drm: rework SET_MASTER and DROP_MASTER perm handling

This commit reworks the permission handling of the two ioctls. In
particular it enforced the CAP_SYS_ADMIN check only, if:
- we're issuing the ioctl from process other than the one which opened
the node, and
- we are, or were master in the past

This ensures that we:
- do not regress the systemd-logind style of DRM_MASTER arbitrator
- allow applications which do not use systemd-logind to drop their
master capabilities (and regain them at later point) ... w/o running as
root.

See the comment above drm_master_check_perm() for more details.

v1:
- Tweak wording, fixup all checks, add igt test

v2:
- Add a few more comments, grammar nitpicks.

Cc: Adam Jackson <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Pekka Paalanen <[e-mail]>
Testcase: igt/core_setmaster/master-drop-set-user
Signed-off-by: Emil Velikov <[e-mail]>
Reviewed-by: Adam Jackson <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_auth.c

···
 		}
 	}
 
+	fpriv->was_master = (ret == 0);
 	return ret;
 }
 
···
 	return ret;
 }
 
+/*
+ * In the olden days the SET/DROP_MASTER ioctls used to return EACCES when
+ * CAP_SYS_ADMIN was not set. This was used to prevent rogue applications
+ * from becoming master and/or failing to release it.
+ *
+ * At the same time, the first client (for a given VT) is _always_ master.
+ * Thus in order for the ioctls to succeed, one had to _explicitly_ run the
+ * application as root or flip the setuid bit.
+ *
+ * If the CAP_SYS_ADMIN was missing, no other client could become master...
+ * EVER :-( Leading to a) the graphics session dying badly or b) a completely
+ * locked session.
+ *
+ *
+ * As some point systemd-logind was introduced to orchestrate and delegate
+ * master as applicable. It does so by opening the fd and passing it to users
+ * while in itself logind a) does the set/drop master per users' request and
+ * b)  * implicitly drops master on VT switch.
+ *
+ * Even though logind looks like the future, there are a few issues:
+ *  - some platforms don't have equivalent (Android, CrOS, some BSDs) so
+ * root is required _solely_ for SET/DROP MASTER.
+ *  - applications may not be updated to use it,
+ *  - any client which fails to drop master* can DoS the application using
+ * logind, to a varying degree.
+ *
+ * * Either due missing CAP_SYS_ADMIN or simply not calling DROP_MASTER.
+ *
+ *
+ * Here we implement the next best thing:
+ *  - ensure the logind style of fd passing works unchanged, and
+ *  - allow a client to drop/set master, iff it is/was master at a given point
+ * in time.
+ *
+ * Note: DROP_MASTER cannot be free for all, as an arbitrator user could:
+ *  - DoS/crash the arbitrator - details would be implementation specific
+ *  - open the node, become master implicitly and cause issues
+ *
+ * As a result this fixes the following when using root-less build w/o logind
+ * - startx
+ * - weston
+ * - various compositors based on wlroots
+ */
+static int
+drm_master_check_perm(struct drm_device *dev, struct drm_file *file_priv)
+{
+	if (file_priv->pid == task_pid(current) && file_priv->was_master)
+		return 0;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EACCES;
+
+	return 0;
+}
+
 int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 			struct drm_file *file_priv)
 {
 	int ret = 0;
 
 	mutex_lock(&dev->master_mutex);
+
+	ret = drm_master_check_perm(dev, file_priv);
+	if (ret)
+		goto out_unlock;
+
 	if (drm_is_current_master(file_priv))
 		goto out_unlock;
 
···
 	int ret = -EINVAL;
 
 	mutex_lock(&dev->master_mutex);
+
+	ret = drm_master_check_perm(dev, file_priv);
+	if (ret)
+		goto out_unlock;
+
+	ret = -EINVAL;
 	if (!drm_is_current_master(file_priv))
 		goto out_unlock;
 

drivers/gpu/drm/drm_ioctl.c

···
 	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_SET_SAREA_CTX, drm_legacy_setsareactx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
 	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_GET_SAREA_CTX, drm_legacy_getsareactx, DRM_AUTH),
 
-	DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl, DRM_ROOT_ONLY),
-	DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl, DRM_ROOT_ONLY),
+	DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl, 0),
+	DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl, 0),
 
 	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_ADD_CTX, drm_legacy_addctx, DRM_AUTH|DRM_ROOT_ONLY),
 	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_RM_CTX, drm_legacy_rmctx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),

include/drm/drm_file.h

···
 	bool writeback_connectors;
 
 	/**
+	 * @was_master:
+	 *
+	 * This client has or had, master capability. Protected by struct
+	 * &drm_device.master_mutex.
+	 *
+	 * This is used to ensure that CAP_SYS_ADMIN is not enforced, if the
+	 * client is or was master in the past.
+	 */
+	bool was_master;
+
+	/**
 	 * @is_master:
 	 *
 	 * This client is the creator of @master. Protected by struct