drm/panthor: Restrict high priorities on group_create

We were allowing any users to create a high priority group without any
permission checks. As a result, this was allowing possible denial of
service.

We now only allow the DRM master or users with the CAP_SYS_NICE
capability to set higher priorities than PANTHOR_GROUP_PRIORITY_MEDIUM.

As the sole user of that uAPI lives in Mesa and hardcode a value of
MEDIUM [1], this should be safe to do.

Additionally, as those checks are performed at the ioctl level,
panthor_group_create now only check for priority level validity.

[1]https://gitlab.freedesktop.org/mesa/mesa/-/blob/f390835074bdf162a63deb0311d1a6de527f9f89/src/gallium/drivers/panfrost/pan_csf.c#L1038

Signed-off-by: Mary Guillemard <mary.guillemard@collabora.com>
Fixes: de85488 ("drm/panthor: Add the scheduler logical block")
Cc: stable@vger.kernel.org
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240903144955.144278-2-mary.guillemard@collabora.com

drivers/gpu/drm/panthor/panthor_drv.c

@@ -10,6 +10,7 @@
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
 
+#include <drm/drm_auth.h>
 #include <drm/drm_debugfs.h>
 #include <drm/drm_drv.h>
 #include <drm/drm_exec.h>
@@ -996,6 +997,24 @@ static int panthor_ioctl_group_destroy(struct drm_device *ddev, void *data,
 	return panthor_group_destroy(pfile, args->group_handle);
 }
 
+static int group_priority_permit(struct drm_file *file,
+				 u8 priority)
+{
+	/* Ensure that priority is valid */
+	if (priority > PANTHOR_GROUP_PRIORITY_HIGH)
+		return -EINVAL;
+
+	/* Medium priority and below are always allowed */
+	if (priority <= PANTHOR_GROUP_PRIORITY_MEDIUM)
+		return 0;
+
+	/* Higher priorities require CAP_SYS_NICE or DRM_MASTER */
+	if (capable(CAP_SYS_NICE) || drm_is_current_master(file))
+		return 0;
+
+	return -EACCES;
+}
+
 static int panthor_ioctl_group_create(struct drm_device *ddev, void *data,
 				      struct drm_file *file)
 {
@@ -1011,6 +1030,10 @@ static int panthor_ioctl_group_create(struct drm_device *ddev, void *data,
 	if (ret)
 		return ret;
 
+	ret = group_priority_permit(file, args->priority);
+	if (ret)
+		return ret;
+
 	ret = panthor_group_create(pfile, args, queue_args);
 	if (ret >= 0) {
 		args->group_handle = ret;

drivers/gpu/drm/panthor/panthor_sched.c

@@ -3092,7 +3092,7 @@ int panthor_group_create(struct panthor_file *pfile,
 	if (group_args->pad)
 		return -EINVAL;
 
-	if (group_args->priority > PANTHOR_CSG_PRIORITY_HIGH)
+	if (group_args->priority >= PANTHOR_CSG_PRIORITY_COUNT)
 		return -EINVAL;
 
 	if ((group_args->compute_core_mask & ~ptdev->gpu_info.shader_present) ||

include/uapi/drm/panthor_drm.h

@@ -692,7 +692,11 @@ enum drm_panthor_group_priority {
 	/** @PANTHOR_GROUP_PRIORITY_MEDIUM: Medium priority group. */
 	PANTHOR_GROUP_PRIORITY_MEDIUM,
 
-	/** @PANTHOR_GROUP_PRIORITY_HIGH: High priority group. */
+	/**
+	 * @PANTHOR_GROUP_PRIORITY_HIGH: High priority group.
+	 *
+	 * Requires CAP_SYS_NICE or DRM_MASTER.
+	 */
 	PANTHOR_GROUP_PRIORITY_HIGH,
 };