Re: [iptables PATCH] xtables: Fix typo in nft_rebuild_cache()

[Phil Sutter <phil@nwl.cc>]

Hi,

On Tue, May 14, 2019 at 10:51:33AM +0200, Phil Sutter wrote:
> Conditional cache flush logic was inverted.
> 
> Fixes: 862818ac3a0de ("xtables: add and use nft_build_cache")
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
>  iptables/nft.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/iptables/nft.c b/iptables/nft.c
> index 6354b7e8e72fe..83e0d9a69b37c 100644
> --- a/iptables/nft.c
> +++ b/iptables/nft.c
> @@ -1541,7 +1541,7 @@ void nft_build_cache(struct nft_handle *h)
>  
>  void nft_rebuild_cache(struct nft_handle *h)
>  {
> -	if (!h->have_cache)
> +	if (h->have_cache)
>  		flush_chain_cache(h, NULL);
>  
>  	__nft_build_cache(h);

So with this change I broke your transaction reload logic. The problem
is that data in h->obj_list potentially sits in cache, too. At least
rules have to be there so insert with index works correctly. If the
cache is flushed before regenerating the batch, use-after-free occurs
which crashes the program.

We need to either keep the old cache around or keep locally generated
entries when flushing (which might require more intelligent cache
update, too).

I also have a fix for your testcase, will submit that in a minute as
well.

Cheers, Phil