Linux kernel: memstick driver can hang removal on rtsx_usb cards

In the Linux kernel, the following vulnerability has been resolved:

memstick: rtsx_usb_ms: complete requests after eject instead of dropping them

memstick_check() can pass its host->removing check just before rtsx_usb_ms_drv_remove() sets eject/removing. Its next request is then silently dropped: rtsx_usb_ms_request() skips schedule_work() once eject is set, and drv_remove's cancel_work_sync() can also cancel a queued handle_req before it picks the request up. Nobody completes card->mrq_complete.

Once memstick core waits for requests without a timeout ("memstick: core: wait for request completion before freeing card"), this hangs removal: memstick_check() never returns and memstick_remove_host() blocks in flush_workqueue():

INFO: task kworker/u10:3:65 blocked for more than 20 seconds. Workqueue: kmemstick memstick_check __wait_for_common memstick_check

INFO: task kworker/1:1:33 blocked for more than 20 seconds. Workqueue: usb_hub_wq hub_event __flush_workqueue memstick_remove_host rtsx_usb_ms_drv_remove

Never drop a request. rtsx_usb_ms_request() always schedules handle_req, and handle_req fails requests with -ENOMEDIUM once eject is set, without touching the device. drv_remove flushes handle_req instead of cancelling it, and cancels it only after memstick_remove_host(), when no new request can arrive, so it cannot run on a freed host.

The host_mutex drain in drv_remove is removed: handle_req always leaves host->req NULL when it finishes, so the drain never had anything to do, and it would now race with handle_req.