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

Andy Lutomirski
2022-08-13T16:03:44Z

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