netdev - Re: [PATCH net-next 5/6] netlink: add mask validation

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Date:   Mon, 05 Oct 2020 22:12:25 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Michal Kubecek <mkubecek@...e.cz>, davem@...emloft.net,
        netdev@...r.kernel.org, kernel-team@...com, jiri@...nulli.us,
        andrew@...n.ch, dsahern@...il.com, pablo@...filter.org
Subject: Re: [PATCH net-next 5/6] netlink: add mask validation

On Mon, 2020-10-05 at 21:53 +0200, Johannes Berg wrote:
> On Mon, 2020-10-05 at 12:40 -0700, Jakub Kicinski wrote:
> 
> > > I would totally support doing that here in the general validation code,
> > > but (again) don't really think NLMSGERR_ATTR_COOKIE is an appropriate
> > > attribute for it.
> > 
> > Hm. Perhaps we can do a partial policy dump into the extack?
> 
> Hm. I like that idea.
> 
> If we have NLMSGERR_ATTR_OFFS we could accompany that with the sub-
> policy for that particular attribute, something like
> 
> [NLMSGERR_ATTR_POLICY] = nested {
>   [NL_POLICY_TYPE_ATTR_TYPE] = ...
>   [NL_POLICY_TYPE_ATTR_MASK] = ...
> }
> 
> which we could basically do by factoring out the inner portion of
> netlink_policy_dump_write():
> 
> 	attr = nla_nest_start(skb, state->attr_idx);
> 	if (!attr)
> 		goto nla_put_failure;
> 	...
> 	nla_nest_end(skb, attr);
> 
> from there into a separate function, give it the pt and the nested
> attribute (what's "state->attr_idx" here) as arguments, and then we call
> it with NLMSGERR_ATTR_POLICY from here, and with "state->attr_idx" from
> netlink_policy_dump_write() :-)
> 
> Nice, easy & useful, maybe I'll code it up tomorrow.

OK I thought about it a bit more and looked at the code, and it's not
actually possible to do easily right now, because we can't actually
point to the bad attribute from the general lib/nlattr.c code ...

Why? Because we don't know right now, e.g. for nla_validate(), where in
the message we started validation, i.e. the offset of the "head" inside
the particular message.

For nlmsg_parse() and friends that's a bit easier, but it needs more
rejiggering than I'm willing to do tonight ;)

johannes