[PATCH v4 1/2] netfilter: Terminate rule eval if protocol=IPv6 and ipv6 module is disabled

[Leonardo Bras <leonardo@linux.ibm.com>]

If IPv6 is disabled on boot (ipv6.disable=1), but nft_fib_inet ends up
dealing with a IPv6 packet, it causes a kernel panic in
fib6_node_lookup_1(), crashing in bad_page_fault.

The panic is caused by trying to deference a very low address (0x38
in ppc64le), due to ipv6.fib6_main_tbl = NULL.
BUG: Kernel NULL pointer dereference at 0x00000038

The kernel panic was reproduced in a host that disabled IPv6 on boot and
have to process guest packets (coming from a bridge) using it's ip6tables.

Terminate rule evaluation when packet protocol is IPv6 but the ipv6 module
is not loaded.

Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
---
 net/netfilter/nft_fib_netdev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/netfilter/nft_fib_netdev.c b/net/netfilter/nft_fib_netdev.c
index 2cf3f32fe6d2..a2e726ae7f07 100644
--- a/net/netfilter/nft_fib_netdev.c
+++ b/net/netfilter/nft_fib_netdev.c
@@ -14,6 +14,7 @@
 #include <linux/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables_core.h>
 #include <net/netfilter/nf_tables.h>
+#include <net/ipv6.h>
 
 #include <net/netfilter/nft_fib.h>
 
@@ -34,6 +35,8 @@ static void nft_fib_netdev_eval(const struct nft_expr *expr,
 		}
 		break;
 	case ETH_P_IPV6:
+		if (!ipv6_mod_enabled())
+			break;
 		switch (priv->result) {
 		case NFT_FIB_RESULT_OIF:
 		case NFT_FIB_RESULT_OIFNAME:
-- 
2.20.1