Re: [PATCH net-next] netfilter: xt_quota: fix the behavior of xt_quota module

Pablo Neira Ayuso
2018-10-02T14:41:04Z

Hi,

On Mon, Oct 01, 2018 at 06:23:08PM -0700, Chenbo Feng wrote:
> From: Chenbo Feng <fengc@google.com>
>
> A major flaw of the current xt_quota module is that quota in a specific
> rule gets reset every time there is a rule change in the same table. It
> makes the xt_quota module not very useful in a table in which iptables
> rules are changed at run time. This fix introduces a new counter that is
> visible to userspace as the remaining quota of the current rule. When
> userspace restores the rules in a table, it can restore the counter to
> the remaining quota instead of resetting it to the full quota.

A few questions, see below.

First one is, don't we need a new match revision for this new option?

> Signed-off-by: Chenbo Feng <fengc@google.com>
> Suggested-by: Maciej Żenczykowski <maze@google.com>
> Reviewed-by: Maciej Żenczykowski <maze@google.com>
> ---
> include/uapi/linux/netfilter/xt_quota.h | 8 +++--
> net/netfilter/xt_quota.c | 55 +++++++++++++--------------------
> 2 files changed, 27 insertions(+), 36 deletions(-)
>
> diff --git a/include/uapi/linux/netfilter/xt_quota.h b/include/uapi/linux/netfilter/xt_quota.h
> index f3ba5d9..d72fd52 100644
> --- a/include/uapi/linux/netfilter/xt_quota.h
> +++ b/include/uapi/linux/netfilter/xt_quota.h
> @@ -15,9 +15,11 @@ struct xt_quota_info {
> __u32 flags;
> __u32 pad;
> __aligned_u64 quota;
> -
> - /* Used internally by the kernel */
> - struct xt_quota_priv *master;
> +#ifdef __KERNEL__
> + atomic64_t counter;
> +#else
> + __aligned_u64 remain;
> +#endif
> };
>
> #endif /* _XT_QUOTA_H */
> diff --git a/net/netfilter/xt_quota.c b/net/netfilter/xt_quota.c
> index 10d61a6..6afa7f4 100644
> --- a/net/netfilter/xt_quota.c
> +++ b/net/netfilter/xt_quota.c
> @@ -11,11 +11,6 @@
> #include <linux/netfilter/xt_quota.h>
> #include <linux/module.h>
>
> -struct xt_quota_priv {
> - spinlock_t lock;
> - uint64_t quota;
> -};
> -
> MODULE_LICENSE("GPL");
> MODULE_AUTHOR("Sam Johnston <samj@samj.net>");
> MODULE_DESCRIPTION("Xtables: countdown quota match");
> @@ -26,54 +21,48 @@ static bool
> quota_mt(const struct sk_buff *skb, struct xt_action_param *par)
> {
> struct xt_quota_info *q = (void *)par->matchinfo;
> - struct xt_quota_priv *priv = q->master;
> + u64 current_count = atomic64_read(&q->counter);
> bool ret = q->flags & XT_QUOTA_INVERT;
> -
> - spin_lock_bh(&priv->lock);
> - if (priv->quota >= skb->len) {
> - priv->quota -= skb->len;
> - ret = !ret;
> - } else {
> - /* we do not allow even small packets from now on */
> - priv->quota = 0;
> - }
> - spin_unlock_bh(&priv->lock);
> -
> - return ret;
> + u64 old_count, new_count;
> +
> + do {
> + if (current_count == 1)
> + return ret;

So 1 means, don't keep updating, quota is depleted?

This current_count = 1 would be exposed to userspace too, right?

Hm, this semantics are going to be a bit awkwards to users I think, I
would prefer to expose this in a different way.

> + if (current_count <= skb->len) {
> + atomic64_set(&q->counter, 1);
> + return ret;
> + }
> + old_count = current_count;
> + new_count = current_count - skb->len;
> + current_count = atomic64_cmpxchg(&q->counter, old_count,
> + new_count);
> + } while (current_count != old_count);

Probably we simplify this via atomic64_add_return()?

I guess problem is userspace may get a current counter that is larger
than the quota, but we could handle this from userspace iptables to
print a value that equals the quota, ie. from userspace, before
printing:

if (consumed > quota)
printf("--consumed %PRIu64 ", quota);
else
printf("--consumed %PRIu64 ", consumed);

> + return !ret;
> }

Thanks !