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 06:52:44PM +0200, Phil Sutter wrote:
> Set expressions' dump callbacks are not concurrency-safe per-se with
> reset bit set. If two CPUs reset the same element at the same time,
> values may underrun at least with element-attached counters and quotas.
> 
> Prevent this by introducing dedicated callbacks for nfnetlink and the
> asynchronous dump handling to serialize access.
> 
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
> Changes since v1:
> - Improved commit description
> - Unrelated chunk moved into a previous patch
> ---
>  net/netfilter/nf_tables_api.c | 105 +++++++++++++++++++++++++++++-----
>  1 file changed, 91 insertions(+), 14 deletions(-)
> 
> diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
> index 1491d4c65fed9..a855b43fe72db 100644
> --- a/net/netfilter/nf_tables_api.c
> +++ b/net/netfilter/nf_tables_api.c
> @@ -5834,10 +5834,6 @@ static int nf_tables_dump_set(struct sk_buff *skb, struct netlink_callback *cb)
>  	nla_nest_end(skb, nest);
>  	nlmsg_end(skb, nlh);
>  
> -	if (dump_ctx->reset && args.iter.count > args.iter.skip)
> -		audit_log_nft_set_reset(table, cb->seq,
> -					args.iter.count - args.iter.skip);
> -
>  	rcu_read_unlock();
>  
>  	if (args.iter.err && args.iter.err != -EMSGSIZE)
> @@ -5853,6 +5849,24 @@ static int nf_tables_dump_set(struct sk_buff *skb, struct netlink_callback *cb)
>  	return -ENOSPC;
>  }
>  
> +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);
> +
> +	return ret;
> +}
> +
>  static int nf_tables_dump_set_start(struct netlink_callback *cb)
>  {
>  	struct nft_set_dump_ctx *dump_ctx = cb->data;
> @@ -6070,7 +6084,6 @@ static int nf_tables_getsetelem(struct sk_buff *skb,
>  	struct nft_set *set;
>  	struct nlattr *attr;
>  	struct nft_ctx ctx;
> -	bool reset = false;
>  
>  	table = nft_table_lookup(net, nla[NFTA_SET_ELEM_LIST_TABLE], family,
>  				 genmask, 0);
> @@ -6085,9 +6098,6 @@ static int nf_tables_getsetelem(struct sk_buff *skb,
>  
>  	nft_ctx_init(&ctx, net, skb, info->nlh, family, table, NULL, nla);
>  
> -	if (NFNL_MSG_TYPE(info->nlh->nlmsg_type) == NFT_MSG_GETSETELEM_RESET)
> -		reset = true;
> -
>  	if (info->nlh->nlmsg_flags & NLM_F_DUMP) {
>  		struct netlink_dump_control c = {
>  			.start = nf_tables_dump_set_start,
> @@ -6098,7 +6108,67 @@ static int nf_tables_getsetelem(struct sk_buff *skb,
>  		struct nft_set_dump_ctx dump_ctx = {
>  			.set = set,
>  			.ctx = ctx,
> -			.reset = reset,
> +			.reset = false,
> +		};
> +
> +		c.data = &dump_ctx;
> +		return nft_netlink_dump_start_rcu(info->sk, skb, info->nlh, &c);
> +	}
> +
> +	if (!nla[NFTA_SET_ELEM_LIST_ELEMENTS])
> +		return -EINVAL;
> +
> +	nla_for_each_nested(attr, nla[NFTA_SET_ELEM_LIST_ELEMENTS], rem) {
> +		err = nft_get_set_elem(&ctx, set, attr, false);
> +		if (err < 0) {
> +			NL_SET_BAD_ATTR(extack, attr);
> +			break;
> +		}
> +		nelems++;
> +	}
> +
> +	return err;
> +}
> +
> +static int nf_tables_getsetelem_reset(struct sk_buff *skb,
> +				      const struct nfnl_info *info,
> +				      const struct nlattr * const nla[])
> +{
> +	struct nftables_pernet *nft_net = nft_pernet(info->net);
> +	struct netlink_ext_ack *extack = info->extack;
> +	u8 genmask = nft_genmask_cur(info->net);
> +	u8 family = info->nfmsg->nfgen_family;
> +	int rem, err = 0, nelems = 0;
> +	struct net *net = info->net;
> +	struct nft_table *table;
> +	struct nft_set *set;
> +	struct nlattr *attr;
> +	struct nft_ctx ctx;
> +
> +	table = nft_table_lookup(net, nla[NFTA_SET_ELEM_LIST_TABLE], family,
> +				 genmask, 0);
> +	if (IS_ERR(table)) {
> +		NL_SET_BAD_ATTR(extack, nla[NFTA_SET_ELEM_LIST_TABLE]);
> +		return PTR_ERR(table);
> +	}
> +
> +	set = nft_set_lookup(table, nla[NFTA_SET_ELEM_LIST_SET], genmask);
> +	if (IS_ERR(set))
> +		return PTR_ERR(set);
> +
> +	nft_ctx_init(&ctx, net, skb, info->nlh, family, table, NULL, nla);
> +
> +	if (info->nlh->nlmsg_flags & NLM_F_DUMP) {
> +		struct netlink_dump_control c = {
> +			.start = nf_tables_dump_set_start,
> +			.dump = nf_tables_dumpreset_set,
> +			.done = nf_tables_dump_set_done,
> +			.module = THIS_MODULE,
> +		};
> +		struct nft_set_dump_ctx dump_ctx = {
> +			.set = set,
> +			.ctx = ctx,
> +			.reset = true,
>  		};
>  
>  		c.data = &dump_ctx;
> @@ -6108,18 +6178,25 @@ static int nf_tables_getsetelem(struct sk_buff *skb,
>  	if (!nla[NFTA_SET_ELEM_LIST_ELEMENTS])
>  		return -EINVAL;
>  
> +	if (!try_module_get(THIS_MODULE))
> +		return -EINVAL;
> +	rcu_read_unlock();
> +	mutex_lock(&nft_net->commit_mutex);
> +	rcu_read_lock();
>  	nla_for_each_nested(attr, nla[NFTA_SET_ELEM_LIST_ELEMENTS], rem) {
> -		err = nft_get_set_elem(&ctx, set, attr, reset);
> +		err = nft_get_set_elem(&ctx, set, attr, true);
>  		if (err < 0) {
>  			NL_SET_BAD_ATTR(extack, attr);
>  			break;
>  		}
>  		nelems++;
>  	}
> +	rcu_read_unlock();
> +	mutex_unlock(&nft_net->commit_mutex);
> +	rcu_read_lock();
> +	module_put(THIS_MODULE);
>  
> -	if (reset)
> -		audit_log_nft_set_reset(table, nft_pernet(net)->base_seq,
> -					nelems);
> +	audit_log_nft_set_reset(table, nft_net->base_seq, nelems);
>  
>  	return err;
>  }
> @@ -9140,7 +9217,7 @@ static const struct nfnl_callback nf_tables_cb[NFT_MSG_MAX] = {
>  		.policy		= nft_set_elem_list_policy,
>  	},
>  	[NFT_MSG_GETSETELEM_RESET] = {
> -		.call		= nf_tables_getsetelem,
> +		.call		= nf_tables_getsetelem_reset,

This diff is weird. This is a complete new function, but it show like
new code in nf_tables_getsetelem(), this is difficult to track at
review.

What did it change for this diff to happen?

>  		.type		= NFNL_CB_RCU,
>  		.attr_count	= NFTA_SET_ELEM_LIST_MAX,
>  		.policy		= nft_set_elem_list_policy,
> -- 
> 2.41.0
>