Re: [PATCH] can: bcm: delay release of struct bcm_op after synchronize_rcu

[Oliver Hartkopp <socketcan@hartkopp.net>]

On 19.06.21 18:18, Thadeu Lima de Souza Cascardo wrote:
> can_rx_register callbacks may be called concurrently to the call to
> can_rx_unregister. The callbacks and callback data, though, are protected by
> RCU and the struct sock reference count.
> 
> So the callback data is really attached to the life of sk, meaning that it
> should be released on sk_destruct. However, bcm_remove_op calls tasklet_kill,
> and RCU callbacks may be called under RCU softirq, so that cannot be used on
> kernels before the introduction of HRTIMER_MODE_SOFT.
> 
> However, bcm_rx_handler is called under RCU protection, so after calling
> can_rx_unregister, we may call synchronize_rcu in order to wait for any RCU
> read-side critical sections to finish. That is, bcm_rx_handler won't be called
> anymore for those ops. So, we only free them, after we do that synchronize_rcu.
> 
> Reported-by: syzbot+0f7e7e5e2f4f40fa89c0@syzkaller.appspotmail.com
> Reported-by: Norbert Slusarek <nslusarek@gmx.net>
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

Fixes: ffd980f976e7 ("[CAN]: Add broadcast manager (bcm) protocol")
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>

Thanks Norbert for reporting and Thadeu for working out the fix!

Best regards,
Oliver

> ---
>   net/can/bcm.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/net/can/bcm.c b/net/can/bcm.c
> index f3e4d9528fa3..c67916020e63 100644
> --- a/net/can/bcm.c
> +++ b/net/can/bcm.c
> @@ -785,6 +785,7 @@ static int bcm_delete_rx_op(struct list_head *ops, struct bcm_msg_head *mh,
>   						  bcm_rx_handler, op);
>   
>   			list_del(&op->list);
> +			synchronize_rcu();
>   			bcm_remove_op(op);
>   			return 1; /* done */
>   		}
> @@ -1533,6 +1534,11 @@ static int bcm_release(struct socket *sock)
>   					  REGMASK(op->can_id),
>   					  bcm_rx_handler, op);
>   
> +	}
> +
> +	synchronize_rcu();
> +
> +	list_for_each_entry_safe(op, next, &bo->rx_ops, list) {
>   		bcm_remove_op(op);
>   	}
>   
>