[PATCH v2 3/6] KVM: x86: Manually check DR4/5 write values to fix SVM intercept priority

[Sean Christopherson <seanjc@google.com>]

Manually (pre)check the values being written to DR4/5, i.e. the DR6/DR7
aliases, instead of relying on ->set_dr() => kvm_set_dr() to signal a #GP.
SVM unfortunately prioritizes all exceptions over an instruction intercept,
i.e. nSVM is relying on the emulator to perform *all* exception checks
prior to attempting to execute the instruction.

Fixes: 3b88e41a4134 ("KVM: SVM: Add intercept check for accessing dr registers")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/emulate.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 4484c5fa19e3..a1bccab0eefe 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -3853,15 +3853,23 @@ static int check_dr_read(struct x86_emulate_ctxt *ctxt)
 static int check_dr_write(struct x86_emulate_ctxt *ctxt)
 {
 	u64 new_val = ctxt->src.val64;
-	int dr = ctxt->modrm_reg;
 	int rc;
 
 	rc = check_dr_read(ctxt);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
 
-	if ((dr == 6 || dr == 7) && (new_val & 0xffffffff00000000ULL))
-		return emulate_gp(ctxt, 0);
+	switch (ctxt->modrm_reg) {
+	case 4:
+	case 5:
+	case 6:
+	case 7:
+		if (new_val & 0xffffffff00000000ULL)
+			return emulate_gp(ctxt, 0);
+		break;
+	default:
+		break;
+	}
 
 	return X86EMUL_CONTINUE;
 }
-- 
2.54.0.1032.g2f8565e1d1-goog