[PATCH] timekeeping: Require permission to set auxiliary clocks

[Daehyeon Ko]

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")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Daehyeon Ko <4ncienth@xxxxxxxxx>
---
AI assistance was used for source analysis, reproducer development, and
validation. The submitter reviewed the resulting change before signing it.

 kernel/time/timekeeping.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ea2e6e55f37bf..1b342825bd8f1 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -14,6 +14,7 @@
 #include <linux/random.h>
 #include <linux/sched/clock.h>
 #include <linux/sched/loadavg.h>
+#include <linux/security.h>
 #include <linux/static_key.h>
 #include <linux/stop_machine.h>
 #include <linux/syscore_ops.h>
@@ -3215,11 +3216,15 @@ static int aux_clock_set(const clockid_t id, const struct timespec64 *tnew)
 	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;
 

base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
-- 
2.55.0