[PATCH net v3] net: bgmac: fix TX DMA unmapping and slot cleanup

[Rosen Penev <rosenp@gmail.com>]

When mapping a fragment fails, bgmac_dma_tx_add() unwinds with the
wrong slots. It passes the failed fragment's slot->dma_addr, which
holds the mapping error, to dma_unmap_single() for the head. The
fragment loop indexes from ring->end instead of ring->end + 1, so it
unmaps the head a second time as a page and leaves the last mapped
fragment mapped. None of the affected slots get their dma_addr cleared
either, which includes the head slot when mapping the head fails.

bgmac_dma_tx_ring_free() unmaps every slot with a non-zero dma_addr,
so tearing down the ring unmaps these addresses again. It also frees
the skbs of pending TX slots but leaves slot->skb and slot->dma_addr
set. The slots survive a down/up cycle, and bgmac_dma_tx_add() only
sets slot->skb on the last slot of a packet. If a slot with a stale
pointer is later used for the head or a fragment of a scatter/gather
packet, bgmac_dma_tx_free() frees the old skb a second time when that
slot completes.

bgmac_dma_tx_ring_free() also picks dma_unmap_single() or
dma_unmap_page() by looking at slot->skb, which is set on the last slot
of a packet, while the head was mapped with dma_map_single() and the
fragments with skb_frag_dma_map().

In the error path, unmap the head and each mapped fragment from their
own slots, and clear dma_addr on every slot the failed packet touched.
In bgmac_dma_tx_ring_free(), choose the unmap function by the SOF flag
of the descriptor like bgmac_dma_tx_free() does, unmap before freeing,
and clear both fields.

This was found through code inspection by an LLM while hitting other
bugs on an Asus RT-N16. It did not hit these ones specifically.

Fixes: 9cde94506eac ("bgmac: implement scatter/gather support")
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 v3: clarify how this was found
 v2: merge two patches into one
 drivers/net/ethernet/broadcom/bgmac.c | 41 ++++++++++++++++-----------
 1 file changed, 24 insertions(+), 17 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/bgmac.c b/drivers/net/ethernet/broadcom/bgmac.c
index e66856337d33..55d126c7d35c 100644
--- a/drivers/net/ethernet/broadcom/bgmac.c
+++ b/drivers/net/ethernet/broadcom/bgmac.c
@@ -205,19 +205,24 @@ static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
 	return NETDEV_TX_OK;
 
 err_dma:
-	dma_unmap_single(dma_dev, slot->dma_addr, skb_headlen(skb),
-			 DMA_TO_DEVICE);
+	slot->dma_addr = 0;
 
 	while (i-- > 0) {
-		int index = (ring->end + i) % BGMAC_TX_RING_SLOTS;
-		struct bgmac_slot_info *slot = &ring->slots[index];
-		u32 ctl1 = le32_to_cpu(ring->cpu_base[index].ctl1);
-		int len = ctl1 & BGMAC_DESC_CTL1_LEN;
-
-		dma_unmap_page(dma_dev, slot->dma_addr, len, DMA_TO_DEVICE);
+		index = (ring->end + 1 + i) % BGMAC_TX_RING_SLOTS;
+		slot = &ring->slots[index];
+		dma_unmap_page(dma_dev, slot->dma_addr,
+			       skb_frag_size(&skb_shinfo(skb)->frags[i]),
+			       DMA_TO_DEVICE);
+		slot->dma_addr = 0;
 	}
 
+	slot = &ring->slots[ring->end % BGMAC_TX_RING_SLOTS];
+	dma_unmap_single(dma_dev, slot->dma_addr, skb_headlen(skb),
+			 DMA_TO_DEVICE);
+
 err_dma_head:
+	slot->dma_addr = 0;
+
 	netdev_err(bgmac->net_dev, "Mapping error of skb on ring 0x%X\n",
 		   ring->mmio_base);
 
@@ -535,17 +540,19 @@ static void bgmac_dma_tx_ring_free(struct bgmac *bgmac,
 		unsigned int len = ctl1 & BGMAC_DESC_CTL1_LEN;
 
 		slot = &ring->slots[i];
-		dev_kfree_skb(slot->skb);
 
-		if (!slot->dma_addr)
-			continue;
+		if (slot->dma_addr) {
+			if (le32_to_cpu(dma_desc[i].ctl0) & BGMAC_DESC_CTL0_SOF)
+				dma_unmap_single(dma_dev, slot->dma_addr,
+						 len, DMA_TO_DEVICE);
+			else
+				dma_unmap_page(dma_dev, slot->dma_addr,
+					       len, DMA_TO_DEVICE);
+			slot->dma_addr = 0;
+		}
 
-		if (slot->skb)
-			dma_unmap_single(dma_dev, slot->dma_addr,
-					 len, DMA_TO_DEVICE);
-		else
-			dma_unmap_page(dma_dev, slot->dma_addr,
-				       len, DMA_TO_DEVICE);
+		dev_kfree_skb(slot->skb);
+		slot->skb = NULL;
 	}
 }
 
-- 
2.56.0