LSM: SafeSetID: Add setgroups() security policy handling

The SafeSetID LSM has functionality for restricting setuid()/setgid()
syscalls based on its configured security policies. This patch adds the
analogous functionality for the setgroups() syscall. Security policy
for the setgroups() syscall follows the same policies that are
installed on the system for setgid() syscalls.
Signed-off-by: Micah Morton <mortonm@chromium.org>

security/safesetid/lsm.c

@@ -97,15 +97,9 @@ static int safesetid_security_capable(const struct cred *cred,
 		return 0;
 
 	/*
-	 * If CAP_SET{U/G}ID is currently used for a setid() syscall, we want to
-	 * let it go through here; the real security check happens later, in the
-	 * task_fix_set{u/g}id hook.
-         *
-         * NOTE:
-         * Until we add support for restricting setgroups() calls, GID security
-         * policies offer no meaningful security since we always return 0 here
-         * when called from within the setgroups() syscall and there is no
-         * additional hook later on to enforce security policies for setgroups().
+	 * If CAP_SET{U/G}ID is currently used for a setid or setgroups syscall, we
+	 * want to let it go through here; the real security check happens later, in
+	 * the task_fix_set{u/g}id or task_fix_setgroups hooks.
 	 */
 	if ((opts & CAP_OPT_INSETID) != 0)
 		return 0;
@@ -241,9 +235,36 @@ static int safesetid_task_fix_setgid(struct cred *new,
 	return -EACCES;
 }
 
+static int safesetid_task_fix_setgroups(struct cred *new, const struct cred *old)
+{
+	int i;
+
+	/* Do nothing if there are no setgid restrictions for our old RGID. */
+	if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
+		return 0;
+
+	get_group_info(new->group_info);
+	for (i = 0; i < new->group_info->ngroups; i++) {
+		if (!id_permitted_for_cred(old, (kid_t){.gid = new->group_info->gid[i]}, GID)) {
+			put_group_info(new->group_info);
+			/*
+			 * Kill this process to avoid potential security vulnerabilities
+			 * that could arise from a missing allowlist entry preventing a
+			 * privileged process from dropping to a lesser-privileged one.
+			 */
+			force_sig(SIGKILL);
+			return -EACCES;
+		}
+	}
+
+	put_group_info(new->group_info);
+	return 0;
+}
+
 static struct security_hook_list safesetid_security_hooks[] = {
 	LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
 	LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid),
+	LSM_HOOK_INIT(task_fix_setgroups, safesetid_task_fix_setgroups),
 	LSM_HOOK_INIT(capable, safesetid_security_capable)
 };