[PATCH] KVM: X86: Also reload the debug registers before kvm_x86->run() when the host is using them

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

When the host is using debug registers but the guest is not using them
nor is the guest in guest-debug state, the kvm code does not reset
the host debug registers before kvm_x86->run().  Rather, it relies on
the hardware vmentry instruction to automatically reset the dr7 registers
which ensures that the host breakpoints do not affect the guest.

But there are still problems:
	o The addresses of the host breakpoints can leak into the guest
	  and the guest may use these information to attack the host.

	o It violates the non-instrumentable nature around VM entry and
	  exit.  For example, when a host breakpoint is set on
	  vcpu->arch.cr2, #DB will hit aftr kvm_guest_enter_irqoff().

Beside the problems, the logic is not consistent either. When the guest
debug registers are active, the host breakpoints are reset before
kvm_x86->run(). But when the guest debug registers are inactive, the
host breakpoints are delayed to be disabled.  The host tracing tools may
see different results depending on there is any guest running or not.

To fix the problems, we also reload the debug registers before
kvm_x86->run() when the host is using them whenever the guest is using
them or not.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b594275d49b5..cce316655d3c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9320,7 +9320,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
 		switch_fpu_return();
 
-	if (unlikely(vcpu->arch.switch_db_regs)) {
+	if (unlikely(vcpu->arch.switch_db_regs || hw_breakpoint_active())) {
 		set_debugreg(0, 7);
 		set_debugreg(vcpu->arch.eff_db[0], 0);
 		set_debugreg(vcpu->arch.eff_db[1], 1);
-- 
2.19.1.6.gb485710b