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

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Date:	Mon, 16 Jun 2014 13:33:27 +0300
From:	Nadav Amit <nadav.amit@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Nadav Amit <namit@...technion.ac.il>
CC:	gleb@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on
 long-mode

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@...il.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@...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

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