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: b256795b3606 ("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>

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 4f6d9e6521878fe9d80422b2940a368eb69611a9..c2c11f2a01e701dc8bad4ba806cbdb646f302864 100644 (file)
--- a/drivers/block/ublk_drv.c
+++ b/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;
 }