[PATCH v2] net/mlx4_en: fix potential use-after-free with dma_unmap_page

[Sarah Newman <srn@prgmr.com>]

When swiotlb is in use, calling dma_unmap_page means that
the original page mapped with dma_map_page must still be valid
as swiotlb will copy data from its internal cache back to the
originally requested DMA location. When GRO is enabled,
all references to the original frag may be put before
mlx4_en_free_frag is called, meaning the page has been freed
before the call to dma_unmap_page in mlx4_en_free_frag.

To fix, unmap the page as soon as possible.

This can be trivially detected by doing the following:

Compile the kernel with DEBUG_PAGEALLOC
Run the kernel as a Xen Dom0
Leave GRO enabled on the interface
Run a 10 second or more test with iperf over the interface.

Signed-off-by: Sarah Newman <srn@prgmr.com>
---
 drivers/net/ethernet/mellanox/mlx4/en_rx.c | 32 +++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
index 844f5ad..abe2b43 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -142,16 +142,17 @@ static void mlx4_en_free_frag(struct mlx4_en_priv *priv,
 			      struct mlx4_en_rx_alloc *frags,
 			      int i)
 {
-	const struct mlx4_en_frag_info *frag_info = &priv->frag_info[i];
-	u32 next_frag_end = frags[i].page_offset + 2 * frag_info->frag_stride;
-
-
-	if (next_frag_end > frags[i].page_size)
-		dma_unmap_page(priv->ddev, frags[i].dma, frags[i].page_size,
-			       frag_info->dma_dir);
+	if (frags[i].page) {
+		const struct mlx4_en_frag_info *frag_info = &priv->frag_info[i];
+		u32 next_frag_end = frags[i].page_offset +
+				2 * frag_info->frag_stride;
 
-	if (frags[i].page)
+		if (next_frag_end > frags[i].page_size) {
+			dma_unmap_page(priv->ddev, frags[i].dma,
+				       frags[i].page_size, frag_info->dma_dir);
+		}
 		put_page(frags[i].page);
+	}
 }
 
 static int mlx4_en_init_allocator(struct mlx4_en_priv *priv,
@@ -586,21 +587,28 @@ static int mlx4_en_complete_rx_desc(struct mlx4_en_priv *priv,
 				    int length)
 {
 	struct skb_frag_struct *skb_frags_rx = skb_shinfo(skb)->frags;
-	struct mlx4_en_frag_info *frag_info;
 	int nr;
 	dma_addr_t dma;
 
 	/* Collect used fragments while replacing them in the HW descriptors */
 	for (nr = 0; nr < priv->num_frags; nr++) {
-		frag_info = &priv->frag_info[nr];
+		struct mlx4_en_frag_info *frag_info = &priv->frag_info[nr];
+		u32 next_frag_end = frags[nr].page_offset +
+				2 * frag_info->frag_stride;
+
 		if (length <= frag_info->frag_prefix_size)
 			break;
 		if (unlikely(!frags[nr].page))
 			goto fail;
 
 		dma = be64_to_cpu(rx_desc->data[nr].addr);
-		dma_sync_single_for_cpu(priv->ddev, dma, frag_info->frag_size,
-					DMA_FROM_DEVICE);
+		if (next_frag_end > frags[nr].page_size)
+			dma_unmap_page(priv->ddev, frags[nr].dma,
+				       frags[nr].page_size, frag_info->dma_dir);
+		else
+			dma_sync_single_for_cpu(priv->ddev, dma,
+						frag_info->frag_size,
+						DMA_FROM_DEVICE);
 
 		/* Save page reference in skb */
 		__skb_frag_set_page(&skb_frags_rx[nr], frags[nr].page);
-- 
1.9.1