bpf: Fix hashtab overflow check on 32-bit arches

The hashtab code relies on roundup_pow_of_two() to compute the number of
hash buckets, and contains an overflow check by checking if the
resulting value is 0. However, on 32-bit arches, the roundup code itself
can overflow by doing a 32-bit left-shift of an unsigned long value,
which is undefined behaviour, so it is not guaranteed to truncate
neatly. This was triggered by syzbot on the DEVMAP_HASH type, which
contains the same check, copied from the hashtab code. So apply the same
fix to hashtab, by moving the overflow check to before the roundup.

Fixes: daaf427c ("bpf: fix arraymap NULL deref and missing overflow and zero size checks")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Message-ID: <20240307120340.99577-3-toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/hashtab.c

@@ -499,7 +499,13 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
 							  num_possible_cpus());
 	}
 
-	/* hash table size must be power of 2 */
+	/* hash table size must be power of 2; roundup_pow_of_two() can overflow
+	 * into UB on 32-bit arches, so check that first
+	 */
+	err = -E2BIG;
+	if (htab->map.max_entries > 1UL << 31)
+		goto free_htab;
+
 	htab->n_buckets = roundup_pow_of_two(htab->map.max_entries);
 
 	htab->elem_size = sizeof(struct htab_elem) +
@@ -509,10 +515,8 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
 	else
 		htab->elem_size += round_up(htab->map.value_size, 8);
 
-	err = -E2BIG;
-	/* prevent zero size kmalloc and check for u32 overflow */
-	if (htab->n_buckets == 0 ||
-	    htab->n_buckets > U32_MAX / sizeof(struct bucket))
+	/* check for u32 overflow */
+	if (htab->n_buckets > U32_MAX / sizeof(struct bucket))
 		goto free_htab;
 
 	err = bpf_map_init_elem_count(&htab->map);