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 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()?
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.
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))