Re: [PATCH] netlink: fix memory leak of dump

[Pablo Neira Ayuso]

On Sun, Jul 22, 2018 at 08:09:10PM +0200, Florian Westphal wrote:
> David Miller <davem@xxxxxxxxxxxxx> wrote:
> > From: Florian Westphal <fw@xxxxxxxxx>
> > Date: Sun, 22 Jul 2018 18:39:25 +0200
> > 
> > > 3. change meaning of ->done() so its always called once ->start()
> > >    was invoked (and returned 0), this requires audit of all
> > >    places that provide .done to make sure they won't trip.
> > > 
> > > 3) seems to be what Tom intended when he added .start, so probably
> > > best to investigate that first.
> > 
> > Hmmm...
> > 
> > Any time ->start() succeeds, we set cb_running to true.
> 
> Right.
> 
> > From that point forward, ->done() will be called at some point at all
> > of the locations that check if cb_running is true and set it to false.
> 
> Also right, thanks for pointing this out, I missed fact that netlink
> core restarts a dump after this.
> 
> So 3) is already true which means we should try to see if we can move
> all dump-related extra magic into ->start().
> 
> Shaochun, can you see if this is possible?
> 
> Something along these lines (totally untested), which makes this
> a netfilter fix:
> 
> diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
> --- a/net/netfilter/nf_tables_api.c
> +++ b/net/netfilter/nf_tables_api.c
> @@ -5010,6 +5013,22 @@ nft_obj_filter_alloc(const struct nlattr * const nla[])
>  	return filter;
>  }
>  
> +static int nf_tables_dump_obj_start(struct netlink_callback *cb)
> +{
> +	const struct nlattr * const *nla = cb->data;
> +	struct nft_obj_filter *filter = NULL;
> +
> +	if (nla[NFTA_OBJ_TABLE] ||
> +	    nla[NFTA_OBJ_TYPE]) {
> +		filter = nft_obj_filter_alloc(nla);
> +		if (IS_ERR(filter))
> +			return -ENOMEM;
> +	}
> +
> +	cb->data = filter;
> +	return 0;
> +}
> +
>  /* called with rcu_read_lock held */
>  static int nf_tables_getobj(struct net *net, struct sock *nlsk,
>  			    struct sk_buff *skb, const struct nlmsghdr *nlh,
> @@ -5028,21 +5047,13 @@ static int nf_tables_getobj(struct net *net, struct sock *nlsk,
>  
>  	if (nlh->nlmsg_flags & NLM_F_DUMP) {
>  		struct netlink_dump_control c = {
> +			.start = nf_tables_dump_obj_start,
>  			.dump = nf_tables_dump_obj,
>  			.done = nf_tables_dump_obj_done,
>  			.module = THIS_MODULE,
> +			.data = (void *)nla,

You cannot do this.

nla is allocated in this stack.

the nla will not be available in the second recv(), it won't be there.