diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 136329b4027b..b02bdc6c2cd1 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -1991,7 +1991,8 @@ static void map_region(sector_t lba, unsigned int len)
 		block = lba + alignment;
 		rem = do_div(block, granularity);
 
-		set_bit(block, map_storep);
+		if (block < map_size)
+			set_bit(block, map_storep);
 
 		lba += granularity - rem;
 	}
@@ -2011,7 +2012,8 @@ static void unmap_region(sector_t lba, unsigned int len)
 		block = lba + alignment;
 		rem = do_div(block, granularity);
 
-		if (rem == 0 && lba + granularity <= end)
+		if (rem == 0 && lba + granularity <= end &&
+		    block < map_size)
 			clear_bit(block, map_storep);
 
 		lba += granularity - rem;