[PATCH] netlink: Fix spectre v1 gadget in netlink_create()

[Jeremy Cline]

'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@xxxxxxxxxx>
Signed-off-by: Jeremy Cline <jcline@xxxxxxxxxx>
---
 net/netlink/af_netlink.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 393573a99a5a..59dac45ad452 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -63,6 +63,7 @@
 #include <linux/hash.h>
 #include <linux/genetlink.h>
 #include <linux/net_namespace.h>
+#include <linux/nospec.h>
 
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
@@ -679,6 +680,7 @@ 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
-- 
2.17.1