Re: [PATCH v3 2/5] KVM: MMU: check guest CR3 reserved bits based on its physical address width.

[Jim Mattson]

On Thu, Aug 24, 2017 at 5:27 AM, Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> Currently, KVM uses CR3_L_MODE_RESERVED_BITS to check the
> reserved bits in CR3. Yet the length of reserved bits in
> guest CR3 should be based on the physical address width
> exposed to the VM. This patch changes CR3 check logic to
> calculate the reserved bits at runtime.
>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  1 -
>  arch/x86/kvm/emulate.c          | 14 ++++++++++++--
>  arch/x86/kvm/mmu.h              |  3 +++
>  arch/x86/kvm/x86.c              |  8 ++++----
>  4 files changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6db0ed9..e716228 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -79,7 +79,6 @@
>                           | X86_CR0_ET | X86_CR0_NE | X86_CR0_WP | X86_CR0_AM \
>                           | X86_CR0_NW | X86_CR0_CD | X86_CR0_PG))
>
> -#define CR3_L_MODE_RESERVED_BITS 0xFFFFFF0000000000ULL
>  #define CR3_PCID_INVD           BIT_64(63)
>  #define CR4_RESERVED_BITS                                               \
>         (~(unsigned long)(X86_CR4_VME | X86_CR4_PVI | X86_CR4_TSD | X86_CR4_DE\
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 319d91f..a89b595 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -28,6 +28,7 @@
>
>  #include "x86.h"
>  #include "tss.h"
> +#include "mmu.h"
>
>  /*
>   * Operand types
> @@ -4097,8 +4098,17 @@ static int check_cr_write(struct x86_emulate_ctxt *ctxt)
>                 u64 rsvd = 0;
>
>                 ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
> -               if (efer & EFER_LMA)
> -                       rsvd = CR3_L_MODE_RESERVED_BITS & ~CR3_PCID_INVD;
> +               if (efer & EFER_LMA) {
> +                       u64 maxphyaddr;
> +                       u32 eax = 0x80000008;
> +
> +                       if (ctxt->ops->get_cpuid(ctxt, &eax, NULL, NULL,
> +                                                NULL, false))

Passing NULL for the address of ecx looks problematic to me.

We have:

static bool emulator_get_cpuid(struct x86_emulate_ctxt *ctxt,
                        u32 *eax, u32 *ebx, u32 *ecx, u32 *edx, bool
check_limit)
{
        return kvm_cpuid(emul_to_vcpu(ctxt), eax, ebx, ecx, edx, check_limit);
}

And:

bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
       u32 *ecx, u32 *edx, bool check_limit)
{
u32 function = *eax, index = *ecx;
struct kvm_cpuid_entry2 *best;
bool entry_found = true;
...

Doesn't this immediately try to dereference a NULL pointer?  How much
testing have you done of this code?

> +                               maxphyaddr = eax & 0xff;
> +                       else
> +                               maxphyaddr = 36;
> +                       rsvd = rsvd_bits(maxphyaddr, 62);
> +               }
>
>                 if (new_val & rsvd)
>                         return emulate_gp(ctxt, 0);
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 3ed6192..67e7ec2 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -48,6 +48,9 @@
>
>  static inline u64 rsvd_bits(int s, int e)
>  {
> +       if (e < s)
> +               return 0;
> +
>         return ((1ULL << (e - s + 1)) - 1) << s;
>  }
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index cc2c7e4..79f5889 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -813,10 +813,10 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>                 return 0;
>         }
>
> -       if (is_long_mode(vcpu)) {
> -               if (cr3 & CR3_L_MODE_RESERVED_BITS)
> -                       return 1;
> -       } else if (is_pae(vcpu) && is_paging(vcpu) &&
> +       if (is_long_mode(vcpu) &&
> +           (cr3 & rsvd_bits(cpuid_maxphyaddr(vcpu), 62)))
> +               return 1;
> +       else if (is_pae(vcpu) && is_paging(vcpu) &&
>                    !load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))
>                 return 1;
>
> --
> 2.5.0
>