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Mark Johnston
96bf349029
This helper wasn't updated in commitbe1f7435ef, 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