can: af_can: 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/can/af_can.c:115 can_get_proto() warn: potential spectre issue 'proto_tab' [w]

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=2

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/can/af_can.c

@@ -59,6 +59,7 @@
 #include <linux/can/core.h>
 #include <linux/can/skb.h>
 #include <linux/ratelimit.h>
+#include <linux/nospec.h>
 #include <net/net_namespace.h>
 #include <net/sock.h>
 
@@ -136,6 +137,7 @@ static int can_create(struct net *net, struct socket *sock, int protocol,
 
 	if (protocol < 0 || protocol >= CAN_NPROTO)
 		return -EINVAL;
+	protocol = array_index_nospec(protocol, CAN_NPROTO);
 
 	cp = can_get_proto(protocol);