Re: [PATCH 1/2] usb: gadget: u_ether: Do not make UDC parent of the net device

[Paul Cercueil]

Hi Sascha, Greg,

I have a breakage in 6.2-rc* that I eventually bisected to this commit,
on a Ingenic SoC (using the jz4740 musb driver) with ECM or RNDIS
configured through gadgetfs.

When plugging the board to my PC, the USB network interface is
recognized, but 'ip link' sees it as 'NO-CARRIER'. With this commit
reverted on v6.2-rc5, everything works fine.

Cheers,
-Paul

Le vendredi 04 novembre 2022 à 14:10 +0100, Sascha Hauer a écrit :
> The UDC is not a suitable parent of the net device as the UDC can
> change or vanish during the lifecycle of the ethernet gadget. This
> can be illustrated with the following:
> 
> mkdir -p /sys/kernel/config/usb_gadget/mygadget
> cd /sys/kernel/config/usb_gadget/mygadget
> mkdir -p configs/c.1/strings/0x409
> echo "C1:Composite Device" > configs/c.1/strings/0x409/configuration
> mkdir -p functions/ecm.usb0
> ln -s functions/ecm.usb0 configs/c.1/
> echo "dummy_udc.0" > UDC
> rmmod dummy_hcd
> 
> The 'rmmod' removes the UDC from the just created gadget, leaving
> the still existing net device with a no longer existing parent.
> 
> Accessing the ethernet device with commands like:
> 
> ip --details link show usb0
> 
> will result in a KASAN splat:
> 
> ==================================================================
> BUG: KASAN: use-after-free in if_nlmsg_size+0x3e8/0x528
> Read of size 4 at addr c5c84754 by task ip/357
> 
> CPU: 3 PID: 357 Comm: ip Not tainted 6.1.0-rc3-00013-gd14953726b24-
> dirty #324
> Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
>  unwind_backtrace from show_stack+0x10/0x14
>  show_stack from dump_stack_lvl+0x58/0x70
>  dump_stack_lvl from print_report+0x134/0x4d4
>  print_report from kasan_report+0x78/0x10c
>  kasan_report from if_nlmsg_size+0x3e8/0x528
>  if_nlmsg_size from rtnl_getlink+0x2b4/0x4d0
>  rtnl_getlink from rtnetlink_rcv_msg+0x1f4/0x674
>  rtnetlink_rcv_msg from netlink_rcv_skb+0xb4/0x1f8
>  netlink_rcv_skb from netlink_unicast+0x294/0x478
>  netlink_unicast from netlink_sendmsg+0x328/0x640
>  netlink_sendmsg from ____sys_sendmsg+0x2a4/0x3b4
>  ____sys_sendmsg from ___sys_sendmsg+0xc8/0x12c
>  ___sys_sendmsg from sys_sendmsg+0xa0/0x120
>  sys_sendmsg from ret_fast_syscall+0x0/0x1c
> 
> Solve this by not setting the parent of the ethernet device.
> 
> Signed-off-by: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
> ---
>  drivers/usb/gadget/function/u_ether.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/u_ether.c
> b/drivers/usb/gadget/function/u_ether.c
> index e06022873df16..8f12f3f8f6eeb 100644
> --- a/drivers/usb/gadget/function/u_ether.c
> +++ b/drivers/usb/gadget/function/u_ether.c
> @@ -798,7 +798,6 @@ struct eth_dev *gether_setup_name(struct
> usb_gadget *g,
>         net->max_mtu = GETHER_MAX_MTU_SIZE;
>  
>         dev->gadget = g;
> -       SET_NETDEV_DEV(net, &g->dev);
>         SET_NETDEV_DEVTYPE(net, &gadget_type);
>  
>         status = register_netdev(net);
> @@ -873,8 +872,6 @@ int gether_register_netdev(struct net_device
> *net)
>         struct usb_gadget *g;
>         int status;
>  
> -       if (!net->dev.parent)
> -               return -EINVAL;
>         dev = netdev_priv(net);
>         g = dev->gadget;
>  
> @@ -905,7 +902,6 @@ void gether_set_gadget(struct net_device *net,
> struct usb_gadget *g)
>  
>         dev = netdev_priv(net);
>         dev->gadget = g;
> -       SET_NETDEV_DEV(net, &g->dev);
>  }
>  EXPORT_SYMBOL_GPL(gether_set_gadget);
>