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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 4 Jan 2024, Oleksandr Tyshchenko wrote:
> 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>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> 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;
>  
> -- 
> 2.34.1
>