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From:   Mark Yacoub <markyacoub@chromium.org>
Date:   Thu, 12 May 2022 10:22:38 -0400
Message-ID: <CAJUqKUpP=YeGdAi5oD7FGjUREc7PRFc3ZvOfenXi3BZJ8_75xw@mail.gmail.com>
Subject: Re: [PATCH] drm: Create support for Write-Only property blob
Cc:     seanpaul@chromium.org, markyacoub@google.com,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
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Precedence: bulk

friendly ping :)

On Tue, May 10, 2022 at 3:08 PM 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.
>
> [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;
>  };
>
>  /**
> --
> 2.36.0.512.ge40c2bad7a-goog
>