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

sys/security/mac_do/mac_do.c

@@ -1992,6 +1992,10 @@ check_proc(void)
 	/*
 	 * Only grant privileges if requested by the right executable.
 	 *
+	 * As MAC/do configuration is per-jail, in order to avoid confused
+	 * deputy situations in chroots (privileged or unprivileged), make sure
+	 * to check the path from the current jail's root.
+	 *
 	 * XXXOC: We may want to base this check on a tunable path and/or
 	 * a specific MAC label.  Going even further, e.g., envisioning to
 	 * completely replace the path check with the latter, we would need to
@@ -2003,7 +2007,7 @@ check_proc(void)
 	 * setting a MAC label per file (perhaps via additions to mtree(1)).  So
 	 * this probably isn't going to happen overnight, if ever.
 	 */
-	if (vn_fullpath(curproc->p_textvp, &path, &to_free) != 0)
+	if (vn_fullpath_jail(curproc->p_textvp, &path, &to_free) != 0)
 		return (EPERM);
 	error = strcmp(path, "/usr/bin/mdo") == 0 ? 0 : EPERM;
 	free(to_free, M_TEMP);