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

[Matthew Auld <matthew.auld@intel.com>]

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))