On 05/12/2025 10:41, Christian König wrote:
On 12/4/25 16:51, Matthew Auld wrote:
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 <[email protected]>
Reviewed-by: Michael J. Ruhl <[email protected]> (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()?
Good point.
It doesn't crash at the moment because page_to_pfn() also works with NULL as
page, but it is clearly not the nicest thing to do.
There does look to be:
https://elixir.bootlin.com/linux/v6.18/source/include/asm-generic/memory_model.h#L56
So not completely sure it can't crash here?
I will switch over to using sg_assign_page() instead.
+ 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.
Oh, indeed.
Thanks,
Christian.
+ 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(©->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))
