Re: [PATCH] KVM: x86: Manually calculate reserved bits when loading PDPTRS

From: Peter Xu
Date: Tue Sep 10 2019 - 02:36:11 EST

On Tue, Sep 03, 2019 at 04:36:45PM -0700, Sean Christopherson wrote:
> Manually generate the PDPTR reserved bit mask when explicitly loading
> PDPTRs. The reserved bits that are being tracked by the MMU reflect the
> current paging mode, which is unlikely to be PAE paging in the vast
> majority of flows that use load_pdptrs(), e.g. CR0 and CR4 emulation,
> __set_sregs(), etc... This can cause KVM to incorrectly signal a bad
> PDPTR, or more likely, miss a reserved bit check and subsequently fail
> a VM-Enter due to a bad VMCS.GUEST_PDPTR.
> Add a one off helper to generate the reserved bits instead of sharing
> code across the MMU's calculations and the PDPTR emulation. The PDPTR
> reserved bits are basically set in stone, and pushing a helper into
> the MMU's calculation adds unnecessary complexity without improving
> readability.
> Oppurtunistically fix/update the comment for load_pdptrs().
> Note, the buggy commit also introduced a deliberate functional change,
> "Also remove bit 5-6 from rsvd_bits_mask per latest SDM.", which was
> effectively (and correctly) reverted by commit cd9ae5fe47df ("KVM: x86:
> Fix page-tables reserved bits"). A bit of SDM archaeology shows that
> the SDM from late 2008 had a bug (likely a copy+paste error) where it
> listed bits 6:5 as AVL and A for PDPTEs used for 4k entries but reserved
> for 2mb entries. I.e. the SDM contradicted itself, and bits 6:5 are and
> always have been reserved.
> Fixes: 20c466b56168d ("KVM: Use rsvd_bits_mask in load_pdptrs()")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
> Reported-by: Doug Reiland <doug.reiland@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

Maybe with a test case would be even better? FWIW:

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>

Peter Xu