Re: [PATCH v3 3/9] KVM: x86: mmu: Add guest physical address check in translate_gpa()

Paolo Bonzini
2020-07-10T17:41:33Z

On 10/07/20 17:48, Mohammed Gamal wrote:
> In case of running a guest with 4-level page tables on a 5-level page
> table host, it might happen that a guest might have a physical address
> with reserved bits set, but the host won't see that and trap it.
>
> Hence, we need to check page faults' physical addresses against the guest's
> maximum physical memory and if it's exceeded, we need to add
> the PFERR_RSVD_MASK bits to the PF's error code.
>
> Also make sure the error code isn't overwritten by the page table walker.
>

New commit message:

KVM: x86: mmu: Add guest physical address check in translate_gpa()

Intel processors of various generations have supported 36, 39, 46 or 52
bits for physical addresses. Until IceLake introduced MAXPHYADDR==52,
running on a machine with higher MAXPHYADDR than the guest more or less
worked, because software that relied on reserved address bits (like KVM)
generally used bit 51 as a marker and therefore the page faults where
generated anyway.

Unfortunately this is not true anymore if the host MAXPHYADDR is 52,
and this can cause problems when migrating from a MAXPHYADDR<52
machine to one with MAXPHYADDR==52. Typically, the latter are machines
that support 5-level page tables, so they can be identified easily from
the LA57 CPUID bit.

When that happens, the guest might have a physical address with reserved
bits set, but the host won't see that and trap it. Hence, we need
to check page faults' physical addresses against the guest's maximum
physical memory and if it's exceeded, we need to add the PFERR_RSVD_MASK
bits to the page fault error code.

This patch does this for the MMU's page walks. The next patches will
ensure that the correct exception and error code is produced whenever
no host-reserved bits are set in page table entries.

Paolo