drm/nouveau: validate vbios size

Without checking, we could detect vbios size as 0, allocate 0-byte array
(kmalloc returns invalid pointer for such allocation) and crash in
nouveau_bios_score while checking for vbios signature.
Reported-by: Heinz Diehl <htd@fritha.org>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/subdev/bios/base.c

@@ -72,7 +72,7 @@ nouveau_bios_shadow_of(struct nouveau_bios *bios)
 	}
 
 	data = of_get_property(dn, "NVDA,BMP", &size);
-	if (data) {
+	if (data && size) {
 		bios->size = size;
 		bios->data = kmalloc(bios->size, GFP_KERNEL);
 		if (bios->data)
@@ -104,6 +104,9 @@ nouveau_bios_shadow_pramin(struct nouveau_bios *bios)
 		goto out;
 
 	bios->size = nv_rd08(bios, 0x700002) * 512;
+	if (!bios->size)
+		goto out;
+
 	bios->data = kmalloc(bios->size, GFP_KERNEL);
 	if (bios->data) {
 		for (i = 0; i < bios->size; i++)
@@ -155,6 +158,9 @@ nouveau_bios_shadow_prom(struct nouveau_bios *bios)
 
 	/* read entire bios image to system memory */
 	bios->size = nv_rd08(bios, 0x300002) * 512;
+	if (!bios->size)
+		goto out;
+
 	bios->data = kmalloc(bios->size, GFP_KERNEL);
 	if (bios->data) {
 		for (i = 0; i < bios->size; i++)
@@ -196,6 +202,8 @@ nouveau_bios_shadow_acpi(struct nouveau_bios *bios)
 	bios->size = 0;
 	if (nouveau_acpi_get_bios_chunk(data, 0, 3) == 3)
 		bios->size = data[2] * 512;
+	if (!bios->size)
+		return;
 
 	bios->data = kmalloc(bios->size, GFP_KERNEL);
 	for (i = 0; bios->data && i < bios->size; i += cnt) {
@@ -231,12 +239,14 @@ nouveau_bios_shadow_pci(struct nouveau_bios *bios)
 static int
 nouveau_bios_score(struct nouveau_bios *bios, const bool writeable)
 {
-	if (!bios->data || bios->data[0] != 0x55 || bios->data[1] != 0xAA) {
+	if (bios->size < 3 || !bios->data || bios->data[0] != 0x55 ||
+			bios->data[1] != 0xAA) {
 		nv_info(bios, "... signature not found\n");
 		return 0;
 	}
 
-	if (nvbios_checksum(bios->data, bios->data[2] * 512)) {
+	if (nvbios_checksum(bios->data,
+			min_t(u32, bios->data[2] * 512, bios->size))) {
 		nv_info(bios, "... checksum invalid\n");
 		/* if a ro image is somewhat bad, it's probably all rubbish */
 		return writeable ? 2 : 1;