Re: [PATCH 5.10 031/102] net: fix dev_ifsioc_locked() race condition

[Greg Kroah-Hartman]

On Mon, Mar 08, 2021 at 01:50:57PM +0100, Pavel Machek wrote:
> Hi!
> 
> > commit 3b23a32a63219f51a5298bc55a65ecee866e79d0 upstream.
> > 
> > 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(). We can not use seqlock here as it does not
> 
> I guess it may fix a race, but... does it leak kernel stack data to
> userland?
> 
> 
> > +++ b/drivers/net/tap.c
> > @@ -1093,10 +1093,9 @@ static long tap_ioctl(struct file *file,
> >  			return -ENOLINK;
> >  		}
> >  		ret = 0;
> > -		u = tap->dev->type;
> > +		dev_get_mac_address(&sa, dev_net(tap->dev), tap->dev->name);
> >  		if (copy_to_user(&ifr->ifr_name, tap->dev->name, IFNAMSIZ) ||
> > -		    copy_to_user(&ifr->ifr_hwaddr.sa_data, tap->dev->dev_addr, ETH_ALEN) ||
> > -		    put_user(u, &ifr->ifr_hwaddr.sa_family))
> > +		    copy_to_user(&ifr->ifr_hwaddr, &sa, sizeof(sa)))
> >  			ret = -EFAULT;
> >  		tap_put_tap_dev(tap);
> >  		rtnl_unlock();
> 
> We copy whole "struct sockaddr".
> 
> > +int dev_get_mac_address(struct sockaddr *sa, struct net *net, char *dev_name)
> > +{
> > +	size_t size = sizeof(sa->sa_data);
> > +	struct net_device *dev;
> > +	int ret = 0;
> ...
> > +	if (!dev->addr_len)
> > +		memset(sa->sa_data, 0, size);
> > +	else
> > +		memcpy(sa->sa_data, dev->dev_addr,
> > +		       min_t(size_t, size, dev->addr_len));
> 
> But we only coppied dev->addr_len bytes in.
> 
> This would be very simple way to plug the leak.
> 
> Signed-off-by: Pavel Machek (CIP) <pavel@xxxxxxx>
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 75ca6c6d01d6..b67ff23a1f0d 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -8714,11 +8714,9 @@ int dev_get_mac_address(struct sockaddr *sa, struct net *net, char *dev_name)
>  		ret = -ENODEV;
>  		goto unlock;
>  	}
> -	if (!dev->addr_len)
> -		memset(sa->sa_data, 0, size);
> -	else
> -		memcpy(sa->sa_data, dev->dev_addr,
> -		       min_t(size_t, size, dev->addr_len));
> +	memset(sa->sa_data, 0, size);
> +	memcpy(sa->sa_data, dev->dev_addr,
> +	       min_t(size_t, size, dev->addr_len));
>  	sa->sa_family = dev->type;
>  
>  unlock:
> 

Please submit this change properly to the networking developers, they
are not going to pick anything up this way.

greg k-h