Re: [nf PATCH v2 8/8] netfilter: nf_tables: Add locking for NFT_MSG_GETSETELEM_RESET requests

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Thu, Sep 28, 2023 at 08:57:45PM +0200, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > On Thu, Sep 28, 2023 at 07:46:30PM +0200, Florian Westphal wrote:
> > > Phil Sutter <phil@nwl.cc> wrote:
> > > > +static int nf_tables_dumpreset_set(struct sk_buff *skb,
> > > > +				   struct netlink_callback *cb)
> > > > +{
> > > > +	struct nftables_pernet *nft_net = nft_pernet(sock_net(skb->sk));
> > > > +	struct nft_set_dump_ctx *dump_ctx = cb->data;
> > > > +	int ret, skip = cb->args[0];
> > > > +
> > > > +	mutex_lock(&nft_net->commit_mutex);
> > > > +	ret = nf_tables_dump_set(skb, cb);
> > > > +	mutex_unlock(&nft_net->commit_mutex);
> > > > +
> > > > +	if (cb->args[0] > skip)
> > > > +		audit_log_nft_set_reset(dump_ctx->ctx.table, cb->seq,
> > > > +					cb->args[0] - skip);
> > > > +
> > > 
> > > Once commit_mutex is dropped, parallel user can
> > > delete table, and ctx.table references garbage.
> > 
> > This path should hold rcu read lock.
> 
> Then it would splat on first mutex_lock().
> 
> > I think spinlock would be better, we would just spin for very little
> > time here for another thread to complete the reset, and the race is
> > fixed.
> > 
> > The use of commit_mutex here is confusing is really misleading to the
> > reader, this is also not the commit path.
> 
> I'd say its semantically the same thing, we alter state.
> 
> We even emit audit records to tell userspace that there is state
> change.

This is a netlink event, how does the mutex help in that regard?

> Also, are you sure spinlock is appropriate here?
> 
> For full-ruleset resets we might be doing quite some
> traverals.

I said several times, we grab and release this for each
netlink_recvmsg(), commit_mutex helps us in no way to fix the "two
concurrent dump-and-reset problem".

One concern might be deadlock due to reordering, but I don't see how
that can happen.

> But, no problem, I'm fine with spinlock as well.