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

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Date:   Tue, 10 May 2022 15:08:21 -0400
From:   Mark Yacoub <markyacoub@...omium.org>
To:     unlisted-recipients:; (no To-header on input)
Cc:     seanpaul@...omium.org, markyacoub@...gle.com,
        markyacoub@...omium.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: [PATCH] drm: Create support for Write-Only property blob

[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@...omium.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