Commit 51db5336 authored by Christian König's avatar Christian König
Browse files

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: default avatarChristian König <christian.koenig@amd.com>
Reviewed-by: default avatarMichael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://lore.kernel.org/r/20251205130604.1582-1-christian.koenig@amd.com
parent 4ebaaa3b

drivers/dma-buf/dma-buf.c

+60 −14
Original line number Diff line number Diff line
@@ -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;

	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 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.

	/*

	 * 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))