netlink: Fix spectre v1 gadget in netlink_create()

BugLink: https://bugs.launchpad.net/bugs/1791953

[ Upstream commit bc5b6c0b ]

'protocol' is a user-controlled value, so sanitize it after the bounds
check to avoid using it for speculative out-of-bounds access to arrays
indexed by it.

This addresses the following accesses detected with the help of smatch:

* net/netlink/af_netlink.c:654 __netlink_create() warn: potential
  spectre issue 'nlk_cb_mutex_keys' [w]

* net/netlink/af_netlink.c:654 __netlink_create() warn: potential
  spectre issue 'nlk_cb_mutex_key_strings' [w]

* net/netlink/af_netlink.c:685 netlink_create() warn: potential spectre
  issue 'nl_table' [w] (local cap)

Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/netlink/af_netlink.c

@@ -62,6 +62,7 @@
 #include <asm/cacheflush.h>
 #include <linux/hash.h>
 #include <linux/genetlink.h>
+#include <linux/nospec.h>
 
 #include <net/net_namespace.h>
 #include <net/sock.h>
@@ -654,6 +655,7 @@ static int netlink_create(struct net *net, struct socket *sock, int protocol,
 
 	if (protocol < 0 || protocol >= MAX_LINKS)
 		return -EPROTONOSUPPORT;
+	protocol = array_index_nospec(protocol, MAX_LINKS);
 
 	netlink_lock_table();
 #ifdef CONFIG_MODULES