[BUG] usb: gadget: f_tcm: NULL-ptr-deref / use-after-free when the UDC is disconnected during in-flight target I/O

[Yu Zhang <yuz08559@gmail.com>]

Hi,

f_tcm frees its per-endpoint USB requests on the teardown path without
waiting for in-flight target_core commands to finish, so a command that
is still being processed by the backend when the gadget is disconnected
completes into freed/NULL request memory.

The teardown path (host disconnect / port reset, "echo '' > UDC", or
set_config 0) reaches the function's ->disable:

  tcm_disable()                                 f_tcm.c:2562
    tcm_cleanup_old_alt()                       f_tcm.c:2366
      uasp_cleanup_old_alt() [UAS]              f_tcm.c:538
      bot_cleanup_old_alt()  [BOT]              f_tcm.c:365

bot_cleanup_old_alt() / uasp_cleanup_old_alt() disable the endpoints and
then free the request objects and NULL the pointers, e.g. (BOT):

  usb_ep_free_request(fu->ep_in, fu->bot_req_in);
  ...
  fu->bot_req_in = NULL;                        f_tcm.c:383

(the UAS path NULLs stream->req_in/req_out/req_status the same way, and
tcm_unbind()/tcm_free() later free descriptors and the f_uas itself).
There is no target_wait_for_sess_cmds() / command drain anywhere on the
disable/unbind path -- the only thing cancelled is the delayed_set_alt
work.

The SCSI session (I_T nexus) is owned by the configfs TPG, not by the
gadget function, so unbinding the UDC does not stop or drain the
in-flight se_cmds: they keep running in target_core and complete
asynchronously. The completion reaches the fabric queue_data_in callback
with no check of endpoint/teardown state:

  target_complete_ok_work()
    se_tfo->queue_data_in == usbg_send_read_response()   f_tcm.c:1175
      bot_send_read_response()   [BOT]                   f_tcm.c:209
          fu->bot_req_in->buf = cmd->data_buf;           f_tcm.c:232
      uasp_send_read_response()/uasp_prepare_r_request() [UAS] f_tcm.c:574

fu->bot_req_in (resp. stream->req_in) has just been freed and set to
NULL by the concurrent teardown, so the write faults. The queue_status
path (usbg_send_status_response) touches the freed req_status in the
same way.

KASAN, reproduced:

  BUG: KASAN: null-ptr-deref in usbg_send_read_response+0x377/0x560
  Write of size 8 at addr 0000000000000038 by task kworker/1:0/23
  Workqueue: target_completion target_complete_ok_work
  Call Trace:
   usbg_send_read_response+0x377/0x560
   target_complete_ok_work+0x499/0x850
   process_one_work+0x46c/0x970
   worker_thread+0x395/0x6e0
   kthread+0x211/0x270
   ret_from_fork+0x333/0x590
   ret_from_fork_asm+0x1a/0x30

(offset 0x38 is req->buf: fu->bot_req_in == NULL after bot_cleanup_old_alt()).
If the teardown has also reached tcm_free(), cmd->fu is dangling and this
is a use-after-free of the f_uas instead of a NULL deref.

== Reproduction ==

f_tcm target on dummy_hcd, KASAN. dummy_hcd defaults to HighSpeed, so the
function runs in BOT mode and the host side binds usb-storage.

  - backstore: brd /dev/ram0 behind dm-delay with a 300ms read delay, so
    a READ stays in-flight in the backend; iblock on the dm-delay device.
  - target: /sys/kernel/config/target/usb_gadget/naa.<wwn>/tpgt_1 with a
    LUN linked to the backstore and a nexus, enabled.
  - gadget: usb_gadget/g1 with functions/tcm.0 linked into a config,
    bound to the dummy UDC; the host enumerates /dev/sda.

Then:
  - task A: continuous O_DIRECT reads of the host /dev/sda (each becomes a
            real READ(10) to the gadget, parked 300ms in the backend).
  - task B: echo "" > g1/UDC   (disconnect while a READ is in-flight)

The in-flight READ completes on the target_completion workqueue after the
teardown freed fu->bot_req_in, and bot_send_read_response() writes through
it. The dm-delay read delay only makes the window deterministic; on real
hardware the window is the backing-store latency, and a plain cable
unplug / host port reset during I/O takes the same ->disable path.

The disable/unbind path frees the endpoint request objects but nothing
waits for the target commands that still reference them to drain.