kern/164402: [pf] pf crashes with a particular set of rules when first matching packet arrives

[Gleb Smirnoff]

On Tue, Apr 17, 2012 at 10:06:15AM +0200, Ermal Lu?i wrote:
E> 2012/4/16 Gleb Smirnoff <glebius at freebsd.org>:
E> > On Sun, Apr 15, 2012 at 12:00:21PM +0000, Gleb Smirnoff wrote:
E> > T>  On Sun, Apr 15, 2012 at 11:10:03AM +0000, Gleb Smirnoff wrote:
E> > T>  T>    I have a vague suspicion on what is happening. Your description of
E> > T>  T>  the problem looks like if a packet processing in the kernel has entered
E> > T>  T>  an endless loop.
E> > T>  T>
E> > T>  T>    Looking at pf_route() I see such possibility. From OpenBSD we have
E> > T>  T>  this protection against endless looping:
E> > T>  T>
E> > T>  T>          if ((*m)->m_pkthdr.pf.routed++ > 3) {
E> > T>  T>                  m0 = *m;
E> > T>  T>                  *m = NULL;
E> > T>  T>                  goto bad;
E> > T>  T>          }
E> > T>  T>
E> > T>  T>  In our code this transforms to:
E> > T>  T>
E> > T>  T>          if (pd->pf_mtag->routed++ > 3) {
E> > T>  T>                  m0 = *m;
E> > T>  T>                  *m = NULL;
E> > T>  T>                  goto bad;
E> > T>  T>          }
E> > T>  T>
E> > T>  T>  The root difference between storing the tag on mbuf and on pfdesc
E> > T>  T>  is that we lose pfdesc, and thus the tag, when we enter pf_test()
E> > T>  T>  recursively. And pf_route() does this recursion:
E> > T>  T>
E> > T>  T>          if (oifp != ifp) {
E> > T>  T>                  if (pf_test(PF_OUT, ifp, &m0, NULL) != PF_PASS) {
E> > T>  T>                          goto bad;
E> > T>  T>          ....
E> > T>
E> > T>  On second look I see that my suspicion may not be true. In the
E> > T>  beginning of pf_test() we do pf_get_mtag() which preserves already
E> > T>  present tag if there is one.
E> >
E> > Further investigation showed that problem exist when route applied
E> > ends in lo0, and packet passes to if_simloop(). There all mtags are
E> > stripped from the mbuf, including the pf mtag. Then packet is again
E> > processed by ip_input() again entering pf(4), if it again matches
E> > a routing rule, then we got an endless loop.
E> >
E> > We can try to fix this applying MTAG_PERSISTENT to the pf(4) tag id.
E> 
E> That seems like the best fix for this case.

In this case crash or freeze is fixed, but still packet is dropped. Example
of such rule:

pass in on igb0 fastroute proto tcp from any to $localip

Anyway, dropping packets is much better than crashing.

-- 
Totus tuus, Glebius.