Re: [PATCH net-next] net: vlan: don't propagate flags on open

[Simon Horman]

On Thu, Mar 13, 2025 at 03:06:57AM -0700, Stanislav Fomichev wrote:
> With the device instance lock, there is now a possibility of a deadlock:

...

> Device setup:
> 
>      netdevsim0 (down)
>      ^        ^
>   bond        netdevsim1.100@netdevsim1 allmulticast=on (down)
> 
> When we enslave the lower device (netdevsim0) which has a vlan, we
> propagate vlan's allmuti/promisc flags during ndo_open. This causes
> (re)locking on of the real_dev.
> 
> Propagate allmulti/promisc on flags change, not on the open. There
> is a slight semantics change that vlans that are down now propagate
> the flags, but this seems unlikely to result in the real issues.
> 
> Reproducer:
> 
>   echo 0 1 > /sys/bus/netdevsim/new_device
> 
>   dev_path=$(ls -d /sys/bus/netdevsim/devices/netdevsim0/net/*)
>   dev=$(echo $dev_path | rev | cut -d/ -f1 | rev)
> 
>   ip link set dev $dev name netdevsim0
>   ip link set dev netdevsim0 up
> 
>   ip link add link netdevsim0 name netdevsim0.100 type vlan id 100
>   ip link set dev netdevsim0.100 allmulticast on down
>   ip link add name bond1 type bond mode 802.3ad
>   ip link set dev netdevsim0 down
>   ip link set dev netdevsim0 master bond1
>   ip link set dev bond1 up
>   ip link show
> 
> Reported-by: syzbot+b0c03d76056ef6cd12a6@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/netdev/Z9CfXjLMKn6VLG5d@mini-arch/T/#m15ba130f53227c883e79fb969687d69d670337a0
> Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>

Reviewed-by: Simon Horman <horms@xxxxxxxxxx>