bpf: Enforce BPF ringbuf size to be the power of 2

BPF ringbuf assumes the size to be a multiple of page size and the power of
2 value. The latter is important to avoid division while calculating position
inside the ring buffer and using (N-1) mask instead. This patch fixes omission
to enforce power-of-2 size rule.

Fixes: 457f4436 ("bpf: Implement BPF ring buffer and verifier support for it")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200630061500.1804799-1-andriin@fb.com

kernel/bpf/ringbuf.c

@@ -132,15 +132,6 @@ static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node)
 {
 	struct bpf_ringbuf *rb;
 
-	if (!data_sz || !PAGE_ALIGNED(data_sz))
-		return ERR_PTR(-EINVAL);
-
-#ifdef CONFIG_64BIT
-	/* on 32-bit arch, it's impossible to overflow record's hdr->pgoff */
-	if (data_sz > RINGBUF_MAX_DATA_SZ)
-		return ERR_PTR(-E2BIG);
-#endif
-
 	rb = bpf_ringbuf_area_alloc(data_sz, numa_node);
 	if (!rb)
 		return ERR_PTR(-ENOMEM);
@@ -166,9 +157,16 @@ static struct bpf_map *ringbuf_map_alloc(union bpf_attr *attr)
 		return ERR_PTR(-EINVAL);
 
 	if (attr->key_size || attr->value_size ||
-	    attr->max_entries == 0 || !PAGE_ALIGNED(attr->max_entries))
+	    !is_power_of_2(attr->max_entries) ||
+	    !PAGE_ALIGNED(attr->max_entries))
 		return ERR_PTR(-EINVAL);
 
+#ifdef CONFIG_64BIT
+	/* on 32-bit arch, it's impossible to overflow record's hdr->pgoff */
+	if (attr->max_entries > RINGBUF_MAX_DATA_SZ)
+		return ERR_PTR(-E2BIG);
+#endif
+
 	rb_map = kzalloc(sizeof(*rb_map), GFP_USER);
 	if (!rb_map)
 		return ERR_PTR(-ENOMEM);