Re: [PATCH v2] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation

[Ben Gardon]

On Thu, Jan 23, 2020 at 12:50 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, though this was partially mitigated by the
> (useless if it weren't for the bug) removal of SPTE_SPECIAL_MASK from
> the spte in get_mmio_spte_generation.  Drop that removal, and replace
> it with a compile-time assertion.
>
> Fixes: 6eeb4ef049e7 ("KVM: x86: assign two bits to track SPTE kinds")
> Reported-by: Ben Gardon <bgardon@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>

> ---
>  arch/x86/kvm/mmu/mmu.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 57e4dbddba72..b9052c7ba43d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -418,22 +418,24 @@ 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)
>
>  #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
> +#define MMIO_SPTE_GEN_HIGH_START       PT64_SECOND_AVAIL_BITS_SHIFT
> +#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_MASK | MMIO_SPTE_GEN_LOW_MASK) & SPTE_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;
> @@ -444,8 +446,6 @@ static u64 get_mmio_spte_generation(u64 spte)
>  {
>         u64 gen;
>
> -       spte &= ~shadow_mmio_mask;
> -
>         gen = (spte & MMIO_SPTE_GEN_LOW_MASK) >> MMIO_SPTE_GEN_LOW_START;
>         gen |= (spte & MMIO_SPTE_GEN_HIGH_MASK) >> MMIO_SPTE_GEN_HIGH_START;
>         return gen;
> --
> 1.8.3.1
>