Re: [PATCH net] netdevsim: check the driver is bound in new_port/del_port

[Hangbin Liu <hangbin.liu@linux.dev>]

Hi Nguyen,
On Sun, Nov 01, 2026 at 08:58:24PM +0700, Nguyen Ngoc Thang wrote:
> new_port and del_port are attributes of the bus device, so they stay
> reachable after the driver is unbound. nsim_drv_remove() clears
> drvdata, yet both handlers pass it straight to priv_to_devlink() and
> devl_lock(), which faults on a small negative pointer:
> 
>   BUG: unable to handle page fault for address: fffffffffffffeb0
>   RIP: 0010:__lock_acquire+0x41/0x2de0 kernel/locking/lockdep.c:5140
>   Call Trace:
>    lock_acquire+0x115/0x350
>    mutex_lock_nested+0x55/0x1d0
>    nsim_drv_port_add+0x5a/0x140 drivers/net/netdevsim/dev.c:1760
>    new_port_store+0x1a5/0x280 drivers/net/netdevsim/bus.c:96
>    kernfs_fop_write_iter+0x3a5/0x540
> 
> The nsim_bus_dev->init check does not help: it is only cleared when
> the whole device is deleted, not on unbind.
> 
> Do what sriov_numvfs already does: take device_lock() so the driver
> cannot be unbound underneath us, and return -ENOENT if drvdata is
> NULL.
> 
> Fixes: 794b2c05ca1c ("netdevsim: extend device attrs to support port addition and deletion")
> Reported-by: syzbot+b3fca27eb4005f0111f4@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b3fca27eb4005f0111f4
> Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>

FYI, Jiacheng already posted a same fix for this issue.

https://lore.kernel.org/netdev/4fe1dcd1.16124.1a01a660a42.Coremail.stitch@zju.edu.cn/

Thanks
Hangbin

> ---
> Reproduced in QEMU (x86_64, KASAN + lockdep) with the syzbot C repro:
> the unpatched kernel oopses in nsim_drv_port_add() right after unbind.
> With the patch, the write to new_port returns -ENOENT, a 40s stress of
> bind/unbind + new_device/del_device racing with new_port/del_port/
> sriov_numvfs runs clean (no splat, no lockdep report, no hung task),
> and new_port/del_port still work as before on a bound device.
>  drivers/net/netdevsim/bus.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/netdevsim/bus.c b/drivers/net/netdevsim/bus.c
> index 5c55c308487b..d843b3c985cf 100644
> --- a/drivers/net/netdevsim/bus.c
> +++ b/drivers/net/netdevsim/bus.c
> @@ -93,8 +93,13 @@ new_port_store(struct device *dev, struct device_attribute *attr,
>  		return -EINVAL;
>  	}
>  
> -	ret = nsim_drv_port_add(nsim_bus_dev, NSIM_DEV_PORT_TYPE_PF, port_index,
> -				addr_set ? eth_addr : NULL);
> +	device_lock(dev);
> +	ret = -ENOENT;
> +	if (dev_get_drvdata(dev))
> +		ret = nsim_drv_port_add(nsim_bus_dev, NSIM_DEV_PORT_TYPE_PF,
> +					port_index, addr_set ? eth_addr : NULL);
> +	device_unlock(dev);
> +
>  	return ret ? ret : count;
>  }
>  
> @@ -115,7 +120,13 @@ del_port_store(struct device *dev, struct device_attribute *attr,
>  	if (ret)
>  		return ret;
>  
> -	ret = nsim_drv_port_del(nsim_bus_dev, NSIM_DEV_PORT_TYPE_PF, port_index);
> +	device_lock(dev);
> +	ret = -ENOENT;
> +	if (dev_get_drvdata(dev))
> +		ret = nsim_drv_port_del(nsim_bus_dev, NSIM_DEV_PORT_TYPE_PF,
> +					port_index);
> +	device_unlock(dev);
> +
>  	return ret ? ret : count;
>  }
>  
> -- 
> 2.43.0
>