Re: [PATCH] iommu/amd: Fix potential out-of-bounds read in iommu_mmio_show

[Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>]

Hi,

On 10/24/2025 1:32 PM, Songtang Liu wrote:
> In iommu_mmio_write(), it validates the user-provided offset with the
> check: `iommu->dbg_mmio_offset > iommu->mmio_phys_end - 4`.
> This assumes a 4-byte access. However, the corresponding
> show handler, iommu_mmio_show(), uses readq() to perform an 8-byte
> (64-bit) read.
> 
> If a user provides an offset equal to `mmio_phys_end - 4`, the check
> passes, and will lead to a 4-byte out-of-bounds read.
> 
> Fix this by adjusting the boundary check to use sizeof(u64), which
> corresponds to the size of the readq() operation.
> 
> Fixes: 7a4ee419e8c1 ("iommu/amd: Add debugfs support to dump IOMMU
> MMIO registers")
> Signed-off-by: Songtang Liu <liusongtang@bytedance.com>
> ---
>   drivers/iommu/amd/debugfs.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/amd/debugfs.c b/drivers/iommu/amd/debugfs.c
> index 8b4d7db7869a4..5d85c47478a57 100644
> --- a/drivers/iommu/amd/debugfs.c
> +++ b/drivers/iommu/amd/debugfs.c
> @@ -37,7 +37,7 @@ static ssize_t iommu_mmio_write(struct file *filp,
> const char __user *ubuf,
>   	if (ret)
>   		return ret;
> 
> -	if (iommu->dbg_mmio_offset > iommu->mmio_phys_end - 4) {
> +	if (iommu->dbg_mmio_offset > iommu->mmio_phys_end - sizeof(u64)) {

LGTM.

Reviewed-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Tested-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>

Thanks
Dheeraj

>   		iommu->dbg_mmio_offset = -1;
>   		return  -EINVAL;
>   	}