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

Nadav Amit
2014-06-16T10:33:27Z

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@gmail.com>
>>
>> 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@cs.technion.ac.il>
>> ---
>> 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