Re: [PATCH v5] dma-buf: Fix silent overflow for phys vec to sgt

[Leon Romanovsky <leon@kernel.org>]

On Mon, Jun 01, 2026 at 08:00:12PM +0000, David Hu 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 `size_t` for `nents` and checking against `UINT_MAX`
> 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: stable@vger.kernel.org
> Cc: iommu@lists.linux.dev
> Reviewed-by: Pranjal Shrivastava <praan@google.com>
> Signed-off-by: David Hu <xuehaohu@google.com>
> ---
> 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).
> 
>  drivers/dma-buf/dma-buf-mapping.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
> index 794acff2546a..607b7998463d 100644
> --- a/drivers/dma-buf/dma-buf-mapping.c
> +++ b/drivers/dma-buf/dma-buf-mapping.c
> @@ -10,7 +10,7 @@ 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++) {
> @@ -36,7 +36,7 @@ static unsigned int calc_sg_nents(struct dma_iova_state *state,
>  				  struct phys_vec *phys_vec, size_t nr_ranges,
>  				  size_t size)
>  {
> -	unsigned int nents = 0;
> +	size_t nents = 0;
>  	size_t i;
>  
>  	if (!state || !dma_use_iova(state)) {
> @@ -51,6 +51,9 @@ static unsigned int calc_sg_nents(struct dma_iova_state *state,
>  		nents = DIV_ROUND_UP(size, UINT_MAX);
>  	}
>  
> +	if (nents > UINT_MAX)

I would suggest to use check_add_overflow() while calculating nents
instead of this check.

> +		return 0;
> +
>  	return nents;
>  }
>  
> @@ -95,9 +98,10 @@ 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 +137,11 @@ 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);
> +	if (!nents) {
> +		ret = -EINVAL;
> +		goto err_free_state;
> +	}

Technically, this hunk is not necessary, since sg_alloc_table() will
return -EINVAL when nents == 0. At least, that is the behavior I relied on.

Thanks

> +
>  	ret = sg_alloc_table(&dma->sgt, nents, GFP_KERNEL | __GFP_ZERO);
>  	if (ret)
>  		goto err_free_state;
> -- 
> 2.54.0.929.g9b7fa37559-goog
>