git: 97b7439e3b06 - releng/15.1 - cred: Fix group_is_primary()

The branch releng/15.1 has been updated by markj:

URL: https://cgit.FreeBSD.org/src/commit/?id=97b7439e3b060d350cbd85321dd7e315907f366a

commit 97b7439e3b060d350cbd85321dd7e315907f366a
Author:     Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2026-08-24 18:13:29 +0000
Commit:     Mark Johnston <markj@FreeBSD.org>
CommitDate: 2026-08-24 19:34:13 +0000

    cred: Fix group_is_primary()
    
    This helper wasn't updated in commit be1f7435ef21, 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 to keep the caller's current primary groups.  This
    means that a rule such as gid=0>uid=0 will permit any credential with no
    supplementary groups.
    
    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:          be1f7435ef21 ("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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sys/sys/ucred.h b/sys/sys/ucred.h
index 4bf48a5e4b87..19b45a5541a7 100644
--- a/sys/sys/ucred.h
+++ b/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);