Re: [PATCHv9 11/14] x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory

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

On Mon, Apr 03, 2023 at 03:28:36PM +0200, Vlastimil Babka wrote:
> On 3/30/23 13:49, 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.
> > 
> > There are three parts to fix this issue and comprehensively avoid access
> > to unaccepted memory. Together these ensure that an extra "guard" page
> > is accepted in addition to the memory that needs to be used.
> > 
> > 1. Implicitly extend the range_contains_unaccepted_memory(start, end)
> >    checks up to end+2M if 'end' is aligned on a 2M boundary. It may
> >    require checking 2M chunk beyond end of RAM. The bitmap allocation is
> >    modified to accommodate this.
> > 2. Implicitly extend accept_memory(start, end) to end+2M if 'end' is
> >    aligned on a 2M boundary.
> > 3. Set PageUnaccepted() on both memory that itself needs to be accepted
> >    *and* memory where the next page needs to be accepted. Essentially,
> >    make PageUnaccepted(page) a marker for whether work needs to be done
> >    to make 'page' usable. That work might include accepting pages in
> >    addition to 'page' itself.
> > 
> > Side note: This leads to something strange. Pages which were accepted
> > 	   at boot, marked by the firmware as accepted and will never
> > 	   _need_ to be accepted might have PageUnaccepted() set on
> > 	   them. PageUnaccepted(page) is a cue to ensure that the next
> > 	   page is accepted before 'page' can be used.
> 
> At least the part about PageUnaccepted() is obsolete in v9, no?

Ah, right. Nice catch.

I removed PageUnaccepted() late in patchset preparation and forgot about
this.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov