Re: [PATCH net] macvlan: inherit needed_headroom and needed_tailroom from lowerdev

[Hangbin Liu <liuhangbin@gmail.com>]

On 06.08.2026 14:19, Eric Dumazet wrote:
>macvlan devices inherit hard_header_len from lowerdev during macvlan_init(),
>but leave needed_headroom and needed_tailroom set to 0.
>
>When the underlying lowerdev requires extra headroom or tailroom for
>headers/trailers (e.g. macsec, ipsec, wireguard, tunnels, or veth with rx
>headroom), upper layers calculating packet headroom and tailroom fail to
>reserve sufficient space.
>
>This can result in reallocation overhead, skb headroom underflows, or KASAN
>slab-use-after-free crashes when dev_hard_header() / macvlan_hard_header()
>prepends header data or when lower devices append tailroom.
>
>Fix this by:
>1. Inheriting needed_headroom and needed_tailroom from lowerdev in macvlan_init().
>2. Propagating needed_headroom and needed_tailroom updates to attached macvlans
>   in macvlan_device_event() when receiving NETDEV_FEAT_CHANGE events.
>
>Fixes: b863ceb7ddce ("[NET]: Add macvlan driver")
>Reported-by: Tangxin Xie <xietangxin@h-partners.com>
>Closes: https://lore.kernel.org/netdev/CANn89i+1EW-sFNK8xoq98gMbPCeLS7e=+rs9gHfLg5Wj+4x0sw@mail.gmail.com/T/#m16adf0ff972cbfd8066c3a8e656e75eaeb12d021
>Signed-off-by: Eric Dumazet <edumazet@google.com>
>---
> drivers/net/macvlan.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
>diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
>index c40fa331836bb2395267914807542ae5094e1a3c..ff2f4bb651b74cdb054f245ea0f4e8cfe2297193 100644
>--- a/drivers/net/macvlan.c
>+++ b/drivers/net/macvlan.c
>@@ -950,6 +950,8 @@ static int macvlan_init(struct net_device *dev)
> 	dev->lltx		= true;
> 	netif_inherit_tso_max(dev, lowerdev);
> 	dev->hard_header_len	= lowerdev->hard_header_len;
>+	dev->needed_headroom	= lowerdev->needed_headroom;
>+	dev->needed_tailroom	= lowerdev->needed_tailroom;
> 	macvlan_set_lockdep_class(dev);
>
> 	vlan->pcpu_stats = netdev_alloc_pcpu_stats(struct vlan_pcpu_stats);
>@@ -1824,6 +1826,8 @@ static int macvlan_device_event(struct notifier_block *unused,
> 	case NETDEV_FEAT_CHANGE:
> 		list_for_each_entry(vlan, &port->vlans, list) {
> 			netif_inherit_tso_max(vlan->dev, dev);
>+			vlan->dev->needed_headroom = dev->needed_headroom;
>+			vlan->dev->needed_tailroom = dev->needed_tailroom;
> 			netdev_update_features(vlan->dev);
> 		}
> 		break;
>-- 
>2.55.0.629.g250fe7f194-goog
>

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>