net/sched: teql: fix NULL pointer dereference in iptunnel_xmit on TEQL slave xmit

teql_master_xmit() calls netdev_start_xmit(skb, slave) to transmit
through slave devices, but does not update skb->dev to the slave device
beforehand.

When a gretap tunnel is a TEQL slave, the transmit path reaches
iptunnel_xmit() which saves dev = skb->dev (still pointing to teql0
master) and later calls iptunnel_xmit_stats(dev, pkt_len). This
function does:

    get_cpu_ptr(dev->tstats)

Since teql_master_setup() does not set dev->pcpu_stat_type to
NETDEV_PCPU_STAT_TSTATS, the core network stack never allocates tstats
for teql0, so dev->tstats is NULL. get_cpu_ptr(NULL) computes
NULL + __per_cpu_offset[cpu], resulting in a page fault.

 BUG: unable to handle page fault for address: ffff8880e6659018
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 68bc067 P4D 68bc067 PUD 0
 Oops: Oops: 0002 [#1] SMP KASAN PTI
 RIP: 0010:iptunnel_xmit (./include/net/ip_tunnels.h:664 net/ipv4/ip_tunnel_core.c:89)
 Call Trace:
  <TASK>
  ip_tunnel_xmit (net/ipv4/ip_tunnel.c:847)
  __gre_xmit (net/ipv4/ip_gre.c:478)
  gre_tap_xmit (net/ipv4/ip_gre.c:779)
  teql_master_xmit (net/sched/sch_teql.c:319)
  dev_hard_start_xmit (net/core/dev.c:3887)
  sch_direct_xmit (net/sched/sch_generic.c:347)
  __dev_queue_xmit (net/core/dev.c:4802)
  neigh_direct_output (net/core/neighbour.c:1660)
  ip_finish_output2 (net/ipv4/ip_output.c:237)
  __ip_finish_output.part.0 (net/ipv4/ip_output.c:315)
  ip_mc_output (net/ipv4/ip_output.c:369)
  ip_send_skb (net/ipv4/ip_output.c:1508)
  udp_send_skb (net/ipv4/udp.c:1195)
  udp_sendmsg (net/ipv4/udp.c:1485)
  inet_sendmsg (net/ipv4/af_inet.c:859)
  __sys_sendto (net/socket.c:2206)

Fix this by setting skb->dev = slave before calling
netdev_start_xmit(), so that tunnel xmit functions see the correct
slave device with properly allocated tstats.

Fixes: 039f506 ("ip_tunnel: Move stats update to iptunnel_xmit()")
Reported-by: Xiang Mei <[email protected]>
Signed-off-by: Weiming Shi <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: winmin

net/sched/sch_teql.c

@@ -315,6 +315,7 @@ static netdev_tx_t teql_master_xmit(struct sk_buff *skb, struct net_device *dev)
 			if (__netif_tx_trylock(slave_txq)) {
 				unsigned int length = qdisc_pkt_len(skb);
 
+				skb->dev = slave;
 				if (!netif_xmit_frozen_or_stopped(slave_txq) &&
 				    netdev_start_xmit(skb, slave, slave_txq, false) ==
 				    NETDEV_TX_OK) {