net: bridge: prevent too big nested attributes in br_fill_linkxstats()

After commit ff205bf ("netlink: add one debug check in nla_nest_end()")
syzbot found that br_fill_linkxstats() can send corrupted netlink packets.

Make sure the nested attribute size is bounded.

Fixes: a60c090 ("bridge: netlink: export per-vlan stats")
Reported-by: syzbot+a35f9259d08f907c06e6@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a0b0da3.050a0220.175f0c.0000.GAE@google.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260520114207.1394241-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/bridge/br_netlink.c

@@ -1824,6 +1824,7 @@ static int br_fill_linkxstats(struct sk_buff *skb,
 			      const struct net_device *dev,
 			      int *prividx, int attr)
 {
+	unsigned int limit = U16_MAX - nla_total_size(0);
 	struct nlattr *nla __maybe_unused;
 	struct net_bridge_port *p = NULL;
 	struct net_bridge_vlan_group *vg;
@@ -1841,6 +1842,7 @@ static int br_fill_linkxstats(struct sk_buff *skb,
 		p = br_port_get_rtnl(dev);
 		if (!p)
 			return 0;
+		limit -= nla_total_size_64bit(sizeof(p->stp_xstats));
 		br = p->br;
 		vg = nbp_vlan_group(p);
 		break;
@@ -1855,13 +1857,21 @@ static int br_fill_linkxstats(struct sk_buff *skb,
 	if (vg) {
 		u16 pvid;
 
+#ifdef CONFIG_BRIDGE_IGMP_SNOOPING
+		limit -= nla_total_size_64bit(sizeof(struct br_mcast_stats));
+#endif
 		pvid = br_get_pvid(vg);
 		list_for_each_entry(v, &vg->vlan_list, vlist) {
 			struct bridge_vlan_xstats vxi;
 			struct pcpu_sw_netstats stats;
 
 			if (++vl_idx < *prividx)
 				continue;
+
+			if (skb_tail_pointer(skb) - (unsigned char *)nest +
+			    nla_total_size(sizeof(vxi)) >= limit)
+				goto nla_put_failure;
+
 			memset(&vxi, 0, sizeof(vxi));
 			vxi.vid = v->vid;
 			vxi.flags = v->flags;

net/core/rtnetlink.c

@@ -6328,8 +6328,9 @@ static int rtnl_stats_get(struct sk_buff *skb, struct nlmsghdr *nlh,
 				  NETLINK_CB(skb).portid, nlh->nlmsg_seq, 0,
 				  0, &filters, &idxattr, &prividx, extack);
 	if (err < 0) {
-		/* -EMSGSIZE implies BUG in if_nlmsg_stats_size */
-		WARN_ON(err == -EMSGSIZE);
+		/* -EMSGSIZE implies BUG in if_nlmsg_stats_size
+		 * or a too big nested attribute.
+		 */
 		kfree_skb(nskb);
 	} else {
 		err = rtnl_unicast(nskb, net, NETLINK_CB(skb).portid);