ethtool: fix a privilege escalation bug

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.

Fixes: f38d138a ("net/ethtool: support set coalesce per queue")
Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>