ublk: don't allow user copy for unprivileged device

UBLK_F_USER_COPY requires userspace to call write() on ublk char
device for filling request buffer, and unprivileged device can't
be trusted.

So don't allow user copy for unprivileged device.

Cc: stable@vger.kernel.org
Fixes: 1172d5b8 ("ublk: support user copy")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20241016134847.2911721-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -2380,10 +2380,19 @@ static int ublk_ctrl_add_dev(struct io_uring_cmd *cmd)
 	 * TODO: provide forward progress for RECOVERY handler, so that
 	 * unprivileged device can benefit from it
 	 */
-	if (info.flags & UBLK_F_UNPRIVILEGED_DEV)
+	if (info.flags & UBLK_F_UNPRIVILEGED_DEV) {
 		info.flags &= ~(UBLK_F_USER_RECOVERY_REISSUE |
 				UBLK_F_USER_RECOVERY);
 
+		/*
+		 * For USER_COPY, we depends on userspace to fill request
+		 * buffer by pwrite() to ublk char device, which can't be
+		 * used for unprivileged device
+		 */
+		if (info.flags & UBLK_F_USER_COPY)
+			return -EINVAL;
+	}
+
 	/* the created device is always owned by current user */
 	ublk_store_owner_uid_gid(&info.owner_uid, &info.owner_gid);
 

include/uapi/linux/ublk_cmd.h

@@ -175,7 +175,13 @@
 /* use ioctl encoding for uring command */
 #define UBLK_F_CMD_IOCTL_ENCODE	(1UL << 6)
 
-/* Copy between request and user buffer by pread()/pwrite() */
+/*
+ *  Copy between request and user buffer by pread()/pwrite()
+ *
+ *  Not available for UBLK_F_UNPRIVILEGED_DEV, otherwise userspace may
+ *  deceive us by not filling request buffer, then kernel uninitialized
+ *  data may be leaked.
+ */
 #define UBLK_F_USER_COPY	(1UL << 7)
 
 /*