cred: Fix group_is_primary()

This helper wasn't updated in commit be1f7435ef, so in reality it was
testing whether "gid" is the first supplemental group.  If a user
doesn't belong to a supplementary group, then it's testing an
uninitialized slot; since ucreds are allocated with M_ZERO, this
typically means that we're testing gid == 0.

group_is_primary() has exactly one use, in mac_do.  There, it's used to
determine whether the requested primary GID can be used in a setcred(2)
call when the ruleset does not explicitly specify a target primary GID.

I believe this is mostly exploitable by daemons which have explicitly
dropped privileges and called setgroups(0, NULL); logged in users will
have a non-empty supplementary group list by virtue of having gone
through initgroups(3).

Fix group_is_primary(), and add a regression test.

Approved by:	so
Security:	FreeBSD-SA-26:59.mac_do
Security:	CVE-2026-58092
Reported by:	Hazley Samsudin of GovTech CSG
Fixes:		be1f7435ef ("kern: start tracking cr_gid outside of cr_groups[]")
Reviewed by:	olce, kevans
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D59051

sys/sys/ucred.h

@@ -267,7 +267,7 @@ bool	cr_xids_subset(struct ucred *active_cred, struct ucred *obj_cred);
 static inline bool
 group_is_primary(const gid_t gid, const struct ucred *const cred)
 {
-	return (gid == cred->cr_groups[0] || gid == cred->cr_rgid ||
+	return (gid == cred->cr_gid || gid == cred->cr_rgid ||
 	    gid == cred->cr_svgid);
 }
 bool	group_is_supplementary(const gid_t gid, const struct ucred *const cred);

tests/sys/mac/do/Makefile

@@ -2,6 +2,8 @@ PACKAGE=	tests
 
 TESTSDIR=	${TESTSBASE}/sys/mac/do
 
+ATF_TESTS_C=	regression
+
 ATF_TESTS_SH+=	valid_configs invalid_configs consistency
 
 ${PACKAGE}FILES+=	common.sh

tests/sys/mac/do/regression.c

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2026 The FreeBSD Foundation
+ *
+ * This software was developed by Mark Johnston under sponsorship from
+ * the FreeBSD Foundation.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <sys/sysctl.h>
+#include <sys/ucred.h>
+
+#include <errno.h>
+#include <libgen.h>
+#include <limits.h>
+#include <pwd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <atf-c.h>
+
+/*
+ * Regression test for a bug which erroneously allowed the setcred() call below.
+ */
+ATF_TC(empty_supplementary_group_list);
+ATF_TC_HEAD(empty_supplementary_group_list, tc)
+{
+	atf_tc_set_md_var(tc, "require.user", "root");
+}
+ATF_TC_BODY(empty_supplementary_group_list, tc)
+{
+	struct setcred cred = SETCRED_INITIALIZER;
+	struct passwd *passwd;
+	const char *user;
+	char path[PATH_MAX], *progname, *rule;
+	int flags;
+
+	if (!atf_tc_has_config_var(tc, "unprivileged_user"))
+		atf_tc_skip("unprivileged_user not set");
+
+	user = atf_tc_get_config_var(tc, "unprivileged_user");
+	passwd = getpwnam(user);
+	ATF_REQUIRE(passwd != NULL);
+	ATF_REQUIRE_MSG(passwd->pw_uid != 0,
+	    "unprivileged user must not be root");
+	ATF_REQUIRE_MSG(passwd->pw_gid != 0,
+	    "unprivileged user group must not be wheel");
+
+	(void)asprintf(&rule, "uid=%d>uid=%d;gid=0>uid=0",
+	    passwd->pw_uid, passwd->pw_uid + 1);
+	ATF_REQUIRE(sysctlbyname("security.mac.do.rules",
+	    NULL, NULL, rule, strlen(rule)) == 0);
+
+	progname = basename(strdup(getprogname()));
+	(void)snprintf(path, sizeof(path), "%s/%s",
+	    atf_tc_get_config_var(tc, "srcdir"), progname);
+	ATF_REQUIRE(sysctlbyname("security.mac.do.exec_paths",
+	    NULL, NULL, path, strlen(path)) == 0);
+
+	ATF_REQUIRE(setgroups(0, NULL) == 0);
+	ATF_REQUIRE(setgid(passwd->pw_gid) == 0);
+	ATF_REQUIRE(setuid(passwd->pw_uid) == 0);
+
+	/*
+	 * Request the UID transition permitted by the first rule while also
+	 * setting all primary GIDs to 0.  MAC/do must reject this because GID 0
+	 * is not a primary GID of the current credential.
+	 */
+	cred.sc_uid = cred.sc_ruid = cred.sc_svuid = passwd->pw_uid + 1;
+	cred.sc_gid = cred.sc_rgid = cred.sc_svgid = 0;
+	flags = SETCREDF_UID | SETCREDF_RUID | SETCREDF_SVUID |
+	    SETCREDF_GID | SETCREDF_RGID | SETCREDF_SVGID;
+	ATF_REQUIRE_ERRNO(EPERM,
+	    setcred(flags, &cred, sizeof(cred)) == -1);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, empty_supplementary_group_list);
+
+	return (atf_no_error());
+}