mb/amd/mandolin: handle invalid intr_index values in init_tables

Make sure that the intr_index is valid to avoid out-of-bounds writes to
the fch_pic_routing and fch_apic_routing arrays.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I45ab115f3814b212243c4f6cf706daf77b6ff3b2
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68848
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>

src/mainboard/amd/mandolin/mainboard.c

@@ -69,6 +69,12 @@ static void init_tables(void)
 	memset(fch_apic_routing, PIRQ_NC, sizeof(fch_apic_routing));
 
 	for (i = 0; i < mb_fch_irq_mapping_table_size; i++) {
+		if (mb_irq_map[i].intr_index >= FCH_IRQ_ROUTING_ENTRIES) {
+			printk(BIOS_WARNING,
+			       "Invalid IRQ index %u in FCH IRQ routing table entry %zu\n",
+			       mb_irq_map[i].intr_index, i);
+			continue;
+		}
 		fch_pic_routing[mb_irq_map[i].intr_index] = mb_irq_map[i].pic_irq_num;
 		fch_apic_routing[mb_irq_map[i].intr_index] = mb_irq_map[i].apic_irq_num;
 	}