[Linaro-mm-sig] Re: [PATCH 1/2] dma-buf: improve sg_table debugging hack v2

On 04/12/2025 14:59, Christian König wrote:

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.

Signed-off-by: Christian König christian.koenig@amd.com Reviewed-by: Michael J. Ruhl michael.j.ruhl@intel.com (v1)

---

drivers/dma-buf/dma-buf.c | 72 +++++++++++++++++++++++++++++++-------- 1 file changed, 58 insertions(+), 14 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 2305bb2cc1f1..8c4afd360b72 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 +834,57 @@ 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_mangle_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) {

• sg_set_page(to_sg, NULL, 0, 0);
  

Are we still allowed to pass NULL page here? There looks to be the recently added:

VM_WARN_ON_ONCE(!page_range_contiguous(page, ALIGN(len + offset, PAGE_SIZE) / PAGE_SIZE));

And if page_range_contiguous() does not just return true, it potentially wants to dereference the page, like with page_to_pfn()?

•            sg_dma_address(to_sg) = sg_dma_address(from_sg);
  

•            sg_dma_len(to_sg) = sg_dma_len(from_sg);
  

Nit: formatting looks off here.

• 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_demangle_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 +1181,9 @@ 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_mangle_sg_table(&sg_table);
• if (ret)

• goto error_unmap;
  

if (IS_ENABLED(CONFIG_DMA_API_DEBUG)) { struct scatterlist *sg; @@ -1220,7 +1264,7 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach, dma_resv_assert_held(attach->dmabuf->resv);

• mangle_sg_table(sg_table);

• dma_buf_demangle_sg_table(&sg_table); attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);

if (dma_buf_pin_on_map(attach))