netdev - Re: [PATCH v4 nf] netfilter: seqadj: Drop the packet directly when fail to add seqadj extension to avoid dereference NULL pointer later

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Date:   Tue, 6 Sep 2016 18:37:57 +0800
From:   Gao Feng <fgao@...ai8.com>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     Netfilter Developer Mailing List 
        <netfilter-devel@...r.kernel.org>, Florian Westphal <fw@...len.de>,
        coreteam@...filter.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 nf] netfilter: seqadj: Drop the packet directly when
 fail to add seqadj extension to avoid dereference NULL pointer later

inline

On Tue, Sep 6, 2016 at 6:17 PM, Pablo Neira Ayuso <pablo@...filter.org> wrote:
> On Tue, Sep 06, 2016 at 09:57:23AM +0800, fgao@...ai8.com wrote:
>> From: Gao Feng <fgao@...ai8.com>
>>
>> When memory is exhausted, nfct_seqadj_ext_add may fail to add the seqadj
>> extension. But the function nf_ct_seqadj_init doesn't check if get valid
>> seqadj pointer by the nfct_seqadj.
>>
>> Now drop the packet directly when fail to add seqadj extension to avoid
>> dereference NULL pointer in nf_ct_seqadj_init.
>>
>> Signed-off-by: Gao Feng <fgao@...ai8.com>
>> ---
>>  v4: Drop the packet directly when fail to add seqadj extension;
>>  v3: Remove the warning log when seqadj is null;
>>  v2: Remove the unnessary seqadj check in nf_ct_seq_adjust
>>  v1: Initial patch
>>
>>  net/netfilter/nf_conntrack_core.c | 6 +++++-
>>  net/netfilter/nf_nat_core.c       | 3 ++-
>>  2 files changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
>> index dd2c43a..dfa76ce 100644
>> --- a/net/netfilter/nf_conntrack_core.c
>> +++ b/net/netfilter/nf_conntrack_core.c
>> @@ -1036,7 +1036,11 @@ init_conntrack(struct net *net, struct nf_conn *tmpl,
>>               return (struct nf_conntrack_tuple_hash *)ct;
>>
>>       if (tmpl && nfct_synproxy(tmpl)) {
>> -             nfct_seqadj_ext_add(ct);
>> +             if (!nfct_seqadj_ext_add(ct)) {
>> +                     nf_conntrack_free(ct);
>> +                     pr_debug("Can't add seqadj extension\n");
>> +                     return NULL;
>> +             }
>>               nfct_synproxy_ext_add(ct);
>
> I think this is part of the same logical change, ie. nf_ct_ext_add()
> returns NULL, then I would also fix nfct_synproxy_ext_add() in this
> go.
>
Sorry, I am not clear well.
Need I modify the logic of this part?

>>       }
>>
>> diff --git a/net/netfilter/nf_nat_core.c b/net/netfilter/nf_nat_core.c
>> index de31818..b82282a 100644
>> --- a/net/netfilter/nf_nat_core.c
>> +++ b/net/netfilter/nf_nat_core.c
>> @@ -441,7 +441,8 @@ nf_nat_setup_info(struct nf_conn *ct,
>>                       ct->status |= IPS_DST_NAT;
>>
>>               if (nfct_help(ct))
>> -                     nfct_seqadj_ext_add(ct);
>> +                     if (!nfct_seqadj_ext_add(ct))
>> +                             return NF_DROP;
>
> ctnetlink may have created a conntrack with seqadj in place by when we
> call nf_nat_setup_info() so NF_ACCEPT would be more conservative, eg.
> via conntrackd state synchronization.
>
> Actually, after a quick look at ctnetlink, I don't see any any call to
> nfct_seqadj_ext_add() from there, so I suspect this is broken since
> SYNPROXY was introduced. It would be great if you can review this and
> send us patches to fix this, if indeed needed.
>
> Thanks!

Let me confirm the problem, or I am afraid I would misunderstand our meaning.
This patch only need to modify the "NF_DROP" to "NF_ACCEPT", is it?

Then I could commit another patch to fix the ctnetlink lost nfct_seqadj_ext_add.

Best Regards
Feng