[PATCH net v3 4/5] ptp: vclock: serialize extended samples with counter updates

[James Hilliard <james.hilliard1@gmail.com>]

The extended and cross-timestamp callbacks sample the physical clock
before taking the virtual clock mutex. A concurrent frequency adjustment
can advance the timecounter and change its multiplier in between, so the
older sample is converted using a different frequency than the one in
effect when it was taken.

Take the mutex before sampling and keep it held through the conversion.
Retain timecounter_cyc2time() and its existing conversion window. A PHC
sample need not advance the counter: for example, e1000e explicitly
allows non-monotonic SYSTIM readings. Treating a slightly older sample
as a forward delta would instead advance virtual time by nearly a full
counter wrap.

This race was found by code inspection of the virtual clock sampling
paths.

Fixes: f0067ebfc42b ("ptp: add gettimex64() to virtual clocks.")
Fixes: 21fad63084c2 ("ptp: add getcrosststamp() to virtual clocks.")
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/ptp/ptp_vclock.c | 32 ++++++++++++++------------------
 1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/drivers/ptp/ptp_vclock.c b/drivers/ptp/ptp_vclock.c
index eea207e5a889..6675466bc307 100644
--- a/drivers/ptp/ptp_vclock.c
+++ b/drivers/ptp/ptp_vclock.c
@@ -113,20 +113,18 @@ static int ptp_vclock_gettimex(struct ptp_clock_info *ptp,
 	struct ptp_clock *pptp = vclock->pclock;
 	struct timespec64 pts;
 	int err;
-	u64 ns;
-
-	err = pptp->info->getcyclesx64(pptp->info, &pts, sts);
-	if (err)
-		return err;
 
 	if (mutex_lock_interruptible(&vclock->lock))
 		return -EINTR;
-	ns = timecounter_cyc2time(&vclock->tc, timespec64_to_ns(&pts));
-	mutex_unlock(&vclock->lock);
+	err = pptp->info->getcyclesx64(pptp->info, &pts, sts);
+	if (!err) {
+		u64 ns = timecounter_cyc2time(&vclock->tc, timespec64_to_ns(&pts));
 
-	*ts = ns_to_timespec64(ns);
+		*ts = ns_to_timespec64(ns);
+	}
+	mutex_unlock(&vclock->lock);
 
-	return 0;
+	return err;
 }
 
 static int ptp_vclock_settime(struct ptp_clock_info *ptp,
@@ -152,20 +150,18 @@ static int ptp_vclock_getcrosststamp(struct ptp_clock_info *ptp,
 	struct ptp_vclock *vclock = info_to_vclock(ptp);
 	struct ptp_clock *pptp = vclock->pclock;
 	int err;
-	u64 ns;
-
-	err = pptp->info->getcrosscycles(pptp->info, xtstamp);
-	if (err)
-		return err;
 
 	if (mutex_lock_interruptible(&vclock->lock))
 		return -EINTR;
-	ns = timecounter_cyc2time(&vclock->tc, ktime_to_ns(xtstamp->device));
-	mutex_unlock(&vclock->lock);
+	err = pptp->info->getcrosscycles(pptp->info, xtstamp);
+	if (!err) {
+		u64 ns = timecounter_cyc2time(&vclock->tc, ktime_to_ns(xtstamp->device));
 
-	xtstamp->device = ns_to_ktime(ns);
+		xtstamp->device = ns_to_ktime(ns);
+	}
+	mutex_unlock(&vclock->lock);
 
-	return 0;
+	return err;
 }
 
 static long ptp_vclock_refresh(struct ptp_clock_info *ptp)

-- 
2.53.0