diff --git a/frontends/monkey/bitmap.c b/frontends/monkey/bitmap.c
index cc13c8b..60104ec 100644
--- a/frontends/monkey/bitmap.c
+++ b/frontends/monkey/bitmap.c
@@ -36,12 +36,16 @@ struct bitmap {
 
 static void *bitmap_create(int width, int height, enum gui_bitmap_flags flags)
 {
+	if (width <= 0 || height <= 0 || width > 8192 || height > 8192 ||
+	    (size_t)width * (size_t)height > 32u * 1024u * 1024u)
+		return NULL;
 	struct bitmap *ret = calloc(1, sizeof(*ret));
 	if (ret == NULL)
 		return NULL;
 
 	ret->width = width;
 	ret->height = height;
+	ret->rowstride = (size_t)width * 4;
 	ret->opaque = (flags & BITMAP_OPAQUE) == BITMAP_OPAQUE;
 
 	ret->ptr = calloc(width, height * 4);