Patch "ptp: annotate data-race around q->head and q->tail" has been added to the 4.19-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    ptp: annotate data-race around q->head and q->tail

to the 4.19-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ptp-annotate-data-race-around-q-head-and-q-tail.patch
and it can be found in the queue-4.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 061696c1e5e484e60c36122b3b82e5681bb196b8
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Thu Nov 9 17:48:59 2023 +0000

    ptp: annotate data-race around q->head and q->tail
    
    [ Upstream commit 73bde5a3294853947252cd9092a3517c7cb0cd2d ]
    
    As I was working on a syzbot report, I found that KCSAN would
    probably complain that reading q->head or q->tail without
    barriers could lead to invalid results.
    
    Add corresponding READ_ONCE() and WRITE_ONCE() to avoid
    load-store tearing.
    
    Fixes: d94ba80ebbea ("ptp: Added a brand new class driver for ptp clocks.")
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Acked-by: Richard Cochran <richardcochran@xxxxxxxxx>
    Link: https://lore.kernel.org/r/20231109174859.3995880-1-edumazet@xxxxxxxxxx
    Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c
index 796eeffdf93b1..8ff2dd5c52e64 100644
--- a/drivers/ptp/ptp_chardev.c
+++ b/drivers/ptp/ptp_chardev.c
@@ -346,7 +346,8 @@ ssize_t ptp_read(struct posix_clock *pc,
 
 	for (i = 0; i < cnt; i++) {
 		event[i] = queue->buf[queue->head];
-		queue->head = (queue->head + 1) % PTP_MAX_TIMESTAMPS;
+		/* Paired with READ_ONCE() in queue_cnt() */
+		WRITE_ONCE(queue->head, (queue->head + 1) % PTP_MAX_TIMESTAMPS);
 	}
 
 	spin_unlock_irqrestore(&queue->lock, flags);
diff --git a/drivers/ptp/ptp_clock.c b/drivers/ptp/ptp_clock.c
index 89632cc9c28fc..1e92bd897aa45 100644
--- a/drivers/ptp/ptp_clock.c
+++ b/drivers/ptp/ptp_clock.c
@@ -68,10 +68,11 @@ static void enqueue_external_timestamp(struct timestamp_event_queue *queue,
 	dst->t.sec = seconds;
 	dst->t.nsec = remainder;
 
+	/* Both WRITE_ONCE() are paired with READ_ONCE() in queue_cnt() */
 	if (!queue_free(queue))
-		queue->head = (queue->head + 1) % PTP_MAX_TIMESTAMPS;
+		WRITE_ONCE(queue->head, (queue->head + 1) % PTP_MAX_TIMESTAMPS);
 
-	queue->tail = (queue->tail + 1) % PTP_MAX_TIMESTAMPS;
+	WRITE_ONCE(queue->tail, (queue->tail + 1) % PTP_MAX_TIMESTAMPS);
 
 	spin_unlock_irqrestore(&queue->lock, flags);
 }
diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h
index 05f6b6a9bbd58..1e02b5ae6127b 100644
--- a/drivers/ptp/ptp_private.h
+++ b/drivers/ptp/ptp_private.h
@@ -68,9 +68,13 @@ struct ptp_clock {
  * that a writer might concurrently increment the tail does not
  * matter, since the queue remains nonempty nonetheless.
  */
-static inline int queue_cnt(struct timestamp_event_queue *q)
+static inline int queue_cnt(const struct timestamp_event_queue *q)
 {
-	int cnt = q->tail - q->head;
+	/*
+	 * Paired with WRITE_ONCE() in enqueue_external_timestamp(),
+	 * ptp_read(), extts_fifo_show().
+	 */
+	int cnt = READ_ONCE(q->tail) - READ_ONCE(q->head);
 	return cnt < 0 ? PTP_MAX_TIMESTAMPS + cnt : cnt;
 }
 
diff --git a/drivers/ptp/ptp_sysfs.c b/drivers/ptp/ptp_sysfs.c
index f97a5eefa2e23..b9e2f08960688 100644
--- a/drivers/ptp/ptp_sysfs.c
+++ b/drivers/ptp/ptp_sysfs.c
@@ -91,7 +91,8 @@ static ssize_t extts_fifo_show(struct device *dev,
 	qcnt = queue_cnt(queue);
 	if (qcnt) {
 		event = queue->buf[queue->head];
-		queue->head = (queue->head + 1) % PTP_MAX_TIMESTAMPS;
+		/* Paired with READ_ONCE() in queue_cnt() */
+		WRITE_ONCE(queue->head, (queue->head + 1) % PTP_MAX_TIMESTAMPS);
 	}
 	spin_unlock_irqrestore(&queue->lock, flags);