Hi Florian,
I'm making another pass on this series, a few thing I would like to
ask, see below.
On Thu, Nov 07, 2024 at 06:44:08PM +0100, Florian Westphal wrote:
> diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
> index bdf5ba21c76d..e96e538fe2eb 100644
> --- a/net/netfilter/nf_tables_api.c
> +++ b/net/netfilter/nf_tables_api.c
> @@ -25,6 +25,7 @@
>
> #define NFT_MODULE_AUTOLOAD_LIMIT (MODULE_NAME_LEN - sizeof("nft-expr-255-"))
> #define NFT_SET_MAX_ANONLEN 16
> +#define NFT_MAX_SET_NELEMS ((2048 - sizeof(struct nft_trans_elem)) / sizeof(struct nft_trans_one_elem))
This NFT_MAX_SET_NELEMS is to stay in a specific kmalloc-X?
What is the logic behind this NFT_MAX_SET_NELEMS?
> unsigned int nf_tables_net_id __read_mostly;
>
> @@ -391,6 +392,69 @@ static void nf_tables_unregister_hook(struct net *net,
> return __nf_tables_unregister_hook(net, table, chain, false);
> }
>
> +static bool nft_trans_collapse_set_elem_allowed(const struct nft_trans_elem *a, const struct nft_trans_elem *b)
> +{
> + return a->set == b->set && a->bound == b->bound && a->nelems < NFT_MAX_SET_NELEMS;
I think this a->bound == b->bound check defensive.
This code is collapsing only two consecutive transactions, the one at
the tail (where nelems > 1) and the new transaction (where nelems ==
1).
bound state should only change in case there is a NEWRULE transaction
in between.
I am trying to find a error scenario where a->bound == b->bound
evaluates false. I considered the following:
newelem -> newrule -> newelem
where newrule has these expressions:
lookup -> error
in this case, newrule error path is exercised:
nft_rule_expr_deactivate(&ctx, rule, NFT_TRANS_PREPARE_ERROR);
this calls nf_tables_deactivate_set() that calls
nft_set_trans_unbind(), then a->bound is restored to false. Rule is
released and no transaction is added.
Because if this succeeds:
newelem -> newrule -> newelem
then no element collapsing can happen, because we only collapse what
is at the tail.
TLDR; Check does not harm, but it looks unlikely to happen to me.
one more comment below.
> +}
> +
> +static bool nft_trans_collapse_set_elem(struct nftables_pernet *nft_net,
> + struct nft_trans_elem *tail,
> + struct nft_trans_elem *trans,
> + gfp_t gfp)
> +{
> + unsigned int nelems, old_nelems = tail->nelems;
> + struct nft_trans_elem *new_trans;
> +
> + if (!nft_trans_collapse_set_elem_allowed(tail, trans))
> + return false;
> +
> + if (WARN_ON_ONCE(trans->nelems != 1))
> + return false;
> +
> + if (check_add_overflow(old_nelems, trans->nelems, &nelems))
> + return false;
> +
> + /* krealloc might free tail which invalidates list pointers */
> + list_del_init(&tail->nft_trans.list);
> +
> + new_trans = krealloc(tail, struct_size(tail, elems, nelems), gfp);
> + if (!new_trans) {
> + list_add_tail(&tail->nft_trans.list, &nft_net->commit_list);
> + return false;
> + }
> +
> + INIT_LIST_HEAD(&new_trans->nft_trans.list);
This initialization is also defensive, this element is added via
list_add_tail().
> + new_trans->nelems = nelems;
> + new_trans->elems[old_nelems] = trans->elems[0];
> + list_add_tail(&new_trans->nft_trans.list, &nft_net->commit_list);
> +
> + return true;
> +}
