Re: [PATCH] dma-buf: fix integer overflow in fill_sg_entry() for buffers >= 8GiB

[Alex Williamson <alex@shazbot.org>]

On Tue, 25 Nov 2025 17:11:18 -0800
Alex Mastro <amastro@fb.com> wrote:

> fill_sg_entry() splits large DMA buffers into multiple scatter-gather
> entries, each holding up to UINT_MAX bytes. When calculating the DMA
> address for entries beyond the second one, the expression (i * UINT_MAX)
> causes integer overflow due to 32-bit arithmetic.
> 
> This manifests when the input arg length >= 8 GiB results in looping for
> i >= 2.
> 
> Fix by casting i to dma_addr_t before multiplication.
> 
> Fixes: 3aa31a8bb11e ("dma-buf: provide phys_vec to scatter-gather mapping routine")
> Signed-off-by: Alex Mastro <amastro@fb.com>
> ---
> More color about how I discovered this in [1] for the commit at [2]:
> 
> [1] https://lore.kernel.org/all/aSZHO6otK0Heh+Qj@devgpu015.cco6.facebook.com
> [2] https://lore.kernel.org/all/20251120-dmabuf-vfio-v9-6-d7f71607f371@nvidia.com
> ---
>  drivers/dma-buf/dma-buf-mapping.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
> index b4819811a64a..b7352e609fbd 100644
> --- a/drivers/dma-buf/dma-buf-mapping.c
> +++ b/drivers/dma-buf/dma-buf-mapping.c
> @@ -24,7 +24,7 @@ static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length,
>  		 * does not require the CPU list for mapping or unmapping.
>  		 */
>  		sg_set_page(sgl, NULL, 0, 0);
> -		sg_dma_address(sgl) = addr + i * UINT_MAX;
> +		sg_dma_address(sgl) = addr + (dma_addr_t)i * UINT_MAX;
>  		sg_dma_len(sgl) = len;
>  		sgl = sg_next(sgl);
>  	}
> 

Applied to vfio next branch for v6.19.  Thanks,

Alex