dma-buf: improve sg_table debugging hack v4

This debugging hack is important to enforce the rule that importers
should *never* touch the underlying struct page of the exporter.

Instead of just mangling the page link create a copy of the sg_table
but only copy over the DMA addresses and not the pages.

This will cause a NULL pointer de-reference if the importer tries to
touch the struct page. Still quite a hack but this at least allows the
exporter to properly keeps it's sg_table intact while allowing the
DMA-buf maintainer to find and fix misbehaving importers and finally
switch over to using a different data structure in the future.

v2: improve the hack further by using a wrapper structure and explaining
the background a bit more in the commit message.
v3: fix some whitespace issues, use sg_assign_page().
v4: give the functions a better name

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://lore.kernel.org/r/20251205130604.1582-1-christian.koenig@amd.com

1 files changed, 60 insertions, 14 deletions

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 2305bb2cc1f1..533ea17b7175 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -35,6 +35,12 @@
 
 #include "dma-buf-sysfs-stats.h"
 
+/* Wrapper to hide the sg_table page link from the importer */
+struct dma_buf_sg_table_wrapper {
+	struct sg_table *original;
+	struct sg_table wrapper;
+};
+
 static inline int is_dma_buf_file(struct file *);
 
 static DEFINE_MUTEX(dmabuf_list_mutex);
@@ -828,21 +834,59 @@ void dma_buf_put(struct dma_buf *dmabuf)
 }
 EXPORT_SYMBOL_NS_GPL(dma_buf_put, "DMA_BUF");
 
-static void mangle_sg_table(struct sg_table *sg_table)
+static int dma_buf_wrap_sg_table(struct sg_table **sg_table)
 {
-#ifdef CONFIG_DMABUF_DEBUG
-	int i;
-	struct scatterlist *sg;
-
-	/* To catch abuse of the underlying struct page by importers mix
-	 * up the bits, but take care to preserve the low SG_ bits to
-	 * not corrupt the sgt. The mixing is undone on unmap
-	 * before passing the sgt back to the exporter.
+	struct scatterlist *to_sg, *from_sg;
+	struct sg_table *from = *sg_table;
+	struct dma_buf_sg_table_wrapper *to;
+	int i, ret;
+
+	if (!IS_ENABLED(CONFIG_DMABUF_DEBUG))
+		return 0;
+
+	/*
+	 * To catch abuse of the underlying struct page by importers copy the
+	 * sg_table without copying the page_link and give only the copy back to
+	 * the importer.
 	 */
-	for_each_sgtable_sg(sg_table, sg, i)
-		sg->page_link ^= ~0xffUL;
-#endif
+	to = kzalloc(sizeof(*to), GFP_KERNEL);
+	if (!to)
+		return -ENOMEM;
+
+	ret = sg_alloc_table(&to->wrapper, from->nents, GFP_KERNEL);
+	if (ret)
+		goto free_to;
+
+	to_sg = to->wrapper.sgl;
+	for_each_sgtable_dma_sg(from, from_sg, i) {
+		to_sg->offset = 0;
+		to_sg->length = 0;
+		sg_assign_page(to_sg, NULL);
+		sg_dma_address(to_sg) = sg_dma_address(from_sg);
+		sg_dma_len(to_sg) = sg_dma_len(from_sg);
+		to_sg = sg_next(to_sg);
+	}
 
+	to->original = from;
+	*sg_table = &to->wrapper;
+	return 0;
+
+free_to:
+	kfree(to);
+	return ret;
+}
+
+static void dma_buf_unwrap_sg_table(struct sg_table **sg_table)
+{
+	struct dma_buf_sg_table_wrapper *copy;
+
+	if (!IS_ENABLED(CONFIG_DMABUF_DEBUG))
+		return;
+
+	copy = container_of(*sg_table, typeof(*copy), wrapper);
+	*sg_table = copy->original;
+	sg_free_table(&copy->wrapper);
+	kfree(copy);
 }
 
 static inline bool
@@ -1139,7 +1183,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
 		if (ret < 0)
 			goto error_unmap;
 	}
-	mangle_sg_table(sg_table);
+	ret = dma_buf_wrap_sg_table(&sg_table);
+	if (ret)
+		goto error_unmap;
 
 	if (IS_ENABLED(CONFIG_DMA_API_DEBUG)) {
 		struct scatterlist *sg;
@@ -1220,7 +1266,7 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
 
 	dma_resv_assert_held(attach->dmabuf->resv);
 
-	mangle_sg_table(sg_table);
+	dma_buf_unwrap_sg_table(&sg_table);
 	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
 
 	if (dma_buf_pin_on_map(attach))