ipv6: sit: fix skb_under_panic with overflowed needed_headroom

When create ipip6 tunnel, if tunnel->parms.link is assigned to the previous
created tunnel device, the dev->needed_headroom will increase based on the
previous one.

If the number of tunnel device is sufficient, the needed_headroom can be
overflowed. The overflow happens like this:

  ipip6_newlink
    ipip6_tunnel_create
      register_netdevice
        ipip6_tunnel_init
          ipip6_tunnel_bind_dev
            t_hlen = tunnel->hlen + sizeof(struct iphdr); // 40
            hlen = tdev->hard_header_len + tdev->needed_headroom; // 65496
            dev->needed_headroom = t_hlen + hlen; // 65536 -> 0

The value of LL_RESERVED_SPACE(rt->dst.dev) may be HH_DATA_MOD, that leads
to a small skb allocated in __ip_append_data(), which triggers a
skb_under_panic:

 ------------[ cut here ]------------
 kernel BUG at net/core/skbuff.c:209!
 Oops: invalid opcode: 0000 [kernel-patches#1] PREEMPT SMP KASAN PTI
 CPU: 0 UID: 0 PID: 23587 Comm: test Tainted: G        W          6.14.0-00624-g2f2d52945852-dirty kernel-patches#15
 Tainted: [W]=WARN
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
 RIP: 0010:skb_panic (net/core/skbuff.c:209 (discriminator 4))
 Call Trace:
  <TASK>
  skb_push (net/core/skbuff.c:2544)
  fou_build_udp (net/ipv4/fou_core.c:1041)
  gue_build_header (net/ipv4/fou_core.c:1085)
  ip_tunnel_xmit (net/ipv4/ip_tunnel.c:780)
  sit_tunnel_xmit__.isra.0 (net/ipv6/sit.c:1065)
  sit_tunnel_xmit (net/ipv6/sit.c:1076)
  dev_hard_start_xmit (net/core/dev.c:3816)
  __dev_queue_xmit (net/core/dev.c:4653)
  neigh_connected_output (net/core/neighbour.c:1543)
  ip_finish_output2 (net/ipv4/ip_output.c:236)
  __ip_finish_output (net/ipv4/ip_output.c:314)
  ip_finish_output (net/ipv4/ip_output.c:324)
  ip_mc_output (net/ipv4/ip_output.c:421)
  ip_send_skb (net/ipv4/ip_output.c:1502)
  udp_send_skb (net/ipv4/udp.c:1197)
  udp_sendmsg (net/ipv4/udp.c:1484)
  udpv6_sendmsg (net/ipv6/udp.c:1545)
  inet6_sendmsg (net/ipv6/af_inet6.c:659)
  ____sys_sendmsg (net/socket.c:2573)
  ___sys_sendmsg (net/socket.c:2629)
  __sys_sendmmsg (net/socket.c:2719)
  __x64_sys_sendmmsg (net/socket.c:2740)
  do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
  </TASK>
 ---[ end trace 0000000000000000 ]---

Fix this by add check for needed_headroom in ipip6_tunnel_bind_dev().

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=4c63f36709a642f801c5
Fixes: c88f8d5 ("sit: update dev->needed_headroom in ipip6_tunnel_bind_dev()")
Signed-off-by: Wang Liang <[email protected]>
Signed-off-by: NipaLocal <nipa@local>

Author: Wang Liang

net/ipv6/sit.c

@@ -1096,7 +1096,7 @@ static netdev_tx_t sit_tunnel_xmit(struct sk_buff *skb,
 
 }
 
-static void ipip6_tunnel_bind_dev(struct net_device *dev)
+static int ipip6_tunnel_bind_dev(struct net_device *dev)
 {
 	struct ip_tunnel *tunnel = netdev_priv(dev);
 	int t_hlen = tunnel->hlen + sizeof(struct iphdr);
@@ -1135,7 +1135,12 @@ static void ipip6_tunnel_bind_dev(struct net_device *dev)
 		WRITE_ONCE(dev->mtu, mtu);
 		hlen = tdev->hard_header_len + tdev->needed_headroom;
 	}
+
+	if (t_hlen + hlen > U16_MAX)
+		return -EOVERFLOW;
+
 	dev->needed_headroom = t_hlen + hlen;
+	return 0;
 }
 
 static void ipip6_tunnel_update(struct ip_tunnel *t,
@@ -1452,7 +1457,9 @@ static int ipip6_tunnel_init(struct net_device *dev)
 	tunnel->dev = dev;
 	strcpy(tunnel->parms.name, dev->name);
 
-	ipip6_tunnel_bind_dev(dev);
+	err = ipip6_tunnel_bind_dev(dev);
+	if (err)
+		return err;
 
 	err = dst_cache_init(&tunnel->dst_cache, GFP_KERNEL);
 	if (err)