ublk: snapshot batch commands before preparing I/O

The batch prepare path rereads its userspace element array when rolling
back a partially prepared batch. Userspace can change an already
processed tag before the second read, causing rollback to reject the
replacement tag and leave earlier I/O slots prepared. The
WARN_ON_ONCE() in the rollback path then fires.

Copy the bounded batch into kernel memory before changing any I/O state
and use the same snapshot for preparation and rollback. Commit and fetch
batches retain the existing chunked userspace walk.

Fixes: b256795 ("ublk: handle UBLK_U_IO_PREP_IO_CMDS")
Reported-by: syzbot+1a67ee1aa79484801ec6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1a67ee1aa79484801ec6
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260630211827.50475-1-alhouseenyousef@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Yousef13710

drivers/block/ublk_drv.c

@@ -3584,6 +3584,7 @@ ublk_batch_auto_buf_reg(const struct ublk_batch_io *uc,
 #define UBLK_CMD_BATCH_TMP_BUF_SZ  (48 * 10)
 struct ublk_batch_io_iter {
 	void __user *uaddr;
+	const u8 *kaddr;
 	unsigned done, total;
 	unsigned char elem_bytes;
 	/* copy to this buffer from user space */
@@ -3632,7 +3633,10 @@ static int ublk_walk_cmd_buf(struct ublk_batch_io_iter *iter,
 	while (iter->done < iter->total) {
 		unsigned int len = min(sizeof(iter->buf), iter->total - iter->done);
 
-		if (copy_from_user(iter->buf, iter->uaddr + iter->done, len)) {
+		if (iter->kaddr) {
+			memcpy(iter->buf, iter->kaddr + iter->done, len);
+		} else if (copy_from_user(iter->buf, iter->uaddr + iter->done,
+				  len)) {
 			pr_warn("ublk%d: read batch cmd buffer failed\n",
 					data->ub->dev_info.dev_id);
 			return -EFAULT;
@@ -3723,14 +3727,21 @@ static int ublk_handle_batch_prep_cmd(const struct ublk_batch_io_data *data)
 		.total = uc->nr_elem * uc->elem_bytes,
 		.elem_bytes = uc->elem_bytes,
 	};
+	void *cmd_buf;
 	int ret;
 
+	cmd_buf = vmemdup_user(iter.uaddr, iter.total);
+	if (IS_ERR(cmd_buf))
+		return PTR_ERR(cmd_buf);
+	iter.kaddr = cmd_buf;
+
 	mutex_lock(&data->ub->mutex);
 	ret = ublk_walk_cmd_buf(&iter, data, ublk_batch_prep_io);
 
 	if (ret && iter.done)
 		ublk_batch_revert_prep_cmd(&iter, data);
 	mutex_unlock(&data->ub->mutex);
+	kvfree(cmd_buf);
 	return ret;
 }