KVM: nSVM: Avoid incorrect injection of SVM_EXIT_CR0_SEL_WRITE

When emulating L2 instructions, svm_check_intercept() checks whether a
write to CR0 should trigger a synthesized #VMEXIT with
SVM_EXIT_CR0_SEL_WRITE. However, it does not check whether L1 enabled
the intercept for SVM_EXIT_WRITE_CR0, which has higher priority
according to the APM (24593—Rev.  3.42—March 2024, Table 15-7):

  When both selective and non-selective CR0-write intercepts are active at
  the same time, the non-selective intercept takes priority. With respect
  to exceptions, the priority of this intercept is the same as the generic
  CR0-write intercept.

Make sure L1 does NOT intercept SVM_EXIT_WRITE_CR0 before checking if
SVM_EXIT_CR0_SEL_WRITE needs to be injected.

Opportunistically tweak the "not CR0" logic to explicitly bail early so
that it's more obvious that only CR0 has a selective intercept, and that
modifying icpt_info.exit_code is functionally necessary so that the call
to nested_svm_exit_handled() checks the correct exit code.

Fixes: cfec82c ("KVM: SVM: Add intercept check for emulated cr accesses")
Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251024192918.3191141-4-yosry.ahmed@linux.dev
[sean: isolate non-CR0 write logic, tweak comments accordingly]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: yosrym93

arch/x86/kvm/svm/svm.c

@@ -4535,15 +4535,29 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
 	case SVM_EXIT_WRITE_CR0: {
 		unsigned long cr0, val;
 
-		if (info->intercept == x86_intercept_cr_write)
+		/*
+		 * Adjust the exit code accordingly if a CR other than CR0 is
+		 * being written, and skip straight to the common handling as
+		 * only CR0 has an additional selective intercept.
+		 */
+		if (info->intercept == x86_intercept_cr_write && info->modrm_reg) {
 			icpt_info.exit_code += info->modrm_reg;
+			break;
+		}
 
-		if (icpt_info.exit_code != SVM_EXIT_WRITE_CR0 ||
-		    info->intercept == x86_intercept_clts)
+		/*
+		 * Convert the exit_code to SVM_EXIT_CR0_SEL_WRITE if a
+		 * selective CR0 intercept is triggered (the common logic will
+		 * treat the selective intercept as being enabled).  Note, the
+		 * unconditional intercept has higher priority, i.e. this is
+		 * only relevant if *only* the selective intercept is enabled.
+		 */
+		if (vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_CR0_WRITE) ||
+		    !(vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_SELECTIVE_CR0)))
 			break;
 
-		if (!(vmcb12_is_intercept(&svm->nested.ctl,
-					INTERCEPT_SELECTIVE_CR0)))
+		/* CLTS never triggers INTERCEPT_SELECTIVE_CR0 */
+		if (info->intercept == x86_intercept_clts)
 			break;
 
 		/* LMSW always triggers INTERCEPT_SELECTIVE_CR0 */