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SubjectRe: [PATCH] netlink: fix memory leak of dump
On Mon, Jul 23, 2018 at 11:28:18AM +0200, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > > 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;
>
> On-Stack input.
> I can't see how its wrong, ->start() happens from same context as
> netlink_dump_start so its valid.
>
> > > + 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;
>
> And this replaced the on-stack input with dynamically
> allocated one, which will be free'd via ->done().
>
> > > /* 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.
>
> Yes.
>
> > the nla will not be available in the second recv(), it won't be there.
>
> Its replaced in ->start().
>
> As David pointed out, once ->start() returns 0 we set cb_running, i.e.
> only after successful ->start() netlink core will call ->dump() again.
>
> So I see no problem setting ->data to onstack cookie and then
> duplicating it to heap via kmemdup in ->start().
>
> As far as I can see netlink core offers all functionality already,
> so we only need to switch netfilter to make use of it.
>
> If you disagree please let me know, otherwise I will cook up
> a patch along this pattern for net/netfilter/*.

Why not just call ->done from netlink_dump_start() when it fails?

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