KVM: s390: fix sthyi error handling

Commit 9fb6c9b3 ("s390/sthyi: add cache to store hypervisor info")
added cache handling for store hypervisor info. This also changed the
possible return code for sthyi_fill().

Instead of only returning a condition code like the sthyi instruction would
do, it can now also return a negative error value (-ENOMEM). handle_styhi()
was not changed accordingly. In case of an error, the negative error value
would incorrectly injected into the guest PSW.

Add proper error handling to prevent this, and update the comment which
describes the possible return values of sthyi_fill().

Fixes: 9fb6c9b3 ("s390/sthyi: add cache to store hypervisor info")
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Link: https://lore.kernel.org/r/20230727182939.2050744-1-hca@linux.ibm.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/sthyi.c

@@ -459,9 +459,9 @@ static int sthyi_update_cache(u64 *rc)
  *
  * Fills the destination with system information returned by the STHYI
  * instruction. The data is generated by emulation or execution of STHYI,
- * if available. The return value is the condition code that would be
- * returned, the rc parameter is the return code which is passed in
- * register R2 + 1.
+ * if available. The return value is either a negative error value or
+ * the condition code that would be returned, the rc parameter is the
+ * return code which is passed in register R2 + 1.
  */
 int sthyi_fill(void *dst, u64 *rc)
 {

arch/s390/kvm/intercept.c

@@ -389,8 +389,8 @@ static int handle_partial_execution(struct kvm_vcpu *vcpu)
  */
 int handle_sthyi(struct kvm_vcpu *vcpu)
 {
-	int reg1, reg2, r = 0;
-	u64 code, addr, cc = 0, rc = 0;
+	int reg1, reg2, cc = 0, r = 0;
+	u64 code, addr, rc = 0;
 	struct sthyi_sctns *sctns = NULL;
 
 	if (!test_kvm_facility(vcpu->kvm, 74))
@@ -421,7 +421,10 @@ int handle_sthyi(struct kvm_vcpu *vcpu)
 		return -ENOMEM;
 
 	cc = sthyi_fill(sctns, &rc);
-
+	if (cc < 0) {
+		free_page((unsigned long)sctns);
+		return cc;
+	}
 out:
 	if (!cc) {
 		if (kvm_s390_pv_cpu_is_protected(vcpu)) {