Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory

From: "Andy Lutomirski" <luto@kernel.org>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: "Borislav Petkov" <bp@alien8.de>,
	"Sean Christopherson" <seanjc@google.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Joerg Roedel" <jroedel@suse.de>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Andi Kleen" <ak@linux.intel.com>,
	"Sathyanarayanan Kuppuswamy"
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	"David Rientjes" <rientjes@google.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Tom Lendacky" <thomas.lendacky@amd.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Varad Gautam" <varad.gautam@suse.com>,
	"Dario Faggioli" <dfaggioli@suse.com>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Mike Rapoport" <rppt@kernel.org>,
	"David Hildenbrand" <david@redhat.com>,
	"Marcelo Henrique Cerri" <marcelo.cerri@canonical.com>,
	tim.gardner@canonical.com, khalid.elmously@canonical.com,
	philip.cox@canonical.com,
	"the arch/x86 maintainers" <x86@kernel.org>,
	linux-mm@kvack.org, linux-coco@lists.linux.dev,
	linux-efi@vger.kernel.org,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory
Date: Sat, 13 Aug 2022 09:03:13 -0700	[thread overview]
Message-ID: <453fb432-ac13-4819-8395-95561bca948b@www.fastmail.com> (raw)
In-Reply-To: <20220809113827.fchtnyzy44z5fuis@box.shutemov.name>

On Tue, Aug 9, 2022, at 4:38 AM, Kirill A. Shutemov wrote:
> On Tue, Jul 26, 2022 at 01:17:13PM -0700, Andy Lutomirski wrote:
>> 
>> 
>> On Tue, Jun 14, 2022, at 5:02 AM, Kirill A. Shutemov wrote:
>> > load_unaligned_zeropad() can lead to unwanted loads across page boundaries.
>> > The unwanted loads are typically harmless. But, they might be made to
>> > totally unrelated or even unmapped memory. load_unaligned_zeropad()
>> > relies on exception fixup (#PF, #GP and now #VE) to recover from these
>> > unwanted loads.
>> >
>> > But, this approach does not work for unaccepted memory. For TDX, a load
>> > from unaccepted memory will not lead to a recoverable exception within
>> > the guest. The guest will exit to the VMM where the only recourse is to
>> > terminate the guest.
>> 
>> Why is unaccepted memory marked present in the direct map in the first place?
>> 
>> Having kernel code assume that every valid address is followed by
>> several bytes of memory that may be read without side effects other than
>> #PF also seems like a mistake, but I probably won’t win that fight. But
>> sticking guard pages in front of definitely-not-logically present pages
>> seems silly to me.  Let’s just not map it.
>
> It would mean no 1G pages in direct mapping for TDX as we accept 2M a
> time.
>
>> (What if MMIO memory is mapped next to regular memory?  Doing random
>> unaligned reads that cross into MMIO seems unwise.)
>
> MMIO is shared, not unaccpted private. We already handle the situation.
> See 1e7769653b06 ("x86/tdx: Handle load_unaligned_zeropad() page-cross to
> a shared page").
>

I don’t mean in a confidential guest — I mean generally. This whole model of “overrun the buffer — no big deal” is just fragile.

> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov