netdev - Re: [Patch nf 3/3] xt_hashlimit: limit the max size of hashtable

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Date:   Fri, 31 Jan 2020 23:08:07 +0100
From:   Florian Westphal <fw@...len.de>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
        syzbot+adf6c6c2be1c3a718121@...kaller.appspotmail.com,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>
Subject: Re: [Patch nf 3/3] xt_hashlimit: limit the max size of hashtable

Cong Wang <xiyou.wangcong@...il.com> wrote:
> The user-specified hashtable size is unbound, this could
> easily lead to an OOM or a hung task as we hold the global
> mutex while allocating and initializing the new hashtable.
> 
> The max value is derived from the max value when chosen by
> the kernel.
> 
> Reported-and-tested-by: syzbot+adf6c6c2be1c3a718121@...kaller.appspotmail.com
> Cc: Pablo Neira Ayuso <pablo@...filter.org>
> Cc: Jozsef Kadlecsik <kadlec@...filter.org>
> Cc: Florian Westphal <fw@...len.de>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/netfilter/xt_hashlimit.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
> index 57a2639bcc22..6327134c5886 100644
> --- a/net/netfilter/xt_hashlimit.c
> +++ b/net/netfilter/xt_hashlimit.c
> @@ -272,6 +272,8 @@ dsthash_free(struct xt_hashlimit_htable *ht, struct dsthash_ent *ent)
>  }
>  static void htable_gc(struct work_struct *work);
>  
> +#define HASHLIMIT_MAX_SIZE 8192
> +
>  static int htable_create(struct net *net, struct hashlimit_cfg3 *cfg,
>  			 const char *name, u_int8_t family,
>  			 struct xt_hashlimit_htable **out_hinfo,
> @@ -290,7 +292,7 @@ static int htable_create(struct net *net, struct hashlimit_cfg3 *cfg,
>  		size = (nr_pages << PAGE_SHIFT) / 16384 /
>  		       sizeof(struct hlist_head);
>  		if (nr_pages > 1024 * 1024 * 1024 / PAGE_SIZE)
> -			size = 8192;
> +			size = HASHLIMIT_MAX_SIZE;
>  		if (size < 16)
>  			size = 16;
>  	}
> @@ -848,6 +850,8 @@ static int hashlimit_mt_check_common(const struct xt_mtchk_param *par,
>  
>  	if (cfg->gc_interval == 0 || cfg->expire == 0)
>  		return -EINVAL;
> +	if (cfg->size > HASHLIMIT_MAX_SIZE)
> +		return -ENOMEM;

Hmm, won't that break restore of rulesets that have something like

--hashlimit-size 10000?

AFAIU this limits the module to vmalloc requests of only 64kbyte.
I'm not opposed to a limit (or a cap), but 64k seems a bit low to me.