Re: [PATCH] batman-adv: Release references to inactive interfaces

["Linus Lüssing" <linus.luessing@c0d3.blue>]

On Sat, Sep 27, 2025 at 08:01:29PM +0200, Sven Eckelmann wrote:
> Trying to dump the originators or the neighbors via netlink for a meshif
> with an inactive primary interface is not allowed. The dump functions were
> checking this correctly but they didn't handle non-existing primary
> interfaces and existing _inactive_ interfaces differently.
> 
> (Primary) batadv_hard_ifaces hold a references to a net_device. And
> accessing them is only allowed when either being in a RCU/spinlock
> protected section or when holding a valid reference to them. The netlink
> dump functions use the latter.
> 
> But because the missing specific error handling for inactive primary
> interfaces, the reference was never dropped. This reference counting error
> was only detected when the interface should have been removed from the
> system:
> 
>   unregister_netdevice: waiting for batadv_slave_0 to become free. Usage count = 2
> 
> Fixes: 50eddf397ac3 ("batman-adv: netlink: reduce duplicate code by returning interfaces")
> Reported-by: syzbot+881d65229ca4f9ae8c84@syzkaller.appspotmail.com
> Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> Signed-off-by: Sven Eckelmann <sven@narfation.org>
> ---

(Not that it'd really matter a year later, after merging, but just
for the record:)

Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>