nfc: af_nfc: Fix Spectre v1 vulnerability

proto is indirectly controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

net/nfc/af_nfc.c:42 nfc_sock_create() warn: potential spectre issue 'proto_tab' [w] (local cap)

Fix this by sanitizing proto before using it to index proto_tab.

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/nfc/af_nfc.c

@@ -21,6 +21,7 @@
 
 #include <linux/nfc.h>
 #include <linux/module.h>
+#include <linux/nospec.h>
 
 #include "nfc.h"
 
@@ -37,6 +38,7 @@ static int nfc_sock_create(struct net *net, struct socket *sock, int proto,
 
 	if (proto < 0 || proto >= NFC_SOCKPROTO_MAX)
 		return -EINVAL;
+	proto = array_index_nospec(proto, NFC_SOCKPROTO_MAX);
 
 	read_lock(&proto_tab_lock);
 	if (proto_tab[proto] &&	try_module_get(proto_tab[proto]->owner)) {