Re: [nf PATCH v3] netfilter: nfnetlink: Fix for interrupted hook dumps

[Phil Sutter <phil@nwl.cc>]

On Tue, Sep 08, 2026 at 02:41:05PM +0200, Pablo Neira Ayuso wrote:
> On Tue, Sep 08, 2026 at 11:23:03AM +0200, Phil Sutter wrote:
> > Hi Pablo,
> > 
> > On Mon, Sep 07, 2026 at 08:19:47PM +0200, Pablo Neira Ayuso wrote:
> > > On Fri, Sep 04, 2026 at 02:52:15PM +0200, Phil Sutter wrote:
> > > > @@ -338,27 +337,47 @@ nfnl_hook_entries_head(u8 pf, unsigned int hook, struct net *net, const char *de
> > > >  }
> > > >  
> > > >  static int nfnl_hook_dump_nat(struct sk_buff *nlskb,
> > > > -			      const struct nfnl_dump_hook_data *ctx,
> > > > -			      const struct nf_hook_ops *ops,
> > > > -			      int family, unsigned int seq)
> > > > +			      struct netlink_callback *cb,
> > > > +			      const struct nf_hook_ops *ops, int family)
> > > >  {
> > > >  	struct nf_nat_lookup_hook_priv *priv = ops->priv;
> > > > -	struct nf_hook_entries *e = rcu_dereference(priv->entries);
> > > > +	struct nfnl_dump_hook_data *ctx = cb->data;
> > > > +	struct net *net = sock_net(nlskb->sk);
> > > >  	struct nf_hook_ops **nat_ops;
> > > > -	int i, err;
> > > > +	unsigned int i = cb->args[1];
> > > > +	struct nf_hook_entries *e;
> > > > +	unsigned int base_seq;
> > > > +	int err = 0;
> > > >  
> > > > +	base_seq = smp_load_acquire(&net->nf.nat_hook_base_seq);
> > > 
> > > Maybe annnotate this base sequence in the .start via:
> > > 
> > > struct netlink_dump_control c = {
> > >         .start = ...;
> > > 
> > > We should probably start doing this in other nfnetlink subsystems too.
> > > 
> > > This will help catch an interference between two netlink recv() calls
> > > which results in calling netlink_dump() which calls this function.
> > 
> > I do not comprehend, sorry. The concurrent hook dumps have distinct cb
> > buffers and net->nf.{nat_,}hook_base_seq is shared but read-only. How
> > does the problematic interference happen?
> 
> See nf_tables_dump_rules_start() for instance. It allocates the struct
> nft_rule_dump_ctx, which is reachable through .start, .dump and .done
> callbacks. I think it should be possible to annotate the current
> base_seq at the beginning of the netlink dump from .start in a ctx
> object. Then, use it from .dump to check if dump is consistent (ie.
> turn on the NLM_F_DUMP_INTR flag).
> 
> So, instead of fetching the current sequence from .dump like this:
> 
>         cb->seq = nft_base_seq(net);
> 
> Use the sequence available in the new ctx object, ie. from .dump path
> you do this:
> 
>         cb->seq = ctx->seq;

But to detect a concurrent hook update between to .dump callback calls
(if skb space was exceeded), the current value in per-net data has to
be fetched, no? So this would have to look like this in .dump callback:

| ctx->seq = nft_base_seq(net);
| cb->seq = ctx->seq;

Then I don't get the detour via struct nfnl_dump_hook_data. Or is it
possible we get multiple concurrent dump requests on the same netlink
socket and thus cb->seq (and cb->prev_seq) gets shared between them?

> Because netlink_dump() is called for each userspace recv() call (netlink
> delivers the chunked listing in several messages), this would allow
> userspace to know that the listing is inconsistent, then optionally
> retry.
> 
> Makes sense to you?

Not quite, sorry.