[PATCH 3/3 v2] net: macb: add missing barriers when reading descriptors

[Anssi Hannula <anssi.hannula@bitwise.fi>]

When reading buffer descriptors on RX or on TX completion, an
RX_USED/TX_USED bit is checked first to ensure that the descriptors have
been populated, i.e. the ownership has been transferred. However, there
are no memory barriers to ensure that the data protected by the
RX_USED/TX_USED bit is up-to-date with respect to that bit.

Specifically:

- TX timestamp descriptors may be loaded before ctrl is loaded for the
  TX_USED check, which is racy as the descriptors may be updated between
  the loads, causing old timestamp descriptor data to be used.

- RX ctrl may be loaded before addr is loaded for the RX_USED check,
  which is racy as a new frame may be written between the loads, causing
  old ctrl descriptor data to be used.
  This issue exists for both macb_rx() and gem_rx() variants.

Fix the races by adding DMA read memory barriers on those paths and
reordering the reads in macb_rx().

I have not observed any actual problems in practice caused by these
being missing, though.

Tested on a ZynqMP based system.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Fixes: 89e5785fc8a6 ("[PATCH] Atmel MACB ethernet driver")
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
---

The discussion is still ongoing on whether I have used the barriers
correctly here, but in the meantime, here's an updated version of the
patch.

v2:
- moved the timestamp protection barrier closer to the timestamp reads
- removed unnecessary move of the addr assignment in gem_rx() to keep
  the patch minimal for maximum clarity
- clarified commit message and comments


 drivers/net/ethernet/cadence/macb_main.c | 13 ++++++++++---
 drivers/net/ethernet/cadence/macb_ptp.c  |  2 ++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 430b7a0f5436..10a2bb44612b 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -1001,11 +1001,15 @@ static int gem_rx(struct macb_queue *queue, int budget)
 
 		rxused = (desc->addr & MACB_BIT(RX_USED)) ? true : false;
 		addr = macb_get_addr(bp, desc);
-		ctrl = desc->ctrl;
 
 		if (!rxused)
 			break;
 
+		/* Ensure ctrl is at least as up-to-date as rxused */
+		dma_rmb();
+
+		ctrl = desc->ctrl;
+
 		queue->rx_tail++;
 		count++;
 
@@ -1180,11 +1184,14 @@ static int macb_rx(struct macb_queue *queue, int budget)
 		/* Make hw descriptor updates visible to CPU */
 		rmb();
 
-		ctrl = desc->ctrl;
-
 		if (!(desc->addr & MACB_BIT(RX_USED)))
 			break;
 
+		/* Ensure ctrl is at least as up-to-date as addr */
+		dma_rmb();
+
+		ctrl = desc->ctrl;
+
 		if (ctrl & MACB_BIT(RX_SOF)) {
 			if (first_frag != -1)
 				discard_partial_frame(queue, first_frag, tail);
diff --git a/drivers/net/ethernet/cadence/macb_ptp.c b/drivers/net/ethernet/cadence/macb_ptp.c
index cd5296b84229..a6dc47edc4cf 100644
--- a/drivers/net/ethernet/cadence/macb_ptp.c
+++ b/drivers/net/ethernet/cadence/macb_ptp.c
@@ -319,6 +319,8 @@ int gem_ptp_txstamp(struct macb_queue *queue, struct sk_buff *skb,
 	desc_ptp = macb_ptp_desc(queue->bp, desc);
 	tx_timestamp = &queue->tx_timestamps[head];
 	tx_timestamp->skb = skb;
+	/* ensure ts_1/ts_2 is loaded after ctrl (TX_USED check) */
+	dma_rmb();
 	tx_timestamp->desc_ptp.ts_1 = desc_ptp->ts_1;
 	tx_timestamp->desc_ptp.ts_2 = desc_ptp->ts_2;
 	/* move head */
-- 
2.17.2