diff -urN mesa-18.1.2.orig/src/gallium/winsys/sw/kms-dri/kms_dri_sw_winsys.c mesa-18.1.2/src/gallium/winsys/sw/kms-dri/kms_dri_sw_winsys.c
--- mesa-18.1.2.orig/src/gallium/winsys/sw/kms-dri/kms_dri_sw_winsys.c	2018-07-04 12:45:31.057803594 +0200
+++ mesa-18.1.2/src/gallium/winsys/sw/kms-dri/kms_dri_sw_winsys.c	2018-07-04 12:47:02.030220461 +0200
@@ -368,10 +368,14 @@
    DEBUG_PRINT("KMS-DEBUG: unmapped buffer %u (was %p)\n", kms_sw_dt->handle, kms_sw_dt->mapped);
    DEBUG_PRINT("KMS-DEBUG: unmapped buffer %u (was %p)\n", kms_sw_dt->handle, kms_sw_dt->ro_mapped);
 
-   munmap(kms_sw_dt->mapped, kms_sw_dt->size);
-   kms_sw_dt->mapped = NULL;
-   munmap(kms_sw_dt->ro_mapped, kms_sw_dt->size);
-   kms_sw_dt->ro_mapped = NULL;
+   if (kms_sw_dt->mapped != NULL) {
+      munmap(kms_sw_dt->mapped, kms_sw_dt->size);
+      kms_sw_dt->mapped = NULL;
+   }
+   if (kms_sw_dt->ro_mapped != NULL) {
+      munmap(kms_sw_dt->ro_mapped, kms_sw_dt->size);
+      kms_sw_dt->ro_mapped = NULL;
+   }
 }
 
 static struct sw_displaytarget *