Re: [PATCH V3] audit: normalize NETFILTER_PKT

[Paul Moore <paul@paul-moore.com>]

On Wed, Mar 1, 2017 at 5:34 PM, Richard Guy Briggs <rgb@redhat.com> wrote:
> On 2017-03-01 17:19, Paul Moore wrote:
>> On Wed, Mar 1, 2017 at 11:28 AM, Richard Guy Briggs <rgb@redhat.com> wrote:
>> > On 2017-02-28 17:22, Paul Moore wrote:
>> >> On Sun, Feb 26, 2017 at 3:49 PM, Richard Guy Briggs <rgb@redhat.com> wrote:
>> >> > Eliminate flipping in and out of message fields, dropping fields in the process.
>> >> >
>> >> > Sample raw message format IPv4 UDP:
>> >> > type=NETFILTER_PKT msg=audit(1487874761.386:228):  mark=0xae8a2732 saddr=127.0.0.1 daddr=127.0.0.1 proto=17^]
>> >> > Sample raw message format IPv6 ICMP6:
>> >> > type=NETFILTER_PKT msg=audit(1487874761.381:227):  mark=0x223894b7 saddr=::1 daddr=::1 proto=58^]
>> >> >
>> >> > Issue: https://github.com/linux-audit/audit-kernel/issues/11
>> >> > Test case: https://github.com/linux-audit/audit-testsuite/issues/43
>> >> >
>> >> > Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
>> >> > ---
>> >> >  net/netfilter/xt_AUDIT.c |  122 ++++++++++-----------------------------------
>> >> >  1 files changed, 27 insertions(+), 95 deletions(-)
>> >> >
>> >> > diff --git a/net/netfilter/xt_AUDIT.c b/net/netfilter/xt_AUDIT.c
>> >> > index 4973cbd..945fa29 100644
>> >> > --- a/net/netfilter/xt_AUDIT.c
>> >> > +++ b/net/netfilter/xt_AUDIT.c
>> >> > @@ -31,146 +31,78 @@ MODULE_ALIAS("ip6t_AUDIT");
>> >>
>> >> ...
>> >>
>> >> > -static void audit_ip4(struct audit_buffer *ab, struct sk_buff *skb)
>> >> > +static bool audit_ip4(struct audit_buffer *ab, struct sk_buff *skb)
>> >> >  {
>> >> >         struct iphdr _iph;
>> >> >         const struct iphdr *ih;
>> >> >
>> >> >         ih = skb_header_pointer(skb, 0, sizeof(_iph), &_iph);
>> >>
>> >> It seems like we should be using skb_network_offset(skb) instead of 0
>> >> above, yes?  Granted, this isn't new, but let's fix it.
>> >
>> > Yes, I agree.  How does this even work now?  Maybe the MAC header hasn't
>> > been added yet (or has already been processed and stripped off) so that
>> > skb->data is already pointing at the network header and hence has an
>> > offset of 0.  Can you be more explicit and elaborate to say if this what
>> > you were thinking?
>>
>> Unfortunately, not really, I haven't thought through of all the
>> situations and it has been a long time since I've had to worry about
>> things like this.  I think we are in agreement that it needs to
>> change, so let's just make the change.
>
> Given Pablo's assurances, this could go either way, fix audit_ip4 to use
> skb_network_offset() or fix audit_ip6 to use zero.  I don't have a
> strong opinion, but using zero would be more efficient while using
> skb_network_offset() would remove the doubt.  Either way, the
> consistency will avoid raising doubt in the future as you have
> (rightfully) done.

Just use skb_network_offset() as it is the safer option and there is
plenty of precedence.  Considering that we expect NETFILTER_PKT to see
limited use, I'm more concerned about it not breaking than some small
loss of performance.

>> >> >  static unsigned int
>> >> >  audit_tg(struct sk_buff *skb, const struct xt_action_param *par)
>> >> >  {
>> >> > -       const struct xt_audit_info *info = par->targinfo;
>> >> >         struct audit_buffer *ab;
>> >> > +       int fam = -1;
>> >> >
>> >> >         if (audit_enabled == 0)
>> >> >                 goto errout;
>> >> > -
>> >> >         ab = audit_log_start(NULL, GFP_ATOMIC, AUDIT_NETFILTER_PKT);
>> >> >         if (ab == NULL)
>> >> >                 goto errout;
>> >> >
>> >> > -       audit_log_format(ab, "action=%hhu hook=%u len=%u inif=%s outif=%s",
>> >> > -                        info->type, par->hooknum, skb->len,
>> >> > -                        par->in ? par->in->name : "?",
>> >> > -                        par->out ? par->out->name : "?");
>> >> > -
>> >> > -       if (skb->mark)
>> >> > -               audit_log_format(ab, " mark=%#x", skb->mark);
>> >> > -
>> >> > -       if (skb->dev && skb->dev->type == ARPHRD_ETHER) {
>> >> > -               audit_log_format(ab, " smac=%pM dmac=%pM macproto=0x%04x",
>> >> > -                                eth_hdr(skb)->h_source, eth_hdr(skb)->h_dest,
>> >> > -                                ntohs(eth_hdr(skb)->h_proto));
>> >> > +       audit_log_format(ab, "mark=%#x", skb->mark ?: -1);
>> >>
>> >> How do Steve's userspace tools like the unset/-1 value represented
>> >> when it is a hex value: -1 or 0xffffffff?
>> >
>> > My understanding is they are set up to cope with this.
>>
>> How does userspace distinguish between an unset nfmark and a nfmark of
>> 0xffffffff?
>
> It never had to deal specifically with nfmark previously because it
> wasn't included if it was blank.  Generally other values that are -1 are
> interpreted by the audit userspace tools as unset (session id, auid,
> etc...)

Yes, I know, let me get straight to the point: should we use "mark=-1"
when the nfmark is unset instead of "mark=0xffffffff"?

-- 
paul moore
www.paul-moore.com