bpf: hashtab: fix 32-bit overflow in memory usage calculation

The intermediate product value_size * num_possible_cpus() is evaluated
in 32-bit arithmetic and only then promoted to 64 bits. On systems with
large value_size and many possible CPUs this can overflow and lead to
an underestimated memory usage.

Cast value_size to u64 before multiplying.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 304849a ("bpf: hashtab memory usage")
Cc: [email protected]
Signed-off-by: Alexei Safin <[email protected]>

Author: Alexei Safin

kernel/bpf/hashtab.c

@@ -2216,7 +2216,7 @@ static u64 htab_map_mem_usage(const struct bpf_map *map)
 		usage += htab->elem_size * num_entries;
 
 		if (percpu)
-			usage += value_size * num_possible_cpus() * num_entries;
+			usage += (u64)value_size * num_possible_cpus() * num_entries;
 		else if (!lru)
 			usage += sizeof(struct htab_elem *) * num_possible_cpus();
 	} else {
@@ -2228,7 +2228,7 @@ static u64 htab_map_mem_usage(const struct bpf_map *map)
 		usage += (htab->elem_size + LLIST_NODE_SZ) * num_entries;
 		if (percpu) {
 			usage += (LLIST_NODE_SZ + sizeof(void *)) * num_entries;
-			usage += value_size * num_possible_cpus() * num_entries;
+			usage += (u64)value_size * num_possible_cpus() * num_entries;
 		}
 	}
 	return usage;