Re: [PATCH nft v2] support for afl++ (american fuzzy lop++) fuzzer

[Florian Westphal <fw@strlen.de>]

Fernando Fernandez Mancera <fmancera@suse.de> wrote:
> On 10/19/25 4:17 PM, Florian Westphal wrote:
> >> In addition I noticed that when a kernel splat happens the ruleset that
> >> triggered it isn't saved anywhere, it would be nice to save them so we have
> >> a reproducer right away.
> > 
> > I had such code but removed it for this version.
> > 
> > I can send a followup patch to re-add it but I think that it is better
> > for kernel fuzzing to extend knft acordingly, as nft is restricted by
> > the input grammar wrt. the nonsense that it can create.
> > 
> 
> That is fine for me, I still have pending to try knft which I might do 
> this week if I have time. If we do not want to save which ruleset 
> generated the kernel splat I would drop netlink-rw mode completely..

Hmmm.... I'm not sure on this.  It would be a bit of a silly limitation.
Its not like -rw adds a huge chunk of code.

The store code wasn't too bad, back then I added some scripting for
allow to e.g. call nft flush ruleset periodically and that was more code
than strictly needed for pure nft (-ro mode) fuzzing.

> Yes, it seems we found the same issue. I do not have a solution on the 
> control plane although I was about to send this patch for data plane.
> 
> diff --git a/net/netfilter/nf_tables_core.c b/net/netfilter/nf_tables_core.c
> index 6557a4018c09..ddc4943d082c 100644
> --- a/net/netfilter/nf_tables_core.c
> +++ b/net/netfilter/nf_tables_core.c
> @@ -251,10 +251,10 @@ nft_do_chain(struct nft_pktinfo *pkt, void *priv)
>   {
>          const struct nft_chain *chain = priv, *basechain = chain;
>          const struct net *net = nft_net(pkt);
> +       unsigned int stackptr = 0, jumps = 0;
>          const struct nft_expr *expr, *last;
>          const struct nft_rule_dp *rule;
>          struct nft_regs regs;
> -       unsigned int stackptr = 0;
>          struct nft_jumpstack jumpstack[NFT_JUMP_STACK_SIZE];
>          bool genbit = READ_ONCE(net->nft.gencursor);
>          struct nft_rule_blob *blob;
> @@ -314,6 +314,9 @@ nft_do_chain(struct nft_pktinfo *pkt, void *priv)
> 
>          switch (regs.verdict.code) {
>          case NFT_JUMP:
> +               jumps++;
> +               if (WARN_ON_ONCE(jumps > 256))
> +                       return NF_DROP;
>                  if (WARN_ON_ONCE(stackptr >= NFT_JUMP_STACK_SIZE))
>                          return NF_DROP;
>                  jumpstack[stackptr].rule = nft_rule_next(rule);
> 
> Currently with enough jumps chained together and traffic generated, CPU 
> can get stuck on nft_do_chain() triggering a kernel splat. If there is a 
> solution on data plane it would be much better than this of course.

There is this patch:
https://patchwork.ozlabs.org/project/netfilter-devel/patch/20250728040315.1014454-1-brady.1345@gmail.com/

I planned to push it upstream in this merge window.