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SubjectRe: [PATCH] netlink: fix memory leak of dump
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/*.

Thanks.

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