time: adjtimex: Validate the ADJ_FREQUENCY values

commit 5e5aeb43 upstream.

Verify that the frequency value from userspace is valid and makes sense.

Unverified values can cause overflows later on.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
[jstultz: Fix up bug for negative values and drop redunent cap check]
Signed-off-by: John Stultz <john.stultz@linaro.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/time/ntp.c

@@ -608,6 +608,13 @@ int do_adjtimex(struct timex *txc)
 			return -EINVAL;
 	}
 
+	if (txc->modes & ADJ_FREQUENCY) {
+		if (LONG_MIN / PPM_SCALE > txc->freq)
+			return -EINVAL;
+		if (LONG_MAX / PPM_SCALE < txc->freq)
+			return -EINVAL;
+	}
+
 	if (txc->modes & ADJ_SETOFFSET) {
 		struct timespec delta;
 		delta.tv_sec  = txc->time.tv_sec;