Subject: Re: [PATCH V4] mISDN: fix use-after-free bugs in l1oip timer handlers

From: Leon Romanovsky <[email-masked]>
Date: Thu, 29 Sep 2022 10:22:07 +0300

On Wed, Sep 28, 2022 at 09:39:38PM +0800, Duoming Zhou wrote:
> The l1oip_cleanup() traverses the l1oip_ilist and calls
> release_card() to cleanup module and stack. However,
> release_card() calls del_timer() to delete the timers
> such as keep_tl and timeout_tl. If the timer handler is
> running, the del_timer() will not stop it and result in
> UAF bugs. One of the processes is shown below:
> 
>     (cleanup routine)          |        (timer handler)
> release_card()                 | l1oip_timeout()
>  ...                           |
>  del_timer()                   | ...
>  ...                           |
>  kfree(hc) //FREE              |
>                                | hc->timeout_on = 0 //USE
> 
> Fix by calling del_timer_sync() in release_card(), which
> makes sure the timer handlers have finished before the
> resources, such as l1oip and so on, have been deallocated.
> 
> What's more, the hc->workq and hc->socket_thread can kick
> those timers right back in. We add a bool flag to show
> if card is released. Then, check this flag in hc->workq
> and hc->socket_thread.
> 
> Fixes: 3712b42d4b1b ("Add layer1 over IP support")
> Signed-off-by: Duoming Zhou <[email-masked]>
> ---
> Changes in v4:
>   - Use bool flag to judge whether card is released.
> 
>  drivers/isdn/mISDN/l1oip.h      |  1 +
>  drivers/isdn/mISDN/l1oip_core.c | 13 +++++++------
>  2 files changed, 8 insertions(+), 6 deletions(-)

It looks like it is ok now, but whole mISDN code doesn't look healthy,
so it is hard to say for sure.

Are you fixing real issue that you saw in field?

Thanks,
Reviewed-by: Leon Romanovsky <[email-masked]>