Re: [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation

[Ben Gardon]

On Tue, Jan 21, 2020 at 8:11 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> The SPTE_MMIO_MASK overlaps with the bits used to track MMIO
> generation number.  A high enough generation number would overwrite the
> SPTE_SPECIAL_MASK region and cause the MMIO SPTE to be misinterpreted;
> likewise, setting bits 52 and 53 would also cause an incorrect generation
> number to be read from the PTE.
>
> Fixes: 6eeb4ef049e7 ("KVM: x86: assign two bits to track SPTE kinds")
> Reported-by: Ben Gardon <bgardon@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 57e4dbddba72..e34ca43d9166 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -418,22 +418,25 @@ static inline bool is_access_track_spte(u64 spte)
>   * requires a full MMU zap).  The flag is instead explicitly queried when
>   * checking for MMIO spte cache hits.
>   */
> -#define MMIO_SPTE_GEN_MASK             GENMASK_ULL(18, 0)
> +#define MMIO_SPTE_GEN_MASK             GENMASK_ULL(17, 0)

I see you're shifting the MMIO high gen mask region to avoid having to
shift it by 2. Looking at the SDM, I believe using bit 62 for the
generation number is safe, but I don't recall why it wasn't used
before.

>
>  #define MMIO_SPTE_GEN_LOW_START                3
>  #define MMIO_SPTE_GEN_LOW_END          11
>  #define MMIO_SPTE_GEN_LOW_MASK         GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
>                                                     MMIO_SPTE_GEN_LOW_START)
>
> -#define MMIO_SPTE_GEN_HIGH_START       52
> -#define MMIO_SPTE_GEN_HIGH_END         61
> +/* Leave room for SPTE_SPECIAL_MASK.  */
> +#define MMIO_SPTE_GEN_HIGH_START       54
> +#define MMIO_SPTE_GEN_HIGH_END         62
>  #define MMIO_SPTE_GEN_HIGH_MASK                GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
>                                                     MMIO_SPTE_GEN_HIGH_START)
> +
>  static u64 generation_mmio_spte_mask(u64 gen)
>  {
>         u64 mask;
>
>         WARN_ON(gen & ~MMIO_SPTE_GEN_MASK);
> +       BUILD_BUG_ON(MMIO_SPTE_GEN_HIGH_START < PT64_SECOND_AVAIL_BITS_SHIFT);

Would it be worth defining the MMIO_SPTE_GEN masks, SPTE_SPECIAL_MASK,
SPTE_AD masks, and SPTE_MMIO_MASK in terms of
PT64_SECOND_AVAIL_BITS_SHIFT? It seems like that might be a more
robust assertion here.

Alternatively, BUILD_BUG_ON((MMIO_SPTE_GEN_HIGH_MASK |
MMIO_SPTE_GEN_LOW_MASK) & SPTE_(MMIO and/or SPECIAL)_MASK)

>
>         mask = (gen << MMIO_SPTE_GEN_LOW_START) & MMIO_SPTE_GEN_LOW_MASK;
>         mask |= (gen << MMIO_SPTE_GEN_HIGH_START) & MMIO_SPTE_GEN_HIGH_MASK;
> --
> 1.8.3.1
>