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

[David Miller <davem@davemloft.net>]

From: Wenwen Wang <wang6495@umn.edu>
Date: Mon,  8 Oct 2018 10:49:35 -0500

> 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@umn.edu>

Applied and queued up for -stable.