Subject: Re: [PATCH net v1] net: stmmac: TSO: Fix unbalanced DMA map/unmap for non-paged SKB data

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 4 Dec 2024 17:45:43 +0000

On Wed, Dec 04, 2024 at 05:02:19PM +0000, Jon Hunter wrote:
> Hi Russell,
> 
> On 04/12/2024 16:39, Russell King (Oracle) wrote:
> > On Wed, Dec 04, 2024 at 04:58:34PM +0100, Thierry Reding wrote:
> > > This doesn't match the location from earlier, but at least there's
> > > something afoot here that needs fixing. I suppose this could simply be
> > > hiding any subsequent errors, so once this is fixed we might see other
> > > similar issues.
> > 
> > Well, having a quick look at this, the first thing which stands out is:
> > 
> > In stmmac_tx_clean(), we have:
> > 
> >                  if (likely(tx_q->tx_skbuff_dma[entry].buf &&
> >                             tx_q->tx_skbuff_dma[entry].buf_type != STMMAC_TXBUF_T
> > _XDP_TX)) {
> >                          if (tx_q->tx_skbuff_dma[entry].map_as_page)
> >                                  dma_unmap_page(priv->device,
> >                                                 tx_q->tx_skbuff_dma[entry].buf,
> >                                                 tx_q->tx_skbuff_dma[entry].len,
> >                                                 DMA_TO_DEVICE);
> >                          else
> >                                  dma_unmap_single(priv->device,
> >                                                   tx_q->tx_skbuff_dma[entry].buf,
> >                                                   tx_q->tx_skbuff_dma[entry].len,
> >                                                   DMA_TO_DEVICE);
> >                          tx_q->tx_skbuff_dma[entry].buf = 0;
> >                          tx_q->tx_skbuff_dma[entry].len = 0;
> >                          tx_q->tx_skbuff_dma[entry].map_as_page = false;
> >                  }
> > 
> > So, tx_skbuff_dma[entry].buf is expected to point appropriately to the
> > DMA region.
> > 
> > Now if we look at stmmac_tso_xmit():
> > 
> >          des = dma_map_single(priv->device, skb->data, skb_headlen(skb),
> >                               DMA_TO_DEVICE);
> >          if (dma_mapping_error(priv->device, des))
> >                  goto dma_map_err;
> > 
> >          if (priv->dma_cap.addr64 <= 32) {
> > ...
> >          } else {
> > ...
> >                  des += proto_hdr_len;
> > ...
> > 	}
> > 
> >          tx_q->tx_skbuff_dma[tx_q->cur_tx].buf = des;
> >          tx_q->tx_skbuff_dma[tx_q->cur_tx].len = skb_headlen(skb);
> >          tx_q->tx_skbuff_dma[tx_q->cur_tx].map_as_page = false;
> >          tx_q->tx_skbuff_dma[tx_q->cur_tx].buf_type = STMMAC_TXBUF_T_SKB;
> > 
> > This will result in stmmac_tx_clean() calling dma_unmap_single() using
> > "des" and "skb_headlen(skb)" as the buffer start and length.
> > 
> > One of the requirements of the DMA mapping API is that the DMA handle
> > returned by the map operation will be passed into the unmap function.
> > Not something that was offset. The length will also be the same.
> > 
> > We can clearly see above that there is a case where the DMA handle has
> > been offset by proto_hdr_len, and when this is so, the value that is
> > passed into the unmap operation no longer matches this requirement.
> > 
> > So, a question to the reporter - what is the value of
> > priv->dma_cap.addr64 in your failing case? You should see the value
> > in the "Using %d/%d bits DMA host/device width" kernel message.
> 
> It is ...
> 
>  dwc-eth-dwmac 2490000.ethernet: Using 40/40 bits DMA host/device width

So yes, "des" is being offset, which will upset the unmap operation.
Please try the following patch, thanks:

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 9b262cdad60b..c81ea8cdfe6e 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4192,8 +4192,8 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct stmmac_txq_stats *txq_stats;
 	struct stmmac_tx_queue *tx_q;
 	u32 pay_len, mss, queue;
+	dma_addr_t tso_des, des;
 	u8 proto_hdr_len, hdr;
-	dma_addr_t des;
 	bool set_ic;
 	int i;
 
@@ -4289,14 +4289,15 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
 
 		/* If needed take extra descriptors to fill the remaining payload */
 		tmp_pay_len = pay_len - TSO_MAX_BUFF_SIZE;
+		tso_des = des;
 	} else {
 		stmmac_set_desc_addr(priv, first, des);
 		tmp_pay_len = pay_len;
-		des += proto_hdr_len;
+		tso_des = des + proto_hdr_len;
 		pay_len = 0;
 	}
 
-	stmmac_tso_allocator(priv, des, tmp_pay_len, (nfrags == 0), queue);
+	stmmac_tso_allocator(priv, tso_des, tmp_pay_len, (nfrags == 0), queue);
 
 	/* In case two or more DMA transmit descriptors are allocated for this
 	 * non-paged SKB data, the DMA buffer address should be saved to

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!