net: introduce DST_NOPEER dst flag

Chris Boot reported crashes occurring in ipv6_select_ident().

[  461.457562] RIP: 0010:[<ffffffff812dde61>]  [<ffffffff812dde61>]
ipv6_select_ident+0x31/0xa7

[  461.578229] Call Trace:
[  461.580742] <IRQ>
[  461.582870]  [<ffffffff812efa7f>] ? udp6_ufo_fragment+0x124/0x1a2
[  461.589054]  [<ffffffff812dbfe0>] ? ipv6_gso_segment+0xc0/0x155
[  461.595140]  [<ffffffff812700c6>] ? skb_gso_segment+0x208/0x28b
[  461.601198]  [<ffffffffa03f236b>] ? ipv6_confirm+0x146/0x15e
[nf_conntrack_ipv6]
[  461.608786]  [<ffffffff81291c4d>] ? nf_iterate+0x41/0x77
[  461.614227]  [<ffffffff81271d64>] ? dev_hard_start_xmit+0x357/0x543
[  461.620659]  [<ffffffff81291cf6>] ? nf_hook_slow+0x73/0x111
[  461.626440]  [<ffffffffa0379745>] ? br_parse_ip_options+0x19a/0x19a
[bridge]
[  461.633581]  [<ffffffff812722ff>] ? dev_queue_xmit+0x3af/0x459
[  461.639577]  [<ffffffffa03747d2>] ? br_dev_queue_push_xmit+0x72/0x76
[bridge]
[  461.646887]  [<ffffffffa03791e3>] ? br_nf_post_routing+0x17d/0x18f
[bridge]
[  461.653997]  [<ffffffff81291c4d>] ? nf_iterate+0x41/0x77
[  461.659473]  [<ffffffffa0374760>] ? br_flood+0xfa/0xfa [bridge]
[  461.665485]  [<ffffffff81291cf6>] ? nf_hook_slow+0x73/0x111
[  461.671234]  [<ffffffffa0374760>] ? br_flood+0xfa/0xfa [bridge]
[  461.677299]  [<ffffffffa0379215>] ?
nf_bridge_update_protocol+0x20/0x20 [bridge]
[  461.684891]  [<ffffffffa03bb0e5>] ? nf_ct_zone+0xa/0x17 [nf_conntrack]
[  461.691520]  [<ffffffffa0374760>] ? br_flood+0xfa/0xfa [bridge]
[  461.697572]  [<ffffffffa0374812>] ? NF_HOOK.constprop.8+0x3c/0x56
[bridge]
[  461.704616]  [<ffffffffa0379031>] ?
nf_bridge_push_encap_header+0x1c/0x26 [bridge]
[  461.712329]  [<ffffffffa037929f>] ? br_nf_forward_finish+0x8a/0x95
[bridge]
[  461.719490]  [<ffffffffa037900a>] ?
nf_bridge_pull_encap_header+0x1c/0x27 [bridge]
[  461.727223]  [<ffffffffa0379974>] ? br_nf_forward_ip+0x1c0/0x1d4 [bridge]
[  461.734292]  [<ffffffff81291c4d>] ? nf_iterate+0x41/0x77
[  461.739758]  [<ffffffffa03748cc>] ? __br_deliver+0xa0/0xa0 [bridge]
[  461.746203]  [<ffffffff81291cf6>] ? nf_hook_slow+0x73/0x111
[  461.751950]  [<ffffffffa03748cc>] ? __br_deliver+0xa0/0xa0 [bridge]
[  461.758378]  [<ffffffffa037533a>] ? NF_HOOK.constprop.4+0x56/0x56
[bridge]

This is caused by bridge netfilter special dst_entry (fake_rtable), a
special shared entry, where attaching an inetpeer makes no sense.

Problem is present since commit 87c48fa3 (ipv6: make fragment
identifications less predictable)

Introduce DST_NOPEER dst flag and make sure ipv6_select_ident() and
__ip_select_ident() fallback to the 'no peer attached' handling.
Reported-by: Chris Boot <bootc@bootc.net>
Tested-by: Chris Boot <bootc@bootc.net>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -53,6 +53,7 @@ struct dst_entry {
 #define DST_NOHASH		0x0008
 #define DST_NOCACHE		0x0010
 #define DST_NOCOUNT		0x0020
+#define DST_NOPEER		0x0040
 
 	short			error;
 	short			obsolete;

net/bridge/br_netfilter.c

@@ -147,7 +147,7 @@ void br_netfilter_rtable_init(struct net_bridge *br)
 	rt->dst.dev = br->dev;
 	rt->dst.path = &rt->dst;
 	dst_init_metrics(&rt->dst, br_dst_default_metrics, true);
-	rt->dst.flags	= DST_NOXFRM;
+	rt->dst.flags	= DST_NOXFRM | DST_NOPEER;
 	rt->dst.ops = &fake_dst_ops;
 }
 

net/ipv4/route.c

@@ -1367,7 +1367,7 @@ void __ip_select_ident(struct iphdr *iph, struct dst_entry *dst, int more)
 {
 	struct rtable *rt = (struct rtable *) dst;
 
-	if (rt) {
+	if (rt && !(rt->dst.flags & DST_NOPEER)) {
 		if (rt->peer == NULL)
 			rt_bind_peer(rt, rt->rt_dst, 1);
 
@@ -1378,7 +1378,7 @@ void __ip_select_ident(struct iphdr *iph, struct dst_entry *dst, int more)
 			iph->id = htons(inet_getid(rt->peer, more));
 			return;
 		}
-	} else
+	} else if (!rt)
 		printk(KERN_DEBUG "rt_bind_peer(0) @%p\n",
 		       __builtin_return_address(0));
 

net/ipv6/ip6_output.c

@@ -603,7 +603,7 @@ void ipv6_select_ident(struct frag_hdr *fhdr, struct rt6_info *rt)
 	static atomic_t ipv6_fragmentation_id;
 	int old, new;
 
-	if (rt) {
+	if (rt && !(rt->dst.flags & DST_NOPEER)) {
 		struct inet_peer *peer;
 
 		if (!rt->rt6i_peer)