Re: [PATCH v3] drm/panthor: use local variables for firmware interface counts

[Liviu Dudau <liviu.dudau@arm.com>]

On Mon, Aug 03, 2026 at 04:11:49PM +0200, Osama Abdelkader wrote:
> The firmware exposes the global group count and per-group stream count in
> the shared control interface. These values are validated before being used
> as loop bounds.
> 
> Read each count once with READ_ONCE() and store it in a local variable.
> This avoids reading the same control section field twice and makes it
> explicit that the loop uses the same value that passed validation
> to protect against self-modifying control sections.
> 
> Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
> Reviewed-by: Steven Price <steven.price@arm.com>

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>

Best regards,
Liviu

> ---
> v3:
> - Add comments explaining the READ_ONCE() usage.
> 
>  drivers/gpu/drm/panthor/panthor_fw.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_fw.c b/drivers/gpu/drm/panthor/panthor_fw.c
> index 8411c468bdac..b2ccb81f280c 100644
> --- a/drivers/gpu/drm/panthor/panthor_fw.c
> +++ b/drivers/gpu/drm/panthor/panthor_fw.c
> @@ -954,6 +954,7 @@ static int panthor_init_csg_iface(struct panthor_device *ptdev,
>  	u64 shared_section_sz = panthor_kernel_bo_size(ptdev->fw->shared_section->mem);
>  	u64 iface_offset = CSF_GROUP_CONTROL_OFFSET +
>  			   ((u64)csg_idx * glb_iface->control->group_stride);
> +	u32 stream_num;
>  	unsigned int i;
>  
>  	if (iface_offset > shared_section_sz ||
> @@ -967,8 +968,13 @@ static int panthor_init_csg_iface(struct panthor_device *ptdev,
>  	csg_iface->output = iface_fw_to_cpu_addr(ptdev, csg_iface->control->output_va,
>  						 sizeof(*csg_iface->output));
>  
> -	if (csg_iface->control->stream_num < MIN_CS_PER_CSG ||
> -	    csg_iface->control->stream_num > MAX_CS_PER_CSG)
> +	/*
> +	 * To protect against self-modifying control sections
> +	 * take a single snapshot from the control section so validation and
> +	 * iteration use the same value.
> +	 */
> +	stream_num = READ_ONCE(csg_iface->control->stream_num);
> +	if (stream_num < MIN_CS_PER_CSG || stream_num > MAX_CS_PER_CSG)
>  		return -EINVAL;
>  
>  	if (!csg_iface->input || !csg_iface->output) {
> @@ -986,7 +992,7 @@ static int panthor_init_csg_iface(struct panthor_device *ptdev,
>  		}
>  	}
>  
> -	for (i = 0; i < csg_iface->control->stream_num; i++) {
> +	for (i = 0; i < stream_num; i++) {
>  		int ret = panthor_init_cs_iface(ptdev, csg_idx, i);
>  
>  		if (ret)
> @@ -1010,6 +1016,7 @@ static int panthor_fw_init_ifaces(struct panthor_device *ptdev)
>  {
>  	struct panthor_fw_global_iface *glb_iface = &ptdev->fw->iface.global;
>  	u64 shared_section_sz = panthor_kernel_bo_size(ptdev->fw->shared_section->mem);
> +	u32 group_num;
>  	unsigned int i;
>  
>  	if (!ptdev->fw->shared_section->mem->kmap)
> @@ -1035,13 +1042,18 @@ static int panthor_fw_init_ifaces(struct panthor_device *ptdev)
>  		return -EINVAL;
>  	}
>  
> -	if (glb_iface->control->group_num > MAX_CSGS ||
> -	    glb_iface->control->group_num < MIN_CSGS) {
> +	/*
> +	 * To protect against self-modifying control sections
> +	 * take a single snapshot from the control section so validation and
> +	 * iteration use the same value.
> +	 */
> +	group_num = READ_ONCE(glb_iface->control->group_num);
> +	if (group_num > MAX_CSGS || group_num < MIN_CSGS) {
>  		drm_err(&ptdev->base, "Invalid number of control groups");
>  		return -EINVAL;
>  	}
>  
> -	for (i = 0; i < glb_iface->control->group_num; i++) {
> +	for (i = 0; i < group_num; i++) {
>  		int ret = panthor_init_csg_iface(ptdev, i);
>  
>  		if (ret)
> -- 
> 2.43.0
> 

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
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