KVM: s390: Fix gmap_helper_zap_one_page() again

A few checks were missing in gmap_helper_zap_one_page(), which can lead
to memory corruption in the guest under specific circumstances.

Add the missing checks.

Fixes: 5deafa2 ("KVM: s390: Fix to clear PTE when discarding a swapped page")
Cc: stable@vger.kernel.org
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: Claudio Imbrenda

arch/s390/mm/gmap_helpers.c

@@ -47,6 +47,7 @@ static void ptep_zap_softleaf_entry(struct mm_struct *mm, softleaf_t entry)
 void gmap_helper_zap_one_page(struct mm_struct *mm, unsigned long vmaddr)
 {
 	struct vm_area_struct *vma;
+	unsigned long pgstev;
 	spinlock_t *ptl;
 	pgste_t pgste;
 	pte_t *ptep;
@@ -65,9 +66,13 @@ void gmap_helper_zap_one_page(struct mm_struct *mm, unsigned long vmaddr)
 	if (pte_swap(*ptep)) {
 		preempt_disable();
 		pgste = pgste_get_lock(ptep);
+		pgstev = pgste_val(pgste);
 
-		ptep_zap_softleaf_entry(mm, softleaf_from_pte(*ptep));
-		pte_clear(mm, vmaddr, ptep);
+		if ((pgstev & _PGSTE_GPS_USAGE_MASK) == _PGSTE_GPS_USAGE_UNUSED ||
+		    (pgstev & _PGSTE_GPS_ZERO)) {
+			ptep_zap_softleaf_entry(mm, softleaf_from_pte(*ptep));
+			pte_clear(mm, vmaddr, ptep);
+		}
 
 		pgste_set_unlock(ptep, pgste);
 		preempt_enable();