Re: [PATCH] netfilter: conntrack: fix calculation of next bucket number in early_drop

[Vasiliy Khoruzhick]

On Wed, Oct 24, 2018 at 9:52 PM, Dmitry Safonov <dima@xxxxxxxxxx> wrote:
> On 10/25/18 4:48 AM, Vasily Khoruzhick wrote:
>>
>> If there's no entry to drop in bucket that corresponds to the hash,
>> early_drop() should look for it in other buckets. But since it increments
>> hash instead of bucket number, it actually looks in the same bucket 8
>> times: hsize is 16k by default (14 bits) and hash is 32-bit value, so
>> reciprocal_scale(hash, hsize) returns the same value for hash..hash+7 in
>> most cases.
>>
>> Fix it by increasing bucket number instead of hash and rename _hash
>> to bucket to avoid future confusion.
>>
>> Fixes: 3e86638e9a0b ("netfilter: conntrack: consider ct netns in
>> early_drop logic")
>> Cc: <stable@xxxxxxxxxxxxxxx> # v4.7+
>> Signed-off-by: Vasily Khoruzhick <vasilykh@xxxxxxxxxx>
>
>
> Nice work!
>
> Reviewed-by: Dmitry Safonov <dima@xxxxxxxxxx>

Oops, found a bug in it - 'bucket' should be moved outside of the
loop. I'll send v2 tomorrow morning.

>
>> ---
>>   net/netfilter/nf_conntrack_core.c | 11 +++++++----
>>   1 file changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/net/netfilter/nf_conntrack_core.c
>> b/net/netfilter/nf_conntrack_core.c
>> index ca1168d67fac..a04af246b184 100644
>> --- a/net/netfilter/nf_conntrack_core.c
>> +++ b/net/netfilter/nf_conntrack_core.c
>> @@ -1073,19 +1073,22 @@ static unsigned int early_drop_list(struct net
>> *net,
>>         return drops;
>>   }
>>   -static noinline int early_drop(struct net *net, unsigned int _hash)
>> +static noinline int early_drop(struct net *net, unsigned int hash)
>>   {
>>         unsigned int i;
>>         for (i = 0; i < NF_CT_EVICTION_RANGE; i++) {
>>                 struct hlist_nulls_head *ct_hash;
>> -               unsigned int hash, hsize, drops;
>> +               unsigned int bucket, hsize, drops;
>>                 rcu_read_lock();
>>                 nf_conntrack_get_ht(&ct_hash, &hsize);
>> -               hash = reciprocal_scale(_hash++, hsize);
>> +               if (!i)
>> +                       bucket = reciprocal_scale(hash, hsize);
>> +               else
>> +                       bucket = (bucket + 1) % hsize;
>>   -             drops = early_drop_list(net, &ct_hash[hash]);
>> +               drops = early_drop_list(net, &ct_hash[bucket]);
>>                 rcu_read_unlock();
>>                 if (drops) {
>>
>
> --
>           Dima