Re: [PATCH] drm: Create support for Write-Only property blob

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 10 May 2022, Mark Yacoub <markyacoub@chromium.org> wrote:
> [Why]
> User space might need to inject data into the kernel without allowing it
> to be read again by any user space.
> An example of where this is particularly useful is secret keys fetched
> by user space and injected into the kernel to enable content protection.

I think we're going to need more than an example in the commit
message. See Documentation/gpu/drm-uapi.rst.

BR,
Jani.


>
> [How]
> Create a DRM_MODE_CREATE_BLOB_WRITE_ONLY flag used by user space to
> create a blob and mark the blob as write only.
> On reading back the blob, data will be not be copied if it's a write
> only blob
>
> Signed-off-by: Mark Yacoub <markyacoub@chromium.org>
>
> ---
>  drivers/gpu/drm/drm_property.c | 3 ++-
>  include/drm/drm_property.h     | 2 ++
>  include/uapi/drm/drm_mode.h    | 6 ++++++
>  3 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_property.c b/drivers/gpu/drm/drm_property.c
> index dfec479830e4..afedf7109d00 100644
> --- a/drivers/gpu/drm/drm_property.c
> +++ b/drivers/gpu/drm/drm_property.c
> @@ -765,7 +765,7 @@ int drm_mode_getblob_ioctl(struct drm_device *dev,
>  	if (!blob)
>  		return -ENOENT;
>  
> -	if (out_resp->length == blob->length) {
> +	if (out_resp->length == blob->length && !blob->is_write_only) {
>  		if (copy_to_user(u64_to_user_ptr(out_resp->data),
>  				 blob->data,
>  				 blob->length)) {
> @@ -800,6 +800,7 @@ int drm_mode_createblob_ioctl(struct drm_device *dev,
>  		ret = -EFAULT;
>  		goto out_blob;
>  	}
> +	blob->is_write_only = out_resp->flags & DRM_MODE_CREATE_BLOB_WRITE_ONLY;
>  
>  	/* Dropping the lock between create_blob and our access here is safe
>  	 * as only the same file_priv can remove the blob; at this point, it is
> diff --git a/include/drm/drm_property.h b/include/drm/drm_property.h
> index 65bc9710a470..700782f021b9 100644
> --- a/include/drm/drm_property.h
> +++ b/include/drm/drm_property.h
> @@ -205,6 +205,7 @@ struct drm_property {
>   * 	&drm_mode_config.property_blob_list.
>   * @head_file: entry on the per-file blob list in &drm_file.blobs list.
>   * @length: size of the blob in bytes, invariant over the lifetime of the object
> + * @is_write_only: user space can't read the blob data.
>   * @data: actual data, embedded at the end of this structure
>   *
>   * Blobs are used to store bigger values than what fits directly into the 64
> @@ -219,6 +220,7 @@ struct drm_property_blob {
>  	struct list_head head_global;
>  	struct list_head head_file;
>  	size_t length;
> +	bool is_write_only;
>  	void *data;
>  };
>  
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 0a0d56a6158e..de192d3813e9 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -1107,6 +1107,9 @@ struct drm_format_modifier {
>  	__u64 modifier;
>  };
>  
> +#define DRM_MODE_CREATE_BLOB_WRITE_ONLY                                        \
> +	(1 << 0) /* data of the blob can't be read by user space */
> +
>  /**
>   * struct drm_mode_create_blob - Create New blob property
>   *
> @@ -1120,6 +1123,9 @@ struct drm_mode_create_blob {
>  	__u32 length;
>  	/** @blob_id: Return: new property ID. */
>  	__u32 blob_id;
> +	/** Flags for special handling. */
> +	__u32 flags;
> +	__u32 pad;
>  };
>  
>  /**

-- 
Jani Nikula, Intel Open Source Graphics Center