net: dcb: Validate netlink message in DCB handler

DCB uses the same handler function for both RTM_GETDCB and RTM_SETDCB
messages. dcb_doit() bounces RTM_SETDCB mesasges if the user does not have
the CAP_NET_ADMIN capability.

However, the operation to be performed is not decided from the DCB message
type, but from the DCB command. Thus DCB_CMD_*_GET commands are used for
reading DCB objects, the corresponding SET and DEL commands are used for
manipulation.

The assumption is that set-like commands will be sent via an RTM_SETDCB
message, and get-like ones via RTM_GETDCB. However, this assumption is not
enforced.

It is therefore possible to manipulate DCB objects without CAP_NET_ADMIN
capability by sending the corresponding command in an RTM_GETDCB message.
That is a bug. Fix it by validating the type of the request message against
the type used for the response.

Fixes: 2f90b865 ("ixgbe: this patch adds support for DCB to the kernel and ixgbe driver")
Signed-off-by: Petr Machata <me@pmachata.org>
Link: https://lore.kernel.org/r/a2a9b88418f3a58ef211b718f2970128ef9e3793.1608673640.git.me@pmachata.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dcb/dcbnl.c

@@ -1765,6 +1765,8 @@ 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)
+		return -EPERM;
 
 	if (!tb[DCB_ATTR_IFNAME])
 		return -EINVAL;