[PATCH net] net: team: fix NULL pointer dereference in team_xmit during mode change

[Weiming Shi <bestswngs@gmail.com>]

__team_change_mode() clears team->ops with memset() before restoring
safe dummy handlers via team_adjust_ops(). A concurrent team_xmit()
running under RCU on another CPU can read team->ops.transmit during
this window and call a NULL function pointer, crashing the kernel.

The race requires CAP_NET_ADMIN (in init_user_ns) to trigger via
TEAM_CMD_OPTIONS_SET, plus AF_PACKET sendto() on a team device with
forced carrier and no ports.

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 Oops: 0010 [#1] SMP KASAN NOPTI
 RIP: 0010:0x0
 Call Trace:
  team_xmit (drivers/net/team/team_core.c:1853)
  dev_hard_start_xmit (net/core/dev.c:3904)
  __dev_queue_xmit (net/core/dev.c:4871)
  packet_sendmsg (net/packet/af_packet.c:3109)
  __sys_sendto (net/socket.c:2265)

Fix this by reading team->ops.transmit with READ_ONCE() into a local
variable and falling back to team_dummy_transmit if NULL. This matches
what team_adjust_ops() would have installed moments later.

Fixes: 3d249d4ca7d0 ("net: introduce ethernet teaming device")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 drivers/net/team/team_core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/team/team_core.c b/drivers/net/team/team_core.c
index 0c87f9972457..3ff08b5deccd 100644
--- a/drivers/net/team/team_core.c
+++ b/drivers/net/team/team_core.c
@@ -1844,8 +1844,14 @@ static netdev_tx_t team_xmit(struct sk_buff *skb, struct net_device *dev)
 	unsigned int len = skb->len;
 
 	tx_success = team_queue_override_transmit(team, skb);
-	if (!tx_success)
-		tx_success = team->ops.transmit(team, skb);
+	if (!tx_success) {
+		bool (*transmit)(struct team *team, struct sk_buff *skb);
+
+		transmit = READ_ONCE(team->ops.transmit);
+		if (unlikely(!transmit))
+			transmit = team_dummy_transmit;
+		tx_success = transmit(team, skb);
+	}
 	if (tx_success) {
 		struct team_pcpu_stats *pcpu_stats;
 
-- 
2.43.0