Re: [PATCH net] tc: Ensure we have enough buffer space when sending filter netlink notifications

[Cong Wang <xiyou.wangcong@gmail.com>]

On Thu, Apr 10, 2025 at 12:28:43PM +0200, Toke Høiland-Jørgensen wrote:
> Cong Wang <xiyou.wangcong@gmail.com> writes:
> 
> > On Mon, Apr 07, 2025 at 12:55:34PM +0200, Toke Høiland-Jørgensen wrote:
> >> +static struct sk_buff *tfilter_notify_prep(struct net *net,
> >> +					   struct sk_buff *oskb,
> >> +					   struct nlmsghdr *n,
> >> +					   struct tcf_proto *tp,
> >> +					   struct tcf_block *block,
> >> +					   struct Qdisc *q, u32 parent,
> >> +					   void *fh, int event,
> >> +					   u32 portid, bool rtnl_held,
> >> +					   struct netlink_ext_ack *extack)
> >> +{
> >> +	unsigned int size = oskb ? max(NLMSG_GOODSIZE, oskb->len) : NLMSG_GOODSIZE;
> >> +	struct sk_buff *skb;
> >> +	int ret;
> >> +
> >> +retry:
> >> +	skb = alloc_skb(size, GFP_KERNEL);
> >> +	if (!skb)
> >> +		return ERR_PTR(-ENOBUFS);
> >> +
> >> +	ret = tcf_fill_node(net, skb, tp, block, q, parent, fh, portid,
> >> +			    n->nlmsg_seq, n->nlmsg_flags, event, false,
> >> +			    rtnl_held, extack);
> >> +	if (ret <= 0) {
> >> +		kfree_skb(skb);
> >> +		if (ret == -EMSGSIZE) {
> >> +			size += NLMSG_GOODSIZE;
> >> +			goto retry;
> >
> > It is a bit concerning to see this technically unbound loop.
> 
> Well, I did think about that. The loop will terminate eventually by
> either succeeding, or failing the allocation. Most likely the former,
> since this is only called after a filter has been successfully
> installed. I.e., it's not like the amount of data being put into the skb
> is unbounded.

Yeah, I totally agree, it is probably just a theoretical problem.

> 
> >> +		}
> >> +		return ERR_PTR(-EINVAL);
> >
> > I think you probably want to propagate the error code from
> > tcf_fill_node() here.
> 
> I just kept the existing return value (of tfilter_notify()) for the same
> error case. tcf_fill_node() always returns -1 on error, so I think it
> makes more sense to keep this?
> 
> Paolo already merged the patch, and I don't think it's worth it to
> follow up with any fixes, cf the above. WDYT?

Not a big deal, we can keep as it is.

Thanks.