wifi: brcmfmac: fix use-after-free when rescheduling brcmf_btcoex_info work

wifi: brcmfmac: fix use-after-free when rescheduling brcmf_btcoex_info work

The brcmf_btcoex_detach() only shuts down the btcoex timer, if the
flag timer_on is false. However, the brcmf_btcoex_timerfunc(), which
runs as timer handler, sets timer_on to false. This creates critical
race conditions:

1.If brcmf_btcoex_detach() is called while brcmf_btcoex_timerfunc()
is executing, it may observe timer_on as false and skip the call to
timer_shutdown_sync().

2.The brcmf_btcoex_timerfunc() may then reschedule the brcmf_btcoex_info
worker after the cancel_work_sync() has been executed, resulting in
use-after-free bugs.

The use-after-free bugs occur in two distinct scenarios, depending on
the timing of when the brcmf_btcoex_info struct is freed relative to
the execution of its worker thread.

Scenario 1: Freed before the worker is scheduled

The brcmf_btcoex_info is deallocated before the worker is scheduled.
A race condition can occur when schedule_work(&bt_local->work) is
called after the target memory has been freed. The sequence of events
is detailed below:

CPU0                           | CPU1
brcmf_btcoex_detach            | brcmf_btcoex_timerfunc
                               |   bt_local->timer_on = false;
  if (cfg->btcoex->timer_on)   |
    ...                        |
  cancel_work_sync();          |
  ...                          |
  kfree(cfg->btcoex); // FREE  |
                               |   schedule_work(&bt_local->work); // USE

Scenario 2: Freed after the worker is scheduled

The brcmf_btcoex_info is freed after the worker has been scheduled
but before or during its execution. In this case, statements within
the brcmf_btcoex_handler() — such as the container_of macro and
subsequent dereferences of the brcmf_btcoex_info object will cause
a use-after-free access. The following timeline illustrates this
scenario:

CPU0                            | CPU1
brcmf_btcoex_detach             | brcmf_btcoex_timerfunc
                                |   bt_local->timer_on = false;
  if (cfg->btcoex->timer_on)    |
    ...                         |
  cancel_work_sync();           |
  ...                           |   schedule_work(); // Reschedule
                                |
  kfree(cfg->btcoex); // FREE   |   brcmf_btcoex_handler() // Worker
  /*                            |     btci = container_of(....); // USE
   The kfree() above could      |     ...
   also occur at any point      |     btci-> // USE
   during the worker's execution|
   */                           |

To resolve the race conditions, drop the conditional check and call
timer_shutdown_sync() directly. It can deactivate the timer reliably,
regardless of its current state. Once stopped, the timer_on state is
then set to false.

Fixes: 61730d4dfffc ("brcmfmac: support critical protocol API for DHCP")
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Link: https://patch.msgid.link/20250822050839.4413-1-duoming@zju.edu.cn
Signed-off-by: Johannes Berg <johannes.berg@intel.com>