nfc: Fix the sockaddr length sanitization in llcp_sock_connect

Fix the sockaddr length verification in the connect() handler of NFC/LLCP
sockets, to compare against the size of the actual structure expected on
input (sockaddr_nfc_llcp) instead of its shorter version (sockaddr_nfc).

Both structures are defined in include/uapi/linux/nfc.h. The fields
specific to the _llcp extended struct are as follows:

   276		__u8 dsap; /* Destination SAP, if known */
   277		__u8 ssap; /* Source SAP to be bound to */
   278		char service_name[NFC_LLCP_MAX_SERVICE_NAME]; /* Service name URI */;
   279		size_t service_name_len;

If the caller doesn't provide a sufficiently long sockaddr buffer, these
fields remain uninitialized (and they currently originate from the stack
frame of the top-level sys_connect handler). They are then copied by
llcp_sock_connect() into internal storage (nfc_llcp_sock structure), and
could be subsequently read back through the user-mode getsockname()
function (handled by llcp_sock_getname()). This would result in the
disclosure of up to ~70 uninitialized bytes from the kernel stack to
user-mode clients capable of creating AFC_NFC sockets.
Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

net/nfc/llcp_sock.c

@@ -662,8 +662,7 @@ static int llcp_sock_connect(struct socket *sock, struct sockaddr *_addr,
 
 	pr_debug("sock %p sk %p flags 0x%x\n", sock, sk, flags);
 
-	if (!addr || len < sizeof(struct sockaddr_nfc) ||
-	    addr->sa_family != AF_NFC)
+	if (!addr || len < sizeof(*addr) || addr->sa_family != AF_NFC)
 		return -EINVAL;
 
 	if (addr->service_name_len == 0 && addr->dsap == 0)