MAC/do: Check executable path from the current jail's root

Contrary to my initial belief, vn_fullpath() does return a vnode's path
from the current chroot, and not from the global root (which would have
been a bug also, but without security consequences).  This enables
a "confused deputy"-like scenario where a chroot(2) can change which
executable can be authorized by MAC/do, which is even more problematic
for unprivileged chroot(2).

This was found by re-examining the code following two close events:
1. Shawn Webb sent a mail to freebsd-hackers@ on 08/05 saying that in
   HardenedBSD they had added a check on P2_NO_NEW_PRIVS (in
   mac_do_priv_grant()), which I responded to on 08/20 saying that
   P2_NO_NEW_PRIVS was not necessary for mac_do(4), with a correct
   reasoning but based on the wrong above-mentioned assumption about
   vn_fullpath().
2. I reviewed some code by Kushagra Srivastava (GSoC 2025 student
   working on mac_do(4)/mdo(1)) adding the ability to specify which
   executables can spawn processes that mac_do(4) may decide to
   authorize (others are simply ignored), which currently is hardcoded
   to '/usr/bin/mdo'.

MFC after:      3 days
Event:          EuroBSDCon 2025
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D52758

(cherry picked from commit 9f269a0a771aff4f0a735211907a52c52fc0661b)

0 files changed, 0 insertions, 0 deletions