netdev - [PATCH net] macvlan: fix error recovery in macvlan_common

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Message-ID: <20260129204359.632556-1-edumazet@google.com>
Date: Thu, 29 Jan 2026 20:43:59 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>, valis <sec@...is.email>, 
	syzbot+7182fbe91e58602ec1fe@...kaller.appspotmail.com, 
	Boudewijn van der Heide <boudewijn@...ta-utec.com>
Subject: [PATCH net] macvlan: fix error recovery in macvlan_common_newlink()

valis provided a nice repro to crash the kernel:

ip link add p1 type veth peer p2
ip link set address 00:00:00:00:00:20 dev p1
ip link set up dev p1
ip link set up dev p2

ip link add mv0 link p2 type macvlan mode source
ip link add invalid% link p2 type macvlan mode source macaddr add 00:00:00:00:00:20

ping -c1 -I p1 1.2.3.4

He also gave a very detailed analysis:

<quote valis>

The issue is triggered when a new macvlan link is created  with
MACVLAN_MODE_SOURCE mode and MACVLAN_MACADDR_ADD (or
MACVLAN_MACADDR_SET) parameter, lower device already has a macvlan
port and register_netdevice() called from macvlan_common_newlink()
fails (e.g. because of the invalid link name).

In this case macvlan_hash_add_source is called from
macvlan_change_sources() / macvlan_common_newlink():

This adds a reference to vlan to the port's vlan_source_hash using
macvlan_source_entry.

vlan is a pointer to the priv data of the link that is being created.

When register_netdevice() fails, the error is returned from
macvlan_newlink() to rtnl_newlink_create():

        if (ops->newlink)
                err = ops->newlink(dev, &params, extack);
        else
                err = register_netdevice(dev);
        if (err < 0) {
                free_netdev(dev);
                goto out;
        }

and free_netdev() is called, causing a kvfree() on the struct
net_device that is still referenced in the source entry attached to
the lower device's macvlan port.

Now all packets sent on the macvlan port with a matching source mac
address will trigger a use-after-free in macvlan_forward_source().

</quote valis>

With all that, my fix is to make sure we call macvlan_flush_sources()
regardless of @create value whenever "goto destroy_macvlan_port;"
path is taken.

Many thanks to valis for following up on this issue.

Fixes: aa5fd0fb7748 ("driver: macvlan: Destroy new macvlan port if macvlan_common_newlink failed.")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: valis <sec@...is.email>
Reported-by: syzbot+7182fbe91e58602ec1fe@...kaller.appspotmail.com
Closes: https: //lore.kernel.org/netdev/695fb1e8.050a0220.1c677c.039f.GAE@...gle.com/T/#u
Cc: Boudewijn van der Heide <boudewijn@...ta-utec.com>
---
 drivers/net/macvlan.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index b4df7e184791d0fe0a60c17522b91b2766847b37..c509228be84d1bb836cc221c1022d4037cd5c883 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -1567,9 +1567,10 @@ int macvlan_common_newlink(struct net_device *dev,
 	/* the macvlan port may be freed by macvlan_uninit when fail to register.
 	 * so we destroy the macvlan port only when it's valid.
 	 */
-	if (create && macvlan_port_get_rtnl(lowerdev)) {
+	if (macvlan_port_get_rtnl(lowerdev)) {
 		macvlan_flush_sources(port, vlan);
-		macvlan_port_destroy(port->dev);
+		if (create)
+			macvlan_port_destroy(port->dev);
 	}
 	return err;
 }
-- 
2.53.0.rc1.225.gd81095ad13-goog