ublk_drv: only allow owner to open unprivileged disk

Owner of one unprivileged ublk device could be one evil user, which
can grant this disk's privilege to other users deliberately, and
this way could be like making one trap and waiting for other users
to be caught.

So only owner to open unprivileged disk even though the owner
grants disk privilege to other user. This way is reasonable too
given anyone can create ublk disk, and no need other's grant.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Fixes: 4093cb5 ("ublk_drv: add mechanism for supporting unprivileged ublk device")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230131040446.214583-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -377,8 +377,50 @@ static void ublk_free_disk(struct gendisk *disk)
 	put_device(&ub->cdev_dev);
 }
 
+static void ublk_store_owner_uid_gid(unsigned int *owner_uid,
+		unsigned int *owner_gid)
+{
+	kuid_t uid;
+	kgid_t gid;
+
+	current_uid_gid(&uid, &gid);
+
+	*owner_uid = from_kuid(&init_user_ns, uid);
+	*owner_gid = from_kgid(&init_user_ns, gid);
+}
+
+static int ublk_open(struct block_device *bdev, fmode_t mode)
+{
+	struct ublk_device *ub = bdev->bd_disk->private_data;
+
+	if (capable(CAP_SYS_ADMIN))
+		return 0;
+
+	/*
+	 * If it is one unprivileged device, only owner can open
+	 * the disk. Otherwise it could be one trap made by one
+	 * evil user who grants this disk's privileges to other
+	 * users deliberately.
+	 *
+	 * This way is reasonable too given anyone can create
+	 * unprivileged device, and no need other's grant.
+	 */
+	if (ub->dev_info.flags & UBLK_F_UNPRIVILEGED_DEV) {
+		unsigned int curr_uid, curr_gid;
+
+		ublk_store_owner_uid_gid(&curr_uid, &curr_gid);
+
+		if (curr_uid != ub->dev_info.owner_uid || curr_gid !=
+				ub->dev_info.owner_gid)
+			return -EPERM;
+	}
+
+	return 0;
+}
+
 static const struct block_device_operations ub_fops = {
 	.owner =	THIS_MODULE,
+	.open =		ublk_open,
 	.free_disk =	ublk_free_disk,
 };
 
@@ -1620,17 +1662,6 @@ static int ublk_ctrl_get_queue_affinity(struct ublk_device *ub,
 	return ret;
 }
 
-static void ublk_store_owner_uid_gid(struct ublksrv_ctrl_dev_info *info)
-{
-	kuid_t uid;
-	kgid_t gid;
-
-	current_uid_gid(&uid, &gid);
-
-	info->owner_uid = from_kuid(&init_user_ns, uid);
-	info->owner_gid = from_kgid(&init_user_ns, gid);
-}
-
 static inline void ublk_dump_dev_info(struct ublksrv_ctrl_dev_info *info)
 {
 	pr_devel("%s: dev id %d flags %llx\n", __func__,
@@ -1664,7 +1695,7 @@ static int ublk_ctrl_add_dev(struct io_uring_cmd *cmd)
 		return -EPERM;
 
 	/* the created device is always owned by current user */
-	ublk_store_owner_uid_gid(&info);
+	ublk_store_owner_uid_gid(&info.owner_uid, &info.owner_gid);
 
 	if (header->dev_id != info.dev_id) {
 		pr_warn("%s: dev id not match %u %u\n",