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SubjectRe: kernel v4.8: iptables logs are truncated with the 4.8 kernel?
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On Mon, 10 Oct 2016, Liping Zhang wrote:
> 2016-10-10 15:02 GMT+08:00 Chris Caputo <ccaputo@alt.net>:
> > 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.

Not specifying nflog-size does not appear to log the whole packet...

If "--nflog-size" is unspecified, and the iptables config is left
unchanged when the kernel is upgraded to 4.8, ulogd-2.0.5 crashes.

If "--nflog-size 0" is used, ulogd-2.0.5 crashes.

If "--nflog-size" is used with size 1 or greater, ulogd-2.0.5 is fine.

> > 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.

I am reporting that the problem does happen simply with an upgrade to
kernel 4.8 and no other changes. When "--nflog-size" is unspecified or
set to 0, the bug in ulogd-2.0.5 gets triggered.

I agree there is a bug in ulogd-2.0.5 that this kernel change exposed, but
I am trying to explain that all ulogd users risk this segfault if they
upgrade to kernel 4.8 and don't either update to a fixed ulogd (possibly
using your patch below) or an unreleased iptables with iptables config
changes to implement nflog-size on each NFLOG target.

> 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);

I agree this will likely fix ulogd, but this misses the point about the
new kernel defaulting to a zero size return when it used to return the
packet.

Thanks,
Chris

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