netdev - Re: [Patch net] net: fix dev_ifsioc

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Date:   Thu, 28 Jan 2021 12:55:29 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, Cong Wang <cong.wang@...edance.com>,
        "Gong, Sishuai" <sishuai@...due.edu>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [Patch net] net: fix dev_ifsioc_locked() race condition

On Sat, 23 Jan 2021 17:30:49 -0800 Cong Wang wrote:
> From: Cong Wang <cong.wang@...edance.com>
> 
> dev_ifsioc_locked() is called with only RCU read lock, so when
> there is a parallel writer changing the mac address, it could
> get a partially updated mac address, as shown below:
> 
> Thread 1			Thread 2
> // eth_commit_mac_addr_change()
> memcpy(dev->dev_addr, addr->sa_data, ETH_ALEN);
> 				// dev_ifsioc_locked()
> 				memcpy(ifr->ifr_hwaddr.sa_data,
> 					dev->dev_addr,...);
> 
> Close this race condition by guarding them with a RW semaphore,
> like netdev_get_name(). The writers take RTNL anyway, so this
> will not affect the slow path.
> 
> Fixes: 3710becf8a58 ("net: RCU locking for simple ioctl()")
> Reported-by: "Gong, Sishuai" <sishuai@...due.edu>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: Cong Wang <cong.wang@...edance.com>

The addition of the write lock scares me a little for a fix, there's a
lot of code which can potentially run under the callbacks and notifiers
there.

What about using a seqlock?

> +static DECLARE_RWSEM(dev_addr_sem);
> +
>  /**
>   *	dev_set_mac_address - Change Media Access Control Address
>   *	@dev: device
> @@ -8729,19 +8731,50 @@ int dev_set_mac_address(struct net_device *dev, struct sockaddr *sa,
>  		return -EINVAL;
>  	if (!netif_device_present(dev))
>  		return -ENODEV;
> +
> +	down_write(&dev_addr_sem);
>  	err = dev_pre_changeaddr_notify(dev, sa->sa_data, extack);
>  	if (err)
> -		return err;
> +		goto out;
>  	err = ops->ndo_set_mac_address(dev, sa);
>  	if (err)
> -		return err;
> +		goto out;
>  	dev->addr_assign_type = NET_ADDR_SET;
>  	call_netdevice_notifiers(NETDEV_CHANGEADDR, dev);
>  	add_device_randomness(dev->dev_addr, dev->addr_len);
> -	return 0;
> +out:
> +	up_write(&dev_addr_sem);
> +	return err;
>  }