[PATCH 6.18.y] timekeeping: Require permission to set auxiliary clocks

[Thomas Gleixner <tglx@kernel.org>]

From: Daehyeon Ko <4ncienth@gmail.com>

commit 524d95c8a2000cba6cb80a173d76cb746c561244 upstream.

The generic clock_settime() entry point delegates permission checks to
each clock-specific callback. aux_clock_set() does not perform one. Once
an administrator enables an auxiliary clock, any local process can
therefore change the global clock offset.

A test process running as UID and GID 65534 with no effective, permitted,
inheritable, or ambient capabilities set CLOCK_AUX0 successfully, and a
separate process observed the new value. The same process was denied by
clock_adjtime(). An AppArmor profile denying sys_time also blocked
CLOCK_REALTIME changes but did not see the auxiliary clock operation.

Call security_settime64() before mutating the auxiliary timekeeper. This
matches CLOCK_REALTIME's permission model, enforces initial-user-namespace
CAP_SYS_TIME, and gives LSMs the settime hook while preserving authorized
clock changes.

Fixes: 60ecc26ec5af ("timekeeping: Provide time setter for auxiliary clocks")
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: shaos <shawnq1@163.com>
Acked-by: John Stultz <jstultz@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261019080324.4147815-1-4ncienth@gmail.com
---
 kernel/time/timekeeping.c |    5 +++++
 1 file changed, 5 insertions(+)
---
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -14,6 +14,7 @@
 #include <linux/sched.h>
 #include <linux/sched/loadavg.h>
 #include <linux/sched/clock.h>
+#include <linux/security.h>
 #include <linux/syscore_ops.h>
 #include <linux/clocksource.h>
 #include <linux/jiffies.h>
@@ -2936,11 +2937,15 @@ static int aux_clock_set(const clockid_t
 	struct tk_data *aux_tkd = aux_get_tk_data(id);
 	struct timekeeper *aux_tks;
 	ktime_t tnow, nsecs;
+	int ret;
 
 	if (!timespec64_valid_settod(tnew))
 		return -EINVAL;
 	if (!aux_tkd)
 		return -ENODEV;
+	ret = security_settime64(tnew, NULL);
+	if (ret)
+		return ret;
 
 	aux_tks = &aux_tkd->shadow_timekeeper;