[PATCH] KVM: x86: DR6/7.RTM cannot be written

Nadav Amit
2014-07-15T14:37:58Z

Haswell and newer Intel CPUs have support for RTM, and in that case DR6.RTM is
not fixed to 1 and DR7.RTM is not fixed to zero. That is not the case in the
current KVM implementation. This bug is apparent only if the MOV-DR instruction
is emulated or the host also debugs the guest.

This patch is a partial fix which enables DR6.RTM and DR7.RTM to be cleared and
set respectively. It also sets DR6.RTM upon every debug exception. Obviously,
it is not a complete fix, as debugging of RTM is still unsupported.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
---
arch/x86/include/asm/kvm_host.h | 8 +++++---
arch/x86/kvm/cpuid.h | 8 ++++++++
arch/x86/kvm/vmx.c | 4 ++--
arch/x86/kvm/x86.c | 22 ++++++++++++++++------
4 files changed, 31 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index b8a4480..a84eaf7 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -152,14 +152,16 @@ enum {

#define DR6_BD (1 << 13)
#define DR6_BS (1 << 14)
-#define DR6_FIXED_1 0xffff0ff0
-#define DR6_VOLATILE 0x0000e00f
+#define DR6_RTM (1 << 16)
+#define DR6_FIXED_1 0xfffe0ff0
+#define DR6_INIT 0xffff0ff0
+#define DR6_VOLATILE 0x0001e00f

#define DR7_BP_EN_MASK 0x000000ff
#define DR7_GE (1 << 9)
#define DR7_GD (1 << 13)
#define DR7_FIXED_1 0x00000400
-#define DR7_VOLATILE 0xffff23ff
+#define DR7_VOLATILE 0xffff2bff

/* apic attention bits */
#define KVM_APIC_CHECK_VAPIC 0
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index f908731..a538059 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -95,4 +95,12 @@ static inline bool guest_cpuid_has_gbpages(struct kvm_vcpu *vcpu)
best = kvm_find_cpuid_entry(vcpu, 0x80000001, 0);
return best && (best->edx & bit(X86_FEATURE_GBPAGES));
}
+
+static inline bool guest_cpuid_has_rtm(struct kvm_vcpu *vcpu)
+{
+ struct kvm_cpuid_entry2 *best;
+
+ best = kvm_find_cpuid_entry(vcpu, 7, 0);
+ return best && (best->ebx & bit(X86_FEATURE_RTM));
+}
#endif
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 0c9569b..1fd3598 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4892,7 +4892,7 @@ static int handle_exception(struct kvm_vcpu *vcpu)
if (!(vcpu->guest_debug &
(KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))) {
vcpu->arch.dr6 &= ~15;
- vcpu->arch.dr6 |= dr6;
+ vcpu->arch.dr6 |= dr6 | DR6_RTM;
if (!(dr6 & ~DR6_RESERVED)) /* icebp */
skip_emulated_instruction(vcpu);

@@ -5151,7 +5151,7 @@ static int handle_dr(struct kvm_vcpu *vcpu)
return 0;
} else {
vcpu->arch.dr7 &= ~DR7_GD;
- vcpu->arch.dr6 |= DR6_BD;
+ vcpu->arch.dr6 |= DR6_BD | DR6_RTM;
vmcs_writel(GUEST_DR7, vcpu->arch.dr7);
kvm_queue_exception(vcpu, DB_VECTOR);
return 1;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f750b69..fae064f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -759,6 +759,15 @@ static void kvm_update_dr7(struct kvm_vcpu *vcpu)
vcpu->arch.switch_db_regs |= KVM_DEBUGREG_BP_ENABLED;
}

+static u64 kvm_dr6_fixed(struct kvm_vcpu *vcpu)
+{
+ u64 fixed = DR6_FIXED_1;
+
+ if (!guest_cpuid_has_rtm(vcpu))
+ fixed |= DR6_RTM;
+ return fixed;
+}
+
static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val)
{
switch (dr) {
@@ -774,7 +783,7 @@ static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val)
case 6:
if (val & 0xffffffff00000000ULL)
return -1; /* #GP */
- vcpu->arch.dr6 = (val & DR6_VOLATILE) | DR6_FIXED_1;
+ vcpu->arch.dr6 = (val & DR6_VOLATILE) | kvm_dr6_fixed(vcpu);
kvm_update_dr6(vcpu);
break;
case 5:
@@ -5115,7 +5124,8 @@ static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflag
*/
if (unlikely(rflags & X86_EFLAGS_TF)) {
if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
- kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1;
+ kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
+ DR6_RTM;
kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
kvm_run->debug.arch.exception = DB_VECTOR;
kvm_run->exit_reason = KVM_EXIT_DEBUG;
@@ -5128,7 +5138,7 @@ static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflag
* cleared by the processor".
*/
vcpu->arch.dr6 &= ~15;
- vcpu->arch.dr6 |= DR6_BS;
+ vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
kvm_queue_exception(vcpu, DB_VECTOR);
}
}
@@ -5147,7 +5157,7 @@ static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
vcpu->arch.eff_db);

if (dr6 != 0) {
- kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1;
+ kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1 | DR6_RTM;
kvm_run->debug.arch.pc = kvm_rip_read(vcpu) +
get_segment_base(vcpu, VCPU_SREG_CS);

@@ -5165,7 +5175,7 @@ static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)

if (dr6 != 0) {
vcpu->arch.dr6 &= ~15;
- vcpu->arch.dr6 |= dr6;
+ vcpu->arch.dr6 |= dr6 | DR6_RTM;
kvm_queue_exception(vcpu, DB_VECTOR);
*r = EMULATE_DONE;
return true;
@@ -6863,7 +6873,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu)
kvm_clear_exception_queue(vcpu);

memset(vcpu->arch.db, 0, sizeof(vcpu->arch.db));
- vcpu->arch.dr6 = DR6_FIXED_1;
+ vcpu->arch.dr6 = DR6_INIT;
kvm_update_dr6(vcpu);
vcpu->arch.dr7 = DR7_FIXED_1;
kvm_update_dr7(vcpu);
--
1.9.1