phonet: af_phonet: Fix Spectre v1 vulnerability

protocol 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/phonet/af_phonet.c:48 phonet_proto_get() warn: potential spectre issue 'proto_tab' [w] (local cap)

Fix this by sanitizing protocol 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/phonet/af_phonet.c

@@ -34,6 +34,8 @@
 #include <net/phonet/phonet.h>
 #include <net/phonet/pn_dev.h>
 
+#include <linux/nospec.h>
+
 /* Transport protocol registration */
 static const struct phonet_protocol *proto_tab[PHONET_NPROTO] __read_mostly;
 
@@ -43,6 +45,7 @@ static const struct phonet_protocol *phonet_proto_get(unsigned int protocol)
 
 	if (protocol >= PHONET_NPROTO)
 		return NULL;
+	protocol = array_index_nospec(protocol, PHONET_NPROTO);
 
 	rcu_read_lock();
 	pp = rcu_dereference(proto_tab[protocol]);