Re: kernel v4.8: iptables logs are truncated with the 4.8 kernel?

[Liping Zhang]

Hi Chris,

2016-10-10 15:02 GMT+08:00 Chris Caputo <ccaputo@xxxxxxx>:
> On Tue, 4 Oct 2016, Justin Piszcz wrote:
>> kernel 4.8 with ulogd-2.0.5- IPs are no longer logged:
>>
>> Oct  4 17:51:30 atom INPUT_BLOCK IN=eth1 OUT=
>> MAC=00:1b:21:9c:3b:fa:3e:94:d5:d2:49:1e:08:00 LEN=0 TOS=00 PREC=0x00
>> TTL=0 ID=0 PROTO=0 MARK=0
>> Oct  4 17:51:31 atom INPUT_BLOCK IN=eth1 OUT=
>> MAC=00:1b:21:9c:3b:fa:3e:94:d5:d2:49:1e:08:00 LEN=0 TOS=00 PREC=0x00
>> TTL=0 ID=0 PROTO=0 MARK=0
>> Oct  4 17:51:32 atom INPUT_BLOCK IN=eth1 OUT=
>> MAC=00:1b:21:9c:3b:fa:3e:94:d5:d2:49:1e:08:00 LEN=0 TOS=00 PREC=0x00
>> TTL=0 ID=0 PROTO=0 MARK=0
>>
>> (reboot back to kernel 4.7, works fine)
>>
>> kernel 4.7 with ulogd-2.0.5:
>> Oct  4 17:56:44 atom INPUT_BLOCK IN=eth1 OUT=
>> MAC=00:1b:21:9c:3b:fa:3e:94:d5:d2:49:1e:08:00 SRC=74.125.22.125
>> DST=1.2.3.4 LEN=397 TOS=00 PREC=0x00 TTL=48 ID=58093 PROTO=TCP
>> SPT=5222 DPT=19804 SEQ=2032644254 ACK=2273184383 WINDOW=55272 ACK PSH
>> URGP=0 MARK=0
>> Oct  4 17:56:45 atom INPUT_BLOCK IN=eth1 OUT=
>> MAC=00:1b:21:9c:3b:fa:3e:94:d5:d2:49:1e:08:00 SRC=74.125.22.125
>> DST=1.2.3.4 LEN=397 TOS=00 PREC=0x00 TTL=48 ID=58725 PROTO=TCP
>> SPT=5222 DPT=19804 SEQ=2032644254 ACK=2273184383 WINDOW=55272 ACK PSH
>> URGP=0 MARK=0
>>
>> Looks like there were some changes in the 4.8 kernel regarding ulogd,
>> has anyone else run into this problem?
>
> For me, kernel 4.8.1 results in segfaults in ulogd-2.0.5 at:
>
>   Program received signal SIGSEGV, Segmentation fault.
>   0x00007ffff65fd18a in _interp_iphdr (pi=0x617f50, len=0) at ulogd_raw2packet_BASE.c:720
>
>   715     static int _interp_iphdr(struct ulogd_pluginstance *pi, uint32_t len)
>   716     {
>   717             struct ulogd_key *ret = pi->output.keys;
>   718             struct iphdr *iph =
>   719                     ikey_get_ptr(&pi->input.keys[INKEY_RAW_PCKT]);
>   720             void *nexthdr = (uint32_t *)iph + iph->ihl;
>
> I believe 7643507fe8b5bd8ab7522f6a81058cc1209d2585 changed previous
> behavior by not always copying IP header data to user space.
>
> On my machine IPv4 log packets result in a ulogd segfault while IPv6
> packets do not.  I'm not sure of the cause of the difference.
>
> The corresponding userspace commit for the 209d2585 kernel change is:
>
>   https://git.netfilter.org/iptables/commit/?id=7070b1f3c88a0c3d4e315c00cca61f05b0fbc882
>
> This adds --nflog-size to iptables.  When --nflog-size is used with my
> iptables NFLOG lines, the ulogd-2.0.5 segfaults cease.

What numbers did you specify after --nflog-size option?
--nflog-size 0 or ...? If you want log the whole packet to
the ulogd, please do not specify this nflog-size option.

>
> I'm surprised to see a kernel change cause unexpected userspace segfaults,
> so further investigation into a kernel fix would seem a good idea.

According to the original user's manual, nflog-range option was
designed to be the number of bytes copied to userspace, but
unfortunately there's a bug from the beginning and it never works,
i.e. in kernel, it just ignored this option.

Try to change the current nflog-range option's semantics may
cause unexpected results(maybe like this ulogd crash) ...

In order to keep compatibility, Vishwanath introduce a new
nflog-size option and keep nflog-range unchanged. If you just
upgrade the kernel, and do not change iptables rules, this
problem will not happen.

So I think this is ulogd's bug, in _interp_iphdr, it try to
dereference the iphdr pointer before validation check, meanwhile
this problem does not exist in ipv6 path.  Can you try this patch:

diff --git a/filter/raw2packet/ulogd_raw2packet_BASE.c
b/filter/raw2packet/ulogd_raw2packet_BASE.c
index 8a6180c..fd2665a 100644
--- a/filter/raw2packet/ulogd_raw2packet_BASE.c
+++ b/filter/raw2packet/ulogd_raw2packet_BASE.c
@@ -717,7 +717,7 @@ static int _interp_iphdr(struct ulogd_pluginstance
*pi, uint32_t len)
        struct ulogd_key *ret = pi->output.keys;
        struct iphdr *iph =
                ikey_get_ptr(&pi->input.keys[INKEY_RAW_PCKT]);
-       void *nexthdr = (uint32_t *)iph + iph->ihl;
+       void *nexthdr;

        if (len < sizeof(struct iphdr) || len <= (uint32_t)(iph->ihl * 4))
                return ULOGD_IRET_OK;
@@ -734,6 +734,7 @@ static int _interp_iphdr(struct ulogd_pluginstance
*pi, uint32_t len)
        okey_set_u16(&ret[KEY_IP_ID], ntohs(iph->id));
        okey_set_u16(&ret[KEY_IP_FRAGOFF], ntohs(iph->frag_off));

+       nexthdr = (uint32_t *)iph + iph->ihl;
        switch (iph->protocol) {
        case IPPROTO_TCP:
                _interp_tcp(pi, nexthdr, len);

Thanks

> Having to add the likes of "--nflog-size 200" (200 simply being what I am
> using) to every NFLOG line in firewall configs is a significant burden for
> many.
>
> Putting out a new release of iptables may help ease this transition if the
> kernel is not patched to fix this.  I had to use the git code since 1.6.0
> doesn't have it.
>
> Chris