ipv6: do not disable temp_address when reaching max_address

A LAN user can remotely disable temporary address which may lead to privacy violatins and information disclosure.

The reason is that the linux kernel uses the 'ipv6.max_addresses' option to specify how many ipv6 addresses and interface may have. The 'ipv6.regen_max_retry' (default value 3) option specifies how many times the kernel will try to create a new address.

But the kernel is not distinguish between the event of reaching max_addresses for an interface and failing to generate a new address. the kernel disable the temporary address after regenerate a new address 'regen_max_retry' times.

According RFC4941 3.3.7:

---------------------------------------

If DAD indicates the address is already in use,
the node must generate a new randomized interface
identifier as described in section 3.2 above, and
repeat the previous steps as appropriate up to
TEMP_IDGEN_RETRIES times.

If after TEMP_IDGEN_RETRIES consecutive attempts no non-unique address was generated, the node must log a system error and must not attempt to generate temporary address for that interface.

------------------------------------------

RFC4941 3.3.7 specifies that disabling the temp_address must happen upon the address is already in use, not reach the max_address, So we have to check the return err and distinguish the correct retry path.

This fixes CVE-2013-0343

---
 net/ipv6/addrconf.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

index da4241c..7b55464 100644
@@ -1134,10 +1134,28 @@ retry:
 	if (IS_ERR_OR_NULL(ift)) {
 		in6_ifa_put(ifp);
 		in6_dev_put(idev);
-		pr_info("%s: retry temporary address regeneration\n", __func__);
-		tmpaddr = &addr;
-		write_lock(&idev->lock);
-		goto retry;
+
+		/* According RFC4941 3.3.7:
+		 * If DAD indicates the address is already in use,
+		 * the node must generate a new randomized interface
+		 * identifier as described in section 3.2 above, and
+		 * repeat the previous steps as appropriate up to
+		 * TEMP_IDGEN_RETRIES times.
+		 * If after TEMP_IDGEN_RETRIES consecutive attempts no
+		 * non-unique address was generated, the node must log
+		 * a system error and must not attempt to generate
+		 * temporary address for that interface.
+		 * So we have to check the return err and distinguish
+		 * the correct retry path.
+		 */
+		if (PTR_ERR(ift) == -EEXIST) {
+			pr_info("%s: retry temporary address regeneration\n", __func__);
+			tmpaddr = &addr;
+			write_lock(&idev->lock);
+			goto retry;
+		}
+		/* do not retry if the err code is not -EEXIST */
+		goto out;
 	}
 
 	spin_lock_bh(&ift->lock);

1.8.2.1