linux-kernel - [PATCH net v2] team: Move team device type change at the end of team_port

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Message-ID: <20251113211142.245216-1-zlatistiv@gmail.com>
Date: Thu, 13 Nov 2025 23:11:42 +0200
From: "Nikola Z. Ivanov" <zlatistiv@...il.com>
To: jiri@...nulli.us,
	andrew+netdev@...n.ch,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com
Cc: netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	skhan@...uxfoundation.org,
	david.hunter.linux@...il.com,
	khalid@...nel.org,
	linux-kernel-mentees@...ts.linuxfoundation.org,
	"Nikola Z. Ivanov" <zlatistiv@...il.com>,
	syzbot+a2a3b519de727b0f7903@...kaller.appspotmail.com
Subject: [PATCH net v2] team: Move team device type change at the end of team_port_add

Attempting to add a port device that is already up will expectedly fail,
but not before modifying the team device header_ops.

In the case of the syzbot reproducer the gre0 device is
already in state UP when it attempts to add it as a
port device of team0, this fails but before that
header_ops->create of team0 is changed from eth_header to ipgre_header
in the call to team_dev_type_check_change.

Later when we end up in ipgre_header() struct *ip_tunnel points to nonsense
as the private data of the device still holds a struct team.

Example sequence of iproute commands to reproduce the hang/BUG():
ip link add dev team0 type team
ip link add dev gre0 type gre
ip link set dev gre0 up
ip link set dev gre0 master team0
ip link set dev team0 up
ping -I team0 1.1.1.1

Move team_dev_type_check_change down where all other checks have passed
as it changes the dev type with no way to restore it in case
one of the checks that follow it fail.

Also make sure to preserve the origial mtu assignment:
  - If port_dev is not the same type as dev, dev takes mtu from port_dev
  - If port_dev is the same type as dev, port_dev takes mtu from dev

Testing:
  - team device driver in-tree selftests
  - Add/remove various devices as slaves of team device
  - syzbot

Reported-by: syzbot+a2a3b519de727b0f7903@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a2a3b519de727b0f7903
Fixes: 1d76efe1577b ("team: add support for non-ethernet devices")
Signed-off-by: Nikola Z. Ivanov <zlatistiv@...il.com>
---
Changes since v1:
  - Add a "Fixes" tag
  - Add a simple reproducer in the commit log
  https://lore.kernel.org/netdev/20251111171341.4c6d69be@kernel.org/T/#u

 drivers/net/team/team_core.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/drivers/net/team/team_core.c b/drivers/net/team/team_core.c
index 29dc04c299a3..94c149e89231 100644
--- a/drivers/net/team/team_core.c
+++ b/drivers/net/team/team_core.c
@@ -1134,10 +1134,6 @@ static int team_port_add(struct team *team, struct net_device *port_dev,
 		return -EPERM;
 	}
 
-	err = team_dev_type_check_change(dev, port_dev);
-	if (err)
-		return err;
-
 	if (port_dev->flags & IFF_UP) {
 		NL_SET_ERR_MSG(extack, "Device is up. Set it down before adding it as a team port");
 		netdev_err(dev, "Device %s is up. Set it down before adding it as a team port\n",
@@ -1155,10 +1151,12 @@ static int team_port_add(struct team *team, struct net_device *port_dev,
 	INIT_LIST_HEAD(&port->qom_list);
 
 	port->orig.mtu = port_dev->mtu;
-	err = dev_set_mtu(port_dev, dev->mtu);
-	if (err) {
-		netdev_dbg(dev, "Error %d calling dev_set_mtu\n", err);
-		goto err_set_mtu;
+	if (dev->type == port_dev->type) {
+		err = dev_set_mtu(port_dev, dev->mtu);
+		if (err) {
+			netdev_dbg(dev, "Error %d calling dev_set_mtu\n", err);
+			goto err_set_mtu;
+		}
 	}
 
 	memcpy(port->orig.dev_addr, port_dev->dev_addr, port_dev->addr_len);
@@ -1233,6 +1231,10 @@ static int team_port_add(struct team *team, struct net_device *port_dev,
 		}
 	}
 
+	err = team_dev_type_check_change(dev, port_dev);
+	if (err)
+		goto err_set_dev_type;
+
 	if (dev->flags & IFF_UP) {
 		netif_addr_lock_bh(dev);
 		dev_uc_sync_multiple(port_dev, dev);
@@ -1251,6 +1253,7 @@ static int team_port_add(struct team *team, struct net_device *port_dev,
 
 	return 0;
 
+err_set_dev_type:
 err_set_slave_promisc:
 	__team_option_inst_del_port(team, port);
 
-- 
2.51.0