misc/165373: Obvious use-after-free in ipfw_nat()

[Arnaud Lacombe]

>Number:         165373
>Category:       misc
>Synopsis:       Obvious use-after-free in ipfw_nat()
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Feb 21 20:30:11 UTC 2012
>Closed-Date:
>Last-Modified:
>Originator:     Arnaud Lacombe
>Release:        10-current
>Organization:
n/a
>Environment:
>Description:
ipfw_nat() reads the following:

        mcl = m_megapullup(m, m->m_pkthdr.len);
        if (mcl == NULL) {
                args->m = NULL;
                return (IP_FW_DENY);
        }
        ip = mtod(mcl, struct ip *);
        [...]
        if (retval == PKT_ALIAS_RESPOND)
                m->m_flags |= M_SKIP_FIREWALL

It is quite obvious that `m' is no longer valid in the second conditional.

I reported this months ago on freebsd-net@ and did provide a patch to fix it, but no developers seems to actually care about such a bug.
>How-To-Repeat:
read the code.
>Fix:
s/m/mcl/

>Release-Note:
>Audit-Trail:
>Unformatted: