Bluetooth: hci_core: Prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "dl" variable is a pointer to "struct hci_dev_list_req" and this
structure ends in a flexible array:

struct hci_dev_list_req {
[...]
struct hci_dev_req dev_req[]; /* hci_dev_req structures */
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + count * size" in
the kzalloc() and copy_to_user() functions.

At the same time, prepare for the coming implementation by GCC and Clang
of the __counted_by attribute. Flexible array members annotated with
__counted_by can have their accesses bounds-checked at run-time via
CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
strcpy/memcpy-family functions).

In this case, it is important to note that the logic needs a little
refactoring to ensure that the "dev_num" member is initialized before
the first access to the flex array. Specifically, add the assignment
before the list_for_each_entry() loop.

Also remove the "size" variable as it is no longer needed.

This way, the code is more readable and safer.

This code was detected with the help of Coccinelle, and audited and
modified manually.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160 [2]
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Erick Archer <[e-mail]>
Signed-off-by: Luiz Augusto von Dentz <[e-mail]>

include/net/bluetooth/hci_sock.h

···
 
 struct hci_dev_list_req {
 	__u16  dev_num;
-	struct hci_dev_req dev_req[];	/* hci_dev_req structures */
+	struct hci_dev_req dev_req[] __counted_by(dev_num);
 };
 
 struct hci_conn_list_req {

net/bluetooth/hci_core.c

···
 	struct hci_dev *hdev;
 	struct hci_dev_list_req *dl;
 	struct hci_dev_req *dr;
-	int n = 0, size, err;
+	int n = 0, err;
 	__u16 dev_num;
 
 	if (get_user(dev_num, (__u16 __user *) arg))
···
 	if (!dev_num || dev_num > (PAGE_SIZE * 2) / sizeof(*dr))
 		return -EINVAL;
 
-	size = sizeof(*dl) + dev_num * sizeof(*dr);
-
-	dl = kzalloc(size, GFP_KERNEL);
+	dl = kzalloc(struct_size(dl, dev_req, dev_num), GFP_KERNEL);
 	if (!dl)
 		return -ENOMEM;
 
+	dl->dev_num = dev_num;
 	dr = dl->dev_req;
 
 	read_lock(&hci_dev_list_lock);
···
 	read_unlock(&hci_dev_list_lock);
 
 	dl->dev_num = n;
-	size = sizeof(*dl) + n * sizeof(*dr);
-
-	err = copy_to_user(arg, dl, size);
+	err = copy_to_user(arg, dl, struct_size(dl, dev_req, n));
 	kfree(dl);
 
 	return err ? -EFAULT : 0;