Re: [PATCH v6] dma-buf: Fix silent overflow for phys vec to sgt

[Leon Romanovsky <leon@kernel.org>]

On Mon, Jun 08, 2026 at 07:43:21PM +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 `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: stable@vger.kernel.org
> Cc: iommu@lists.linux.dev
> Reviewed-by: Pranjal Shrivastava <praan@google.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: David Hu <xuehaohu@google.com>
> ---
> 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 | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
> index 794acff2546a..67a8ff52fb8f 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 +41,11 @@ 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))

An additional blank line should be inserted between variable initialization
and the subsequent code block.

Aside from that,
Reviewed-by: Leon Romanovsky <leon@kernel.org>