[PATCH] KVM: VMX: Check cpl before emulating debug register access

[PATCH] KVM: VMX: Check cpl before emulating debug register access

[Avi Kivity]

Debug registers may only be accessed from cpl 0.  Unfortunately, vmx will
code to emulate the instruction even though it was issued from guest
userspace, possibly leading to an unexpected trap later.

Cc: stable@kernel.org
Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/include/asm/kvm_host.h |    1 +
 arch/x86/kvm/vmx.c              |    2 ++
 arch/x86/kvm/x86.c              |   13 +++++++++++++
 3 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index a3f637f..6046e6f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -621,6 +621,7 @@ void kvm_queue_exception(struct kvm_vcpu *vcpu, unsigned nr);
 void kvm_queue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code);
 void kvm_inject_page_fault(struct kvm_vcpu *vcpu, unsigned long cr2,
 			   u32 error_code);
+bool kvm_require_cpl(struct kvm_vcpu *vcpu, int required_cpl);
 
 int kvm_pic_set_irq(void *opaque, int irq, int level);
 
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 78101dd..05cd554 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2931,6 +2931,8 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 	unsigned long val;
 	int dr, reg;
 
+	if (!kvm_require_cpl(vcpu, 0))
+		return 1;
 	dr = vmcs_readl(GUEST_DR7);
 	if (dr & DR7_GD) {
 		/*
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8b3a169..891234b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -223,6 +223,19 @@ void kvm_queue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code)
 EXPORT_SYMBOL_GPL(kvm_queue_exception_e);
 
 /*
+ * Checks if cpl <= required_cpl; if true, return true.  Otherwise queue
+ * a #GP and return false.
+ */
+bool kvm_require_cpl(struct kvm_vcpu *vcpu, int required_cpl)
+{
+	if (kvm_x86_ops->get_cpl(vcpu) <= required_cpl)
+		return true;
+	kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
+	return false;
+}
+EXPORT_SYMBOL_GPL(kvm_require_cpl);
+
+/*
  * Load the pae pdptrs.  Return true is they are all valid.
  */
 int load_pdptrs(struct kvm_vcpu *vcpu, unsigned long cr3)
-- 
1.6.4.1

Re: [PATCH] KVM: VMX: Check cpl before emulating debug register access

[Marcelo Tosatti]

On Tue, Sep 01, 2009 at 12:03:25PM +0300, Avi Kivity wrote:
> Debug registers may only be accessed from cpl 0.  Unfortunately, vmx will
> code to emulate the instruction even though it was issued from guest
> userspace, possibly leading to an unexpected trap later.
> 
> Cc: stable@kernel.org
> Signed-off-by: Avi Kivity <avi@redhat.com>

Applied, thanks.