Re: [PATCH v7] dma-buf: Fix silent overflow for phys vec to sgt

On Tue, Jun 9, 2026 at 12:40 PM David Hu <[email protected]> wrote:
>
> In case MMIO size is bigger than 4G and peer2peer DMA goes
> through host bridge, we trigger a code path that assigns the
> total linked IOVA (which is greater than 4G) to mapped_len.
>
> Previously, `mapped_len` was declared as 32-bit `unsigned int`.
> When accumulating `size_t` lengths, this leads to a silent wrap-around.
> This truncation causes truncated lengths to be passed to functions
> like `fill_sg_entry()`.
>
> Fix this by changing `mapped_len` to `size_t` (64-bit). While
> at it, fix similar potential overflow issues in `calc_sg_nents`
> by using `check_add_overflow()` for `nents` and using
> `unsigned int` for the loop iterator in `fill_sg_entry` to match.
>
> Fixes: 3aa31a8bb11e ("dma-buf: provide phys_vec to scatter-gather mapping routine")
> Cc: [email protected]
> Cc: [email protected]
> Reviewed-by: Pranjal Shrivastava <[email protected]>
> Reviewed-by: Kevin Tian <[email protected]>
> Reviewed-by: Leon Romanovsky <[email protected]>
> Signed-off-by: David Hu <[email protected]>
> ---
> Changes in v7:
>  - Added a missing blank line after local variable declaration in
>    `calc_sg_nents()` (Leon).
>  - Collected Reviewed-by from Leon Romanovsky.
>
> Changes in v6:
>  - Used `check_add_overflow()` in `calc_sg_nents()` for safer
>    accumulation (Leon).
>  - Dropped explicit `!nents` check and added a comment noting that
>    `sg_alloc_table` handles `nents == 0` (Leon).
>  - Collected Reviewed-by from Kevin Tian.
>
> Changes in v5:
>  - Removed WARN_ON_ONCE from calc_sg_nents() to avoid log noise (Jason).
>  - Added explicit check for `!nents` in dma_buf_phys_vec_to_sgt() to
>    cleanly return -EINVAL on overflow (Jason).
>
> Changes in v4:
>  - Added WARN_ON_ONCE() to the nents overflow check to prevent silent
>    failures (Claude Bot).
>
> Changes in v3:
>  - Removed leftover sentence fragment from the commit message.
>  - Kept `nents = 0` initialization (previously stated as removed in the
>    v2 changelog) as it is strictly required for the `+=` accumulation
>    loop in `calc_sg_nents()`.
>
> Changes in v2:
>  - Fixed 'IVOA' -> 'IOVA' typo and expanded commit message (Claude Bot).
>  - Added Reverse Xmas tree formatting (Pranjal).
>  - Folded in extra bounds checking for calc_sg_nents() (Pranjal).
>  - Folded in type consistency fix for fill_sg_entry() (Pranjal).
>  - Collected Reviewed-by from Pranjal Shrivastava.
>
>  drivers/dma-buf/dma-buf-mapping.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
> index 794acff2546a..80f6ab2f4809 100644
> --- a/drivers/dma-buf/dma-buf-mapping.c
> +++ b/drivers/dma-buf/dma-buf-mapping.c
> @@ -5,12 +5,13 @@
>   */
>  #include <linux/dma-buf-mapping.h>
>  #include <linux/dma-resv.h>
> +#include <linux/overflow.h>
>
>  static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length,
>                                          dma_addr_t addr)
>  {
>         unsigned int len, nents;
> -       int i;
> +       unsigned int i;
>
>         nents = DIV_ROUND_UP(length, UINT_MAX);
>         for (i = 0; i < nents; i++) {
> @@ -40,8 +41,12 @@ static unsigned int calc_sg_nents(struct dma_iova_state *state,
>         size_t i;
>
>         if (!state || !dma_use_iova(state)) {
> -               for (i = 0; i < nr_ranges; i++)
> -                       nents += DIV_ROUND_UP(phys_vec[i].len, UINT_MAX);
> +               for (i = 0; i < nr_ranges; i++) {
> +                       unsigned int added = DIV_ROUND_UP(phys_vec[i].len, UINT_MAX);
> +
> +                       if (check_add_overflow(nents, added, &nents))
> +                               return 0;
> +               }
>         } else {
>                 /*
>                  * In IOVA case, there is only one SG entry which spans
> @@ -95,9 +100,10 @@ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
>                                          size_t nr_ranges, size_t size,
>                                          enum dma_data_direction dir)
>  {
> -       unsigned int nents, mapped_len = 0;
>         struct dma_buf_dma *dma;
>         struct scatterlist *sgl;
> +       size_t mapped_len = 0;
> +       unsigned int nents;
>         dma_addr_t addr;
>         size_t i;
>         int ret;
> @@ -133,6 +139,8 @@ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
>         }
>
>         nents = calc_sg_nents(dma->state, phys_vec, nr_ranges, size);
> +
> +       /* sg_alloc_table will cleanly fail and return -EINVAL if nents == 0 */
>         ret = sg_alloc_table(&dma->sgt, nents, GFP_KERNEL | __GFP_ZERO);
>         if (ret)
>                 goto err_free_state;
> --
> 2.54.0.1064.gd145956f57-goog
>

Hi Christian et al,

Gentle nudge on this. LMK if there is anything else needed.

Regards,
David