Re: [PATCH] xen/gntdev: Fix the abuse of underlying struct page in DMA-buf import

[Christian König]

Am 04.01.24 um 19:53 schrieb Oleksandr Tyshchenko:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>
> DO NOT access the underlying struct page of an sg table exported
> by DMA-buf in dmabuf_imp_to_refs(), this is not allowed.
> Please see drivers/dma-buf/dma-buf.c:mangle_sg_table() for details.
>
> Fortunately, here (for special Xen device) we can avoid using
> pages and calculate gfns directly from dma addresses provided by
> the sg table.
>
> Suggested-by: Daniel Vetter <daniel@xxxxxxxx>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

I can't say that I can judge the full technical background, but that 
looks reasonable to me.

Acked-by: Christian König <christian.koenig@xxxxxxx>

> ---
> Please note, I didn't manage to test the patch against the latest master branch
> on real HW (patch was only build tested there). Patch was tested on Arm64
> guests using Linux v5.10.41 from vendor's BSP, this is the environment where
> running this use-case is possible and to which I have an access (Xen PV display
> with zero-copy and backend domain as a buffer provider - be-alloc=1, so dma-buf
> import part was involved). A little bit old, but the dma-buf import code
> in gntdev-dmabuf.c hasn't been changed much since that time, all context
> remains allmost the same according to my code inspection.
> ---
> ---
>   drivers/xen/gntdev-dmabuf.c | 42 +++++++++++++++----------------------
>   1 file changed, 17 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
> index 4440e626b797..0dde49fca9a5 100644
> --- a/drivers/xen/gntdev-dmabuf.c
> +++ b/drivers/xen/gntdev-dmabuf.c
> @@ -11,6 +11,7 @@
>   #include <linux/kernel.h>
>   #include <linux/errno.h>
>   #include <linux/dma-buf.h>
> +#include <linux/dma-direct.h>
>   #include <linux/slab.h>
>   #include <linux/types.h>
>   #include <linux/uaccess.h>
> @@ -50,7 +51,7 @@ struct gntdev_dmabuf {
>   
>   	/* Number of pages this buffer has. */
>   	int nr_pages;
> -	/* Pages of this buffer. */
> +	/* Pages of this buffer (only for dma-buf export). */
>   	struct page **pages;
>   };
>   
> @@ -484,7 +485,7 @@ static int dmabuf_exp_from_refs(struct gntdev_priv *priv, int flags,
>   /* DMA buffer import support. */
>   
>   static int
> -dmabuf_imp_grant_foreign_access(struct page **pages, u32 *refs,
> +dmabuf_imp_grant_foreign_access(unsigned long *gfns, u32 *refs,
>   				int count, int domid)
>   {
>   	grant_ref_t priv_gref_head;
> @@ -507,7 +508,7 @@ dmabuf_imp_grant_foreign_access(struct page **pages, u32 *refs,
>   		}
>   
>   		gnttab_grant_foreign_access_ref(cur_ref, domid,
> -						xen_page_to_gfn(pages[i]), 0);
> +						gfns[i], 0);
>   		refs[i] = cur_ref;
>   	}
>   
> @@ -529,7 +530,6 @@ static void dmabuf_imp_end_foreign_access(u32 *refs, int count)
>   
>   static void dmabuf_imp_free_storage(struct gntdev_dmabuf *gntdev_dmabuf)
>   {
> -	kfree(gntdev_dmabuf->pages);
>   	kfree(gntdev_dmabuf->u.imp.refs);
>   	kfree(gntdev_dmabuf);
>   }
> @@ -549,12 +549,6 @@ static struct gntdev_dmabuf *dmabuf_imp_alloc_storage(int count)
>   	if (!gntdev_dmabuf->u.imp.refs)
>   		goto fail;
>   
> -	gntdev_dmabuf->pages = kcalloc(count,
> -				       sizeof(gntdev_dmabuf->pages[0]),
> -				       GFP_KERNEL);
> -	if (!gntdev_dmabuf->pages)
> -		goto fail;
> -
>   	gntdev_dmabuf->nr_pages = count;
>   
>   	for (i = 0; i < count; i++)
> @@ -576,7 +570,8 @@ dmabuf_imp_to_refs(struct gntdev_dmabuf_priv *priv, struct device *dev,
>   	struct dma_buf *dma_buf;
>   	struct dma_buf_attachment *attach;
>   	struct sg_table *sgt;
> -	struct sg_page_iter sg_iter;
> +	struct sg_dma_page_iter sg_iter;
> +	unsigned long *gfns;
>   	int i;
>   
>   	dma_buf = dma_buf_get(fd);
> @@ -624,26 +619,23 @@ dmabuf_imp_to_refs(struct gntdev_dmabuf_priv *priv, struct device *dev,
>   
>   	gntdev_dmabuf->u.imp.sgt = sgt;
>   
> -	/* Now convert sgt to array of pages and check for page validity. */
> +	gfns = kcalloc(count, sizeof(*gfns), GFP_KERNEL);
> +	if (!gfns)
> +		goto fail_unmap;
> +
> +	/* Now convert sgt to array of gfns without accessing underlying pages. */
>   	i = 0;
> -	for_each_sgtable_page(sgt, &sg_iter, 0) {
> -		struct page *page = sg_page_iter_page(&sg_iter);
> -		/*
> -		 * Check if page is valid: this can happen if we are given
> -		 * a page from VRAM or other resources which are not backed
> -		 * by a struct page.
> -		 */
> -		if (!pfn_valid(page_to_pfn(page))) {
> -			ret = ERR_PTR(-EINVAL);
> -			goto fail_unmap;
> -		}
> +	for_each_sgtable_dma_page(sgt, &sg_iter, 0) {
> +		dma_addr_t addr = sg_page_iter_dma_address(&sg_iter);
> +		unsigned long pfn = bfn_to_pfn(XEN_PFN_DOWN(dma_to_phys(dev, addr)));
>   
> -		gntdev_dmabuf->pages[i++] = page;
> +		gfns[i++] = pfn_to_gfn(pfn);
>   	}
>   
> -	ret = ERR_PTR(dmabuf_imp_grant_foreign_access(gntdev_dmabuf->pages,
> +	ret = ERR_PTR(dmabuf_imp_grant_foreign_access(gfns,
>   						      gntdev_dmabuf->u.imp.refs,
>   						      count, domid));
> +	kfree(gfns);
>   	if (IS_ERR(ret))
>   		goto fail_end_access;
>