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

[Yu Zhang]

On 9/16/2017 7:19 AM, Jim Mattson wrote:
> 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?

Thanks Jim.
I have tested this code in a simulator to successfully boot a VM in 
shadow mode.
Seems this code is not covered(but I am now still perplexed why this is 
not covered).
Any possibility that the check_cr_write() is not triggered when 
emulating the cr
operations?

Anyway, this should be a bug and thanks for pointing this out, and I'll 
send out the
fix later.

BR
Yu