Re: [PATCH] brcmfmac: btcoex: Fix use-after-free when rescheduling brcmf_btcoex_info work

[Arend van Spriel]

On 8/21/2025 6:32 AM, Duoming Zhou wrote:
> 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 a critical
> race condition:
>
> 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.
>
> 3.Subsequently, the brcmf_btcoex_info structure is freed.
>
> 4.Finally, the rescheduled worker attempts to execute, leading to
> use-after-free bugs by accessing the freed brcmf_btcoex_info object.

Thanks for the patch. Being a nit picker just wanted to day that the 
use-after-free happens a bit earlier as the worker itself is contained 
in struct brcmf_btcoex_info. Also the diagram below does not add much 
more than the textual description above.

> The following diagram illustrates this sequence of events:
>
> 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-> //use
>
> To resolve this race condition, 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.

However, no reason to stop this patch from going in so...

Acked-by: Arend van Spriel <arend.vanspriel@xxxxxxxxxxxx>

> Fixes: 61730d4dfffc ("brcmfmac: support critical protocol API for DHCP")
> Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
> ---
>   drivers/net/wireless/broadcom/brcm80211/brcmfmac/btcoex.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)