Bluetooth: hci_sock: Correctly bounds check and pad HCI_MON_NEW_INDEX name

The code pattern of memcpy(dst, src, strlen(src)) is almost always
wrong. In this case it is wrong because it leaves memory uninitialized
if it is less than sizeof(ni->name), and overflows ni->name when longer.

Normally strtomem_pad() could be used here, but since ni->name is a
trailing array in struct hci_mon_new_index, compilers that don't support
-fstrict-flex-arrays=3 can't tell how large this array is via
__builtin_object_size(). Instead, open-code the helper and use sizeof()
since it will work correctly.

Additionally mark ni->name as __nonstring since it appears to not be a
%NUL terminated C string.

Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Edward AD <twuufnxlz@gmail.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-bluetooth@vger.kernel.org
Cc: netdev@vger.kernel.org
Fixes: 18f547f3 ("Bluetooth: hci_sock: fix slab oob read in create_monitor_event")
Link: https://lore.kernel.org/lkml/202310110908.F2639D3276@keescook/Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

include/net/bluetooth/hci_mon.h

@@ -56,7 +56,7 @@ struct hci_mon_new_index {
 	__u8		type;
 	__u8		bus;
 	bdaddr_t	bdaddr;
-	char		name[8];
+	char		name[8] __nonstring;
 } __packed;
 #define HCI_MON_NEW_INDEX_SIZE 16
 

net/bluetooth/hci_sock.c

@@ -488,7 +488,8 @@ static struct sk_buff *create_monitor_event(struct hci_dev *hdev, int event)
 		ni->type = hdev->dev_type;
 		ni->bus = hdev->bus;
 		bacpy(&ni->bdaddr, &hdev->bdaddr);
-		memcpy(ni->name, hdev->name, strlen(hdev->name));
+		memcpy_and_pad(ni->name, sizeof(ni->name), hdev->name,
+			       strnlen(hdev->name, sizeof(ni->name)), '\0');
 
 		opcode = cpu_to_le16(HCI_MON_NEW_INDEX);
 		break;