Re: [PATCH] ethtool: fix a privilege escalation bug

[Michal Kubecek]

On Mon, Oct 08, 2018 at 10:49:35AM -0500, Wenwen Wang wrote:
> In dev_ethtool(), the eth command 'ethcmd' is firstly copied from the
> use-space buffer 'useraddr' and checked to see whether it is
> ETHTOOL_PERQUEUE. If yes, the sub-command 'sub_cmd' is further copied from
> the user space. Otherwise, 'sub_cmd' is the same as 'ethcmd'. Next,
> according to 'sub_cmd', a permission check is enforced through the function
> ns_capable(). For example, the permission check is required if 'sub_cmd' is
> ETHTOOL_SCOALESCE, but it is not necessary if 'sub_cmd' is
> ETHTOOL_GCOALESCE, as suggested in the comment "Allow some commands to be
> done by anyone". The following execution invokes different handlers
> according to 'ethcmd'. Specifically, if 'ethcmd' is ETHTOOL_PERQUEUE,
> ethtool_set_per_queue() is called. In ethtool_set_per_queue(), the kernel
> object 'per_queue_opt' is copied again from the user-space buffer
> 'useraddr' and 'per_queue_opt.sub_command' is used to determine which
> operation should be performed. Given that the buffer 'useraddr' is in the
> user space, a malicious user can race to change the sub-command between the
> two copies. In particular, the attacker can supply ETHTOOL_PERQUEUE and
> ETHTOOL_GCOALESCE to bypass the permission check in dev_ethtool(). Then
> before ethtool_set_per_queue() is called, the attacker changes
> ETHTOOL_GCOALESCE to ETHTOOL_SCOALESCE. In this way, the attacker can
> bypass the permission check and execute ETHTOOL_SCOALESCE.
> 
> This patch enforces a check in ethtool_set_per_queue() after the second
> copy from 'useraddr'. If the sub-command is different from the one obtained
> in the first copy in dev_ethtool(), an error code EINVAL will be returned.
> 
> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>

Reviewed-by: Michal Kubecek <mkubecek@xxxxxxx>

I'm just not sure if "privilege escalation" is an appropriate term but
at least some sources define it loosely enough to cover also a simple
permission check bypass like this.

Michal Kubecek

> ---
>  net/core/ethtool.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/ethtool.c b/net/core/ethtool.c
> index c9993c6..ccb337e 100644
> --- a/net/core/ethtool.c
> +++ b/net/core/ethtool.c
> @@ -2462,13 +2462,17 @@ static int ethtool_set_per_queue_coalesce(struct net_device *dev,
>  	return ret;
>  }
>  
> -static int ethtool_set_per_queue(struct net_device *dev, void __user *useraddr)
> +static int ethtool_set_per_queue(struct net_device *dev,
> +				 void __user *useraddr, u32 sub_cmd)
>  {
>  	struct ethtool_per_queue_op per_queue_opt;
>  
>  	if (copy_from_user(&per_queue_opt, useraddr, sizeof(per_queue_opt)))
>  		return -EFAULT;
>  
> +	if (per_queue_opt.sub_command != sub_cmd)
> +		return -EINVAL;
> +
>  	switch (per_queue_opt.sub_command) {
>  	case ETHTOOL_GCOALESCE:
>  		return ethtool_get_per_queue_coalesce(dev, useraddr, &per_queue_opt);
> @@ -2838,7 +2842,7 @@ int dev_ethtool(struct net *net, struct ifreq *ifr)
>  		rc = ethtool_get_phy_stats(dev, useraddr);
>  		break;
>  	case ETHTOOL_PERQUEUE:
> -		rc = ethtool_set_per_queue(dev, useraddr);
> +		rc = ethtool_set_per_queue(dev, useraddr, sub_cmd);
>  		break;
>  	case ETHTOOL_GLINKSETTINGS:
>  		rc = ethtool_get_link_ksettings(dev, useraddr);
> -- 
> 2.7.4
>