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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Sat, Aug 13, 2022 at 09:03:13AM -0700, Andy Lutomirski wrote:
> 
> 
> 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.

As of now, we don't have a way to recover direct mapping from
fragmentation. So once we split 1G to 2M it stays this way.

> >> (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.

If you want to remove load_unaligned_zeropad(), I would not object. It can
make life easier.

I presumed that optimization it brings has measuarable benefit (otherwise,
why bother).

-- 
  Kiryl Shutsemau / Kirill A. Shutemov