net: dcb: Accept RTM_GETDCB messages carrying set-like DCB commands

In commit 826f328e ("net: dcb: Validate netlink message in DCB
handler"), Linux started rejecting RTM_GETDCB netlink messages if they
contained a set-like DCB_CMD_ command.

The reason was that privileges were only verified for RTM_SETDCB messages,
but the value that determined the action to be taken is the command, not
the message type. And validation of message type against the DCB command
was the obvious missing piece.

Unfortunately it turns out that mlnx_qos, a somewhat widely deployed tool
for configuration of DCB, accesses the DCB set-like APIs through
RTM_GETDCB.

Therefore do not bounce the discrepancy between message type and command.
Instead, in addition to validating privileges based on the actual message
type, validate them also based on the expected message type. This closes
the loophole of allowing DCB configuration on non-admin accounts, while
maintaining backward compatibility.

Fixes: 2f90b865 ("ixgbe: this patch adds support for DCB to the kernel and ixgbe driver")
Fixes: 826f328e ("net: dcb: Validate netlink message in DCB handler")
Signed-off-by: NPetr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/a3edcfda0825f2aa2591801c5232f2bbf2d8a554.1610384801.git.me@pmachata.orgSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/dcb/dcbnl.c

@@ -1765,7 +1765,7 @@ static int dcb_doit(struct sk_buff *skb, struct nlmsghdr *nlh,
 	fn = &reply_funcs[dcb->cmd];
 	if (!fn->cb)
 		return -EOPNOTSUPP;
-	if (fn->type != nlh->nlmsg_type)
+	if (fn->type == RTM_SETDCB && !netlink_capable(skb, CAP_NET_ADMIN))
 		return -EPERM;
 
 	if (!tb[DCB_ATTR_IFNAME])