[PATCH net v4] 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 a mode change (CAP_NET_ADMIN) concurrent with
transmit on the team device.

 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)

The original code assumed that no ports means no traffic, so mode
changes could freely memset()/memcpy() the ops.  AF_PACKET with
forced carrier breaks that assumption.

Prevent the race instead of making it safe: replace memset()/memcpy()
with per-field updates that never touch transmit or receive.  Those
two handlers are managed solely by team_adjust_ops(), which already
installs dummies when tx_en_port_count == 0 (always true during mode
change since no ports are present).  WRITE_ONCE/READ_ONCE prevent
store/load tearing on the handler pointers.

synchronize_net() before exit_op() drains in-flight readers that may
still reference old mode state from before port removal switched the
handlers to dummies.

Fixes: 3d249d4ca7d0 ("net: introduce ethernet teaming device")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
v4:
 - Prevent the race instead of making it safe, as suggested by
   Jakub: never touch transmit/receive during mode change, let
   team_adjust_ops() manage them based on port count.
 - Drop smp_store_release()/smp_load_acquire() in favor of
   WRITE_ONCE/READ_ONCE.

v3:
 - Clarify barrier ordering in commit message.
 - Drop AF_PACKET mention from commit message and code comment.

v2:
 - Move fix from data path (reader-side NULL fallback) to
   configuration path (writer-side per-field updates), as suggested
   by the reviewer.
 - Use smp_store_release()/smp_load_acquire() instead of plain
   stores/loads for proper ordering on weakly-ordered architectures.
 - Add synchronize_net() before exit_op() to drain in-flight readers
   and prevent use-after-free of mode private state.

 drivers/net/team/team_core.c | 45 +++++++++++++++++++++++++-----------
 1 file changed, 32 insertions(+), 13 deletions(-)

diff --git a/drivers/net/team/team_core.c b/drivers/net/team/team_core.c
index 0c87f9972457..89a3f71b7a71 100644
--- a/drivers/net/team/team_core.c
+++ b/drivers/net/team/team_core.c
@@ -534,21 +534,23 @@ static void team_adjust_ops(struct team *team)
 
 	if (!team->tx_en_port_count || !team_is_mode_set(team) ||
 	    !team->mode->ops->transmit)
-		team->ops.transmit = team_dummy_transmit;
+		WRITE_ONCE(team->ops.transmit, team_dummy_transmit);
 	else
-		team->ops.transmit = team->mode->ops->transmit;
+		WRITE_ONCE(team->ops.transmit, team->mode->ops->transmit);
 
 	if (!team->rx_en_port_count || !team_is_mode_set(team) ||
 	    !team->mode->ops->receive)
-		team->ops.receive = team_dummy_receive;
+		WRITE_ONCE(team->ops.receive, team_dummy_receive);
 	else
-		team->ops.receive = team->mode->ops->receive;
+		WRITE_ONCE(team->ops.receive, team->mode->ops->receive);
 }
 
 /*
- * We can benefit from the fact that it's ensured no port is present
- * at the time of mode change. Therefore no packets are in fly so there's no
- * need to set mode operations in any special way.
+ * team_change_mode() ensures no ports are present during mode change,
+ * but lockless readers can still reach team_xmit().  Avoid touching
+ * transmit/receive -- they are already set to dummies by
+ * team_adjust_ops() since no ports are enabled.  synchronize_net()
+ * drains in-flight readers before destroying old mode state.
  */
 static int __team_change_mode(struct team *team,
 			      const struct team_mode *new_mode)
@@ -557,9 +559,21 @@ static int __team_change_mode(struct team *team,
 	if (team_is_mode_set(team)) {
 		void (*exit_op)(struct team *team) = team->ops.exit;
 
-		/* Clear ops area so no callback is called any longer */
-		memset(&team->ops, 0, sizeof(struct team_mode_ops));
-		team_adjust_ops(team);
+		/* Clear cold-path ops used only under RTNL.  transmit and
+		 * receive are already dummies (no ports) so leave them
+		 * alone -- overwriting them is the source of the race.
+		 */
+		team->ops.init = NULL;
+		team->ops.exit = NULL;
+		team->ops.port_enter = NULL;
+		team->ops.port_leave = NULL;
+		team->ops.port_change_dev_addr = NULL;
+		team->ops.port_tx_disabled = NULL;
+
+		/* Wait for in-flight readers before tearing down mode
+		 * state they may reference.
+		 */
+		synchronize_net();
 
 		if (exit_op)
 			exit_op(team);
@@ -582,7 +596,12 @@ static int __team_change_mode(struct team *team,
 	}
 
 	team->mode = new_mode;
-	memcpy(&team->ops, new_mode->ops, sizeof(struct team_mode_ops));
+	team->ops.init = new_mode->ops->init;
+	team->ops.exit = new_mode->ops->exit;
+	team->ops.port_enter = new_mode->ops->port_enter;
+	team->ops.port_leave = new_mode->ops->port_leave;
+	team->ops.port_change_dev_addr = new_mode->ops->port_change_dev_addr;
+	team->ops.port_tx_disabled = new_mode->ops->port_tx_disabled;
 	team_adjust_ops(team);
 
 	return 0;
@@ -743,7 +762,7 @@ static rx_handler_result_t team_handle_frame(struct sk_buff **pskb)
 		/* allow exact match delivery for disabled ports */
 		res = RX_HANDLER_EXACT;
 	} else {
-		res = team->ops.receive(team, port, skb);
+		res = READ_ONCE(team->ops.receive)(team, port, skb);
 	}
 	if (res == RX_HANDLER_ANOTHER) {
 		struct team_pcpu_stats *pcpu_stats;
@@ -1845,7 +1864,7 @@ static netdev_tx_t team_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	tx_success = team_queue_override_transmit(team, skb);
 	if (!tx_success)
-		tx_success = team->ops.transmit(team, skb);
+		tx_success = READ_ONCE(team->ops.transmit)(team, skb);
 	if (tx_success) {
 		struct team_pcpu_stats *pcpu_stats;
 
-- 
2.43.0