Re: [PATCH] firewire: net: fix use after free in fwnet_finish_incoming_packet()

[Takashi Sakamoto]

Hi,

Thanks for the fix.

On Fri, Jun 23, 2023 at 01:39:35PM +0800, Zhang Shurong wrote:
> The netif_rx() function frees the skb so we can't dereference it to
> save the skb->len.
> 
> Signed-off-by: Zhang Shurong <zhang_shurong@xxxxxxxxxxx>
> ---
>  drivers/firewire/net.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/firewire/net.c b/drivers/firewire/net.c
> index 538bd677c254..7a4d1a478e33 100644
> --- a/drivers/firewire/net.c
> +++ b/drivers/firewire/net.c
> @@ -479,7 +479,7 @@ static int fwnet_finish_incoming_packet(struct net_device *net,
>  					struct sk_buff *skb, u16 source_node_id,
>  					bool is_broadcast, u16 ether_type)
>  {
> -	int status;
> +	int status, len;
>  
>  	switch (ether_type) {
>  	case ETH_P_ARP:
> @@ -533,13 +533,15 @@ static int fwnet_finish_incoming_packet(struct net_device *net,
>  		}
>  		skb->protocol = protocol;
>  	}
> +
> +	len = skb->len;
>  	status = netif_rx(skb);
>  	if (status == NET_RX_DROP) {
>  		net->stats.rx_errors++;
>  		net->stats.rx_dropped++;
>  	} else {
>  		net->stats.rx_packets++;
> -		net->stats.rx_bytes += skb->len;
> +		net->stats.rx_bytes += len;
>  	}
>  
>  	return 0;

I'm not good at network subsystem, but as long as reading network core,
I think it better not to access members of sk_buff structure after the
call of netif_rx() since the ownership is already delegated from
firewire-net to the network core to dispatch network protocol. The patch
looks good to me.

Reviewed-by: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>

The most of code in firewire-net comes from old driver
(drivers/ieee1394/eth1394.c), thus the most of linux kernel which
supports ieee1394/firewire subsystem affects the issue, however the
potential issue has never appeared since the skb is released enough later
in the different context. I applied the patch to linux-next branch for
v6.5-rc1.


Thanks

Takashi Sakamoto