Subject: Re: Soft lock-ups caused by iptables

From: Pablo Neira Ayuso <[email-masked]>
Date: Thu, 20 Nov 2025 22:01:54 +0100

On Thu, Nov 20, 2025 at 10:34:46AM +0100, Florian Westphal wrote:
> Pablo Neira Ayuso <[email-masked]> wrote:
> > > > Yes, but you also need to annotate the type of the last base chain origin,
> > > > else you might skip validation of 'chain foo' because its depth value says its
> > > > fine but new caller is coming from filter, not nat, and chain foo had
> > > > masquerade expression.
> > 
> > You could also have chains being called from different levels.
> 
> But thats not an issue.  If you see a jump from c1 to c2, and c2
> has been validated for a level of 5, then you need to revalidate
> only if c1->depth >= 5.

OK, you could also have a jump to chain from filter and nat basechain
chains, does this optimization below works in that case too?

Validation is two-folded:

- Search for cycles.
- Ensure expression can be called from basechains that can reach it.

> Do you see any issue with this? (it still lacks annotation for
> the calling basechains type, so this cannot be applied as-is):
> 
> netfilter: nf_tables: avoid chain re-validation if possible
> 
> Consider:
> 
>       input -> j2 -> j3
>       input -> j2 -> j3
>       input -> j1 -> j2 -> j3
> 
> Then the second rule does not need to revalidate j2, and, by extension j3.
> 
> We need to validate it only for rule 3.
> 
> This is needed because chain loop detection also ensures we do not
> exceed the jump stack: Just because we know that j2 is cycle free, its
> last jump might now exceed the allowed stack.  We also need to update
> the new largest call depth for all the reachable nodes.
> 
> diff --git a/include/net/netfilter/nf_tables.h b/include/net/netfilter/nf_tables.h
> --- a/include/net/netfilter/nf_tables.h
> +++ b/include/net/netfilter/nf_tables.h
> @@ -1109,6 +1109,7 @@ struct nft_rule_blob {
>   *	@udlen: user data length
>   *	@udata: user data in the chain
>   *	@blob_next: rule blob pointer to the next in the chain
> + *	@depth: chain was validated for call level <= depth
>   */
>  struct nft_chain {
>  	struct nft_rule_blob		__rcu *blob_gen_0;
> @@ -1128,9 +1129,10 @@ struct nft_chain {
>  
>  	/* Only used during control plane commit phase: */
>  	struct nft_rule_blob		*blob_next;
> +	u8				depth;
>  };
>  
> -int nft_chain_validate(const struct nft_ctx *ctx, const struct nft_chain *chain);
> +int nft_chain_validate(const struct nft_ctx *ctx, struct nft_chain *chain);
>  int nft_setelem_validate(const struct nft_ctx *ctx, struct nft_set *set,
>  			 const struct nft_set_iter *iter,
>  			 struct nft_elem_priv *elem_priv);
> diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
> --- a/net/netfilter/nf_tables_api.c
> +++ b/net/netfilter/nf_tables_api.c
> @@ -4088,15 +4088,26 @@ static void nf_tables_rule_release(const struct nft_ctx *ctx, struct nft_rule *r
>   * and set lookups until either the jump limit is hit or all reachable
>   * chains have been validated.
>   */
> -int nft_chain_validate(const struct nft_ctx *ctx, const struct nft_chain *chain)
> +int nft_chain_validate(const struct nft_ctx *ctx, struct nft_chain *chain)
>  {
>  	struct nft_expr *expr, *last;
>  	struct nft_rule *rule;
>  	int err;
>  
> +	BUILD_BUG_ON(NFT_JUMP_STACK_SIZE > 255);
>  	if (ctx->level == NFT_JUMP_STACK_SIZE)
>  		return -EMLINK;
>  
> +	/* jumps to base chains are not allowed, this is already
> +	 * validated by nft_verdict_init().
> +	 *
> +	 * Chain must be re-validated if we are entering for first
> +	 * time or if the current jumpstack usage is higher than on
> +	 * previous check.
> +	 */
> +	if (ctx->level && chain->depth >= ctx->level)
> +		return 0;
> +
>  	list_for_each_entry(rule, &chain->rules, list) {
>  		if (fatal_signal_pending(current))
>  			return -EINTR;
> @@ -4117,6 +4128,10 @@ int nft_chain_validate(const struct nft_ctx *ctx, const struct nft_chain *chain)
>  		}
>  	}
>  
> +	/* Chain needs no re-validation if called again
> +	 * from a path that doesn't exceed level.
> +	 */
> +	chain->depth = ctx->level;
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(nft_chain_validate);
> @@ -4128,7 +4143,7 @@ static int nft_table_validate(struct net *net, const struct nft_table *table)
>  		.net	= net,
>  		.family	= table->family,
>  	};
> -	int err;
> +	int err = 0;
>  
>  	list_for_each_entry(chain, &table->chains, list) {
>  		if (!nft_is_base_chain(chain))
> @@ -4137,12 +4152,16 @@ static int nft_table_validate(struct net *net, const struct nft_table *table)
>  		ctx.chain = chain;
>  		err = nft_chain_validate(&ctx, chain);
>  		if (err < 0)
> -			return err;
> +			goto err;
>  
>  		cond_resched();
>  	}
>  
> -	return 0;
> +err:
> +	list_for_each_entry(chain, &table->chains, list)
> +		chain->depth = 0;
> +
> +	return err;
>  }
>  
>  int nft_setelem_validate(const struct nft_ctx *ctx, struct nft_set *set,
>