Re: [PATCH v2] xen/gntdev: Fix the abuse of underlying struct page in DMA-buf import

[Daniel Vetter]

On Sun, 7 Jan 2024 at 11:35, Oleksandr Tyshchenko <olekstysh@xxxxxxxxx> 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>
> Acked-by: Christian König <christian.koenig@xxxxxxx>
> 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.
>
>   v2:
>    - add R-b and A-b
>    - fix build warning noticed by kernel test robot by initializing
>      "ret" in case of error
>      https://lore.kernel.org/oe-kbuild-all/202401062122.it6zvLG0-lkp@xxxxxxxxx/
> ---
> ---
>  drivers/xen/gntdev-dmabuf.c | 44 ++++++++++++++++---------------------
>  1 file changed, 19 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
> index 4440e626b797..272c0ab01ef5 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 +619,25 @@ 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) {
> +               ret = ERR_PTR(-ENOMEM);
> +               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) {

Maybe add a comment here to explain why this is done and why it's ok?
Either way:

Acked-by: Daniel Vetter <daniel@xxxxxxxx>


> +               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
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch