux-security-module@vger.kernel.org>,
	 Khalid Aziz <khalid@gonehiking.org>,
	 Andrew Morton <akpm@linux-foundation.org>,
	 Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	 Dave Hansen <dave@sr71.net>, Borislav Petkov <bp@alien8.de>,
	 "H. Peter Anvin" <hpa@zytor.com>,
	Arjan van de Ven <arjan@infradead.org>,
	 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)
Date: Thu, 18 Apr 2019 08:14:48 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1904180811570.3174@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CAHk-=whUwOjFW6RjHVM8kNOv1QVLJuHj2Dda0=mpLPdJ1UyatQ@mail.gmail.com>

On Wed, 17 Apr 2019, Linus Torvalds wrote:
> On Wed, Apr 17, 2019 at 4:42 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Wed, 17 Apr 2019, Linus Torvalds wrote:
> > > With SMEP, user space pages are always NX.
> >
> > We talk past each other. The user space page in the ring3 valid virtual
> > address space (non negative) is of course protected by SMEP.
> >
> > The attack utilizes the kernel linear mapping of the physical
> > memory. I.e. user space address 0x43210 has a kernel equivalent at
> > 0xfxxxxxxxxxx. So if the attack manages to trick the kernel to that valid
> > kernel address and that is mapped X --> game over. SMEP does not help
> > there.
> 
> Oh, agreed.
> 
> But that would simply be a kernel bug. We should only map kernel pages
> executable when we have kernel code in them, and we should certainly
> not allow those pages to be mapped writably in user space.
> 
> That kind of "executable in kernel, writable in user" would be a
> horrendous and major bug.
> 
> So i think it's a non-issue.

Pretty much.

> > From the top of my head I'd say this is a non issue as those kernel address
> > space mappings _should_ be NX, but we got bitten by _should_ in the past:)
> 
> I do agree that bugs can happen, obviously, and we might have missed something.
>
> But in the context of XPFO, I would argue (*very* strongly) that the
> likelihood of the above kind of bug is absolutely *miniscule* compared
> to the likelihood that we'd have something wrong in the software
> implementation of XPFO.
> 
> So if the argument is "we might have bugs in software", then I think
> that's an argument _against_ XPFO rather than for it.

No argument from my side. We better spend time to make sure that a bogus
kernel side X mapping is caught, like we catch other things.

Thanks,

	tglx