Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on long-mode

[Nadav Amit]

On 6/16/14, 1:17 PM, Paolo Bonzini wrote:
> Il 15/06/2014 15:13, Nadav Amit ha scritto:
> > From: Nadav Amit <nadav.amit@xxxxxxxxx>
> >
> > When the guest sets DR6 and DR7, KVM asserts the high 32-bits are
> > clear, and
> > otherwise injects a #GP exception. This exception should only be
> > injected only
> > if running in long-mode.
> >
> > Signed-off-by: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 57eac30..71fe841 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -756,6 +756,15 @@ static void kvm_update_dr7(struct kvm_vcpu *vcpu)
> >          vcpu->arch.switch_db_regs |= KVM_DEBUGREG_BP_ENABLED;
> >  }
> >
> > +static bool is_64_bit_mode(struct kvm_vcpu *vcpu)
> > +{
> > +    int cs_db, cs_l;
> > +    if (!is_long_mode(vcpu))
> > +        return false;
> > +    kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
> > +    return cs_l;
> > +}
> > +
> >  static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long
> > val)
> >  {
> >      switch (dr) {
> > @@ -769,7 +778,7 @@ static int __kvm_set_dr(struct kvm_vcpu *vcpu, int
> > dr, unsigned long val)
> >              return 1; /* #UD */
> >          /* fall through */
> >      case 6:
> > -        if (val & 0xffffffff00000000ULL)
> > +        if ((val & 0xffffffff00000000ULL) && is_64_bit_mode(vcpu))
> >              return -1; /* #GP */
> >          vcpu->arch.dr6 = (val & DR6_VOLATILE) | DR6_FIXED_1;
> >          kvm_update_dr6(vcpu);
> > @@ -779,7 +788,7 @@ static int __kvm_set_dr(struct kvm_vcpu *vcpu, int
> > dr, unsigned long val)
> >              return 1; /* #UD */
> >          /* fall through */
> >      default: /* 7 */
> > -        if (val & 0xffffffff00000000ULL)
> > +        if ((val & 0xffffffff00000000ULL) && is_64_bit_mode(vcpu))
> >              return -1; /* #GP */
> >          vcpu->arch.dr7 = (val & DR7_VOLATILE) | DR7_FIXED_1;
> >          kvm_update_dr7(vcpu);
>
> Do you get this if the input register has bit 31 set?
No. To be frank, the scenario may be considered a bit synthetic: the 
guest assigns a value to a general-purpose register in 64-bit mode, 
setting the high 32-bits to some non-zero value. Then, later, in 32-bit 
mode, the guest performs MOV DR instruction. In between the two 
assignments, the general purpose register is unmodified, so the high 
32-bits of the general purpose registers are still set.

Note that this scenario does not occur when MOV DR is emulated, but when 
handle_dr() is called. In this case, the entire 64-bits of the general 
purpose register used for MOV DR are read, regardless to the execution 
mode of the guest.

Nadav

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