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    SubjectRe: [PATCH V2] mISDN: fix use-after-free bugs in l1oip timer handlers
    On Fri, Sep 23, 2022 at 10:25:14PM +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 use del_timer_sync(&hc->keep_tl)
    > and cancel_work_sync(&hc->keep_tl) twice to stop keep_tl timer
    > and hc->workq. Then, we add del_timer_sync(&hc->timeout_tl)
    > behind l1oip_socket_close() to stop timeout_tl timer.
    >
    > Fixes: 3712b42d4b1b ("Add layer1 over IP support")
    > Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
    > ---
    > Changes in v2:
    > - Solve the problem that timers could be restarted by other threads.
    >
    > drivers/isdn/mISDN/l1oip_core.c | 11 +++++------
    > 1 file changed, 5 insertions(+), 6 deletions(-)
    >
    > diff --git a/drivers/isdn/mISDN/l1oip_core.c b/drivers/isdn/mISDN/l1oip_core.c
    > index 2c40412466e..7b89d98a781 100644
    > --- a/drivers/isdn/mISDN/l1oip_core.c
    > +++ b/drivers/isdn/mISDN/l1oip_core.c
    > @@ -1232,17 +1232,16 @@ release_card(struct l1oip *hc)
    > {
    > int ch;
    >
    > - if (timer_pending(&hc->keep_tl))
    > - del_timer(&hc->keep_tl);
    > -
    > - if (timer_pending(&hc->timeout_tl))
    > - del_timer(&hc->timeout_tl);
    > -
    > + del_timer_sync(&hc->keep_tl);
    > + cancel_work_sync(&hc->workq);
    > + del_timer_sync(&hc->keep_tl);
    > cancel_work_sync(&hc->workq);

    It is racy, the call twice to del_timer_sync and cancel_work_sync
    doesn't solve "the problem that timers could be restarted by other
    threads."

    Thanks

    >
    > if (hc->socket_thread)
    > l1oip_socket_close(hc);
    >
    > + del_timer_sync(&hc->timeout_tl);
    > +
    > if (hc->registered && hc->chan[hc->d_idx].dch)
    > mISDN_unregister_device(&hc->chan[hc->d_idx].dch->dev);
    > for (ch = 0; ch < 128; ch++) {
    > --
    > 2.17.1
    >

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