[PATCH net] net: ibm: emac: fix TX skb double free in emac_full_tx_reset()

[Rosen Penev <rosenp@gmail.com>]

emac_full_tx_reset() cleans the TX ring without the TX lock.
emac_netif_stop() only keeps MAL NAPI from polling RX for this EMAC,
mal_poll() still calls emac_poll_tx() for it. A descriptor cleaned by
emac_clean_tx_ring() has MAL_TX_CTRL_READY clear and looks completed,
so emac_poll_tx() can free the same skb a second time and both race on
tx_cnt and ack_slot.

On a Cisco Meraki MX60, each MTU change to or from a jumbo MTU does a full
TX reset. Toggling the MTU while forwarding traffic corrupted skbs within
minutes (list_del corruption in GRO). With the ring cleaned under the TX
lock it survived 10 minutes of the same test.

Fixes: 1d3bb996481e ("Device tree aware EMAC driver")
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 drivers/net/ethernet/ibm/emac/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/ethernet/ibm/emac/core.c b/drivers/net/ethernet/ibm/emac/core.c
index b676b286e339..75e51ace16f7 100644
--- a/drivers/net/ethernet/ibm/emac/core.c
+++ b/drivers/net/ethernet/ibm/emac/core.c
@@ -745,8 +745,15 @@ static void emac_full_tx_reset(struct emac_instance *dev)
 
 	emac_tx_disable(dev);
 	mal_disable_tx_channel(dev->mal, dev->mal_tx_chan);
+
+	/* MAL NAPI runs emac_poll_tx() even while polling of this EMAC is
+	 * disabled. A cleaned descriptor reads as completed, so clean the
+	 * ring under the TX lock or emac_poll_tx() frees the skbs again.
+	 */
+	netif_tx_lock_bh(dev->ndev);
 	emac_clean_tx_ring(dev);
 	dev->tx_cnt = dev->tx_slot = dev->ack_slot = 0;
+	netif_tx_unlock_bh(dev->ndev);
 
 	emac_configure(dev);
 
-- 
2.56.0