Re: [net,2/2] netfilter: nf_tables: unconditionally bump set->nelems before insertion

[Florian Westphal <fw@strlen.de>]

Jakub Kicinski <kuba@kernel.org> wrote:
> This is an AI-generated review of your patch. The human sending this
> email says "Is this legit?"

Looks legit to me.

> For local reproduction steps see https://netdev-ai.bots.linux.dev/ai-local.html
> ---
> netfilter: nf_tables: unconditionally bump set->nelems before insertion
> 
> This commit addresses a race condition where a new element gets published
> then removed without waiting for the RCU grace period when the set is full.
> The fix moves the nelems increment before insertion and adds cleanup logic
> using the abort path to safely unwind the set.
> 
> > diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
> > index 0c5a4855b97d..834736237b09 100644
> > --- a/net/netfilter/nf_tables_api.c
> > +++ b/net/netfilter/nf_tables_api.c
> > @@ -7171,6 +7171,7 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
> >  	struct nft_data_desc desc;
> >  	enum nft_registers dreg;
> >  	struct nft_trans *trans;
> > +	bool set_full = false;
> >  	u64 expiration;
> >  	u64 timeout;
> >  	int err, i;
> > @@ -7462,10 +7463,18 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
> >  	if (err < 0)
> >  		goto err_elem_free;
> >
> > +	if (!(flags & NFT_SET_ELEM_CATCHALL)) {
> > +		unsigned int max = nft_set_maxsize(set), nelems;
> > +
> > +		nelems = atomic_inc_return(&set->nelems);
> > +		if (nelems > max)
> > +			set_full = true;
> > +	}
> > +
> >  	trans = nft_trans_elem_alloc(ctx, NFT_MSG_NEWSETELEM, set);
> >  	if (trans == NULL) {
> >  		err = -ENOMEM;
> > -		goto err_elem_free;
> > +		goto err_set_size;
> >  	}
> >
> >  	ext->genmask = nft_genmask_cur(ctx->net);
> > @@ -7517,7 +7526,7 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
> >
> >  					ue->priv = elem_priv;
> >  					nft_trans_commit_list_add_elem(ctx->net, trans);
> > -					goto err_elem_free;
> > +					goto err_set_size;
> >  				}
> >  			}
> >  		}
> > @@ -7535,23 +7544,16 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
> >  		goto err_element_clash;
> >  	}
> >
> > -	if (!(flags & NFT_SET_ELEM_CATCHALL)) {
> > -		unsigned int max = nft_set_maxsize(set);
> > -
> > -		if (!atomic_add_unless(&set->nelems, 1, max)) {
> > -			err = -ENFILE;
> > -			goto err_set_full;
> > -		}
> > -	}
> > -
> >  	nft_trans_container_elem(trans)->elems[0].priv = elem.priv;
> >  	nft_trans_commit_list_add_elem(ctx->net, trans);
> > -	return 0;
> >
> > -err_set_full:
> > -	nft_setelem_remove(ctx->net, set, elem.priv);
> > +	return set_full ? -ENFILE : 0;
> > +
> >  err_element_clash:
> >  	kfree(trans);
> > +err_set_size:
> > +	if (!(flags & NFT_SET_ELEM_CATCHALL) && !set_full)
> > +		atomic_dec(&set->nelems);
> 
> Does this properly handle the element update path when the set is full?

Pablo, why does that check !set_full?  The increment is unconditional,
so why is the decrement asymmetric?

err_set_size:
        if (!(flags & NFT_SET_ELEM_CATCHALL))
                atomic_dec(&set->nelems);

?