netdev - [PATCH net] ptp: annotate data-race around q->head and q->tail

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Date: Thu,  9 Nov 2023 17:48:59 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>, Richard Cochran <richardcochran@...il.com>
Subject: [PATCH net] ptp: annotate data-race around q->head and q->tail

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@...gle.com>
Cc: Richard Cochran <richardcochran@...il.com>
---
 drivers/ptp/ptp_chardev.c | 3 ++-
 drivers/ptp/ptp_clock.c   | 5 +++--
 drivers/ptp/ptp_private.h | 8 ++++++--
 drivers/ptp/ptp_sysfs.c   | 3 ++-
 4 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c
index 3f7a7478880240a2d256caf624b61dcc8e7054af..7513018c9f9ac72d5c1b0055b55ae9ff36e710b0 100644
--- a/drivers/ptp/ptp_chardev.c
+++ b/drivers/ptp/ptp_chardev.c
@@ -572,7 +572,8 @@ ssize_t ptp_read(struct posix_clock_context *pccontext, uint rdflags,
 
 	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 3134568af622d396f6ab15049cd1a3ace3243269..15b804ba48685ee11a34b88df1ae738a136d17a1 100644
--- a/drivers/ptp/ptp_clock.c
+++ b/drivers/ptp/ptp_clock.c
@@ -57,10 +57,11 @@ 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 35fde0a0574606a04d6bdf0ab42a204da5fa6532..45f9002a5dcaea2c588c001fa83317fc318500ee 100644
--- a/drivers/ptp/ptp_private.h
+++ b/drivers/ptp/ptp_private.h
@@ -85,9 +85,13 @@ struct ptp_vclock {
  * 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 7d023d9d0acbfb3d128be09578753588fa59e84d..f7a499a1bd39ec22edf6c77407a48736e137f277 100644
--- a/drivers/ptp/ptp_sysfs.c
+++ b/drivers/ptp/ptp_sysfs.c
@@ -94,7 +94,8 @@ 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);
 
-- 
2.42.0.869.gea05f2083d-goog