timekeeping: Audit clock adjustments

Emit an audit record whenever the system clock is changed (i.e. shifted
by a non-zero offset) by a syscall from userspace. The syscalls than can
(at the time of writing) trigger such record are:
  - settimeofday(2), stime(2), clock_settime(2) -- via
    do_settimeofday64()
  - adjtimex(2), clock_adjtime(2) -- via do_adjtimex()

The new records have type AUDIT_TIME_INJOFFSET and contain the following
fields:
  - sec -- the 'seconds' part of the offset
  - nsec -- the 'nanoseconds' part of the offset

Example record (time was shifted backwards by ~15.875 seconds):

type=TIME_INJOFFSET msg=audit(1530616049.652:13): sec=-16 nsec=124887145

The records of this type will be associated with the corresponding
syscall records.
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
[PM: fixed a line width problem in __audit_tk_injoffset()]
Signed-off-by: Paul Moore <paul@paul-moore.com>

include/linux/audit.h

@@ -365,6 +365,7 @@ extern void __audit_log_capset(const struct cred *new, const struct cred *old);
 extern void __audit_mmap_fd(int fd, int flags);
 extern void __audit_log_kern_module(char *name);
 extern void __audit_fanotify(unsigned int response);
+extern void __audit_tk_injoffset(struct timespec64 offset);
 
 static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
 {
@@ -467,6 +468,16 @@ static inline void audit_fanotify(unsigned int response)
 		__audit_fanotify(response);
 }
 
+static inline void audit_tk_injoffset(struct timespec64 offset)
+{
+	/* ignore no-op events */
+	if (offset.tv_sec == 0 && offset.tv_nsec == 0)
+		return;
+
+	if (!audit_dummy_context())
+		__audit_tk_injoffset(offset);
+}
+
 extern int audit_n_rules;
 extern int audit_signals;
 #else /* CONFIG_AUDITSYSCALL */
@@ -580,6 +591,9 @@ static inline void audit_log_kern_module(char *name)
 static inline void audit_fanotify(unsigned int response)
 { }
 
+static inline void audit_tk_injoffset(struct timespec64 offset)
+{ }
+
 static inline void audit_ptrace(struct task_struct *t)
 { }
 #define audit_n_rules 0

include/uapi/linux/audit.h

@@ -114,6 +114,7 @@
 #define AUDIT_REPLACE		1329	/* Replace auditd if this packet unanswerd */
 #define AUDIT_KERN_MODULE	1330	/* Kernel Module events */
 #define AUDIT_FANOTIFY		1331	/* Fanotify access decision */
+#define AUDIT_TIME_INJOFFSET	1332	/* Timekeeping offset injected */
 
 #define AUDIT_AVC		1400	/* SE Linux avc denial or grant */
 #define AUDIT_SELINUX_ERR	1401	/* Internal SE Linux Errors */

kernel/auditsc.c

@@ -2512,6 +2512,13 @@ void __audit_fanotify(unsigned int response)
 		AUDIT_FANOTIFY,	"resp=%u", response);
 }
 
+void __audit_tk_injoffset(struct timespec64 offset)
+{
+	audit_log(audit_context(), GFP_KERNEL, AUDIT_TIME_INJOFFSET,
+		  "sec=%lli nsec=%li",
+		  (long long)offset.tv_sec, offset.tv_nsec);
+}
+
 static void audit_log_task(struct audit_buffer *ab)
 {
 	kuid_t auid, uid;

kernel/time/timekeeping.c

@@ -21,6 +21,7 @@
 #include <linux/stop_machine.h>
 #include <linux/pvclock_gtod.h>
 #include <linux/compiler.h>
+#include <linux/audit.h>
 
 #include "tick-internal.h"
 #include "ntp_internal.h"
@@ -1250,6 +1251,9 @@ int do_settimeofday64(const struct timespec64 *ts)
 	/* signal hrtimers about time change */
 	clock_was_set();
 
+	if (!ret)
+		audit_tk_injoffset(ts_delta);
+
 	return ret;
 }
 EXPORT_SYMBOL(do_settimeofday64);
@@ -2322,6 +2326,8 @@ int do_adjtimex(struct __kernel_timex *txc)
 		ret = timekeeping_inject_offset(&delta);
 		if (ret)
 			return ret;
+
+		audit_tk_injoffset(delta);
 	}
 
 	ktime_get_real_ts64(&ts);