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Date: Fri, 26 Oct 2012 08:20:52 -0700
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Guillaume Juan <guillaume.juan@...emcom.com>
Cc: linux-serial@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] n_gsm: prevent crash due to dereferencing NULL gsm->tty
On Fri, Oct 26, 2012 at 10:11:45AM +0200, Guillaume Juan wrote:
> From: Guillaume Juan <guillaume.juan@...emcom.com>
>
> If gsm->tty happens to be NULL in gsmld_output, avoid crashing the kernel (the crash is replaced by a warning dump).
How can ->tty be NULL here?
> Prevent at earlier level such situation:
> - gsmtty_hangup does no longer call gsm_dlci_begin_close when called synchronously from gsm_cleanup_mux, because this would arm a timer that will asynchronously lead to use gms->tty, potentially after it is nullified.
> - in gsm_cleanup_mux, release DLCIs (other than DLCI#0 which is the control one) before closing mux. Else, it can happen that the T2 timer is rearmed by another thread scheduled between del_timer_sync and gsm_dlci release
> (for example by the following call-stack: fput => tty_release => tty_port_close_start => tty_port_lower_dtr_rts => gsm_dtr_rts => gsmtty_modem_update)
Please wrap your changelog comments at 72 columns.
>
> Signed-off-by: Guillaume Juan <guillaume.juan@...emcom.com>
> ---
> drivers/tty/n_gsm.c | 35 ++++++++++++++++++++++++++---------
> 1 files changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index 1e8e8ce..fe3c6ad 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -1510,8 +1510,8 @@ static void gsm_dlci_begin_open(struct gsm_dlci *dlci)
> }
>
> /**
> - * gsm_dlci_begin_close - start channel open procedure
> - * @dlci: DLCI to open
> + * gsm_dlci_begin_close - start channel close procedure
> + * @dlci: DLCI to close
> *
> * Commence closing a DLCI from the Linux side. We issue DISC messages
> * to the modem which should then reply with a UA, at which point we
> @@ -2031,6 +2031,13 @@ void gsm_cleanup_mux(struct gsm_mux *gsm)
> spin_unlock(&gsm_mux_lock);
> WARN_ON(i == MAX_MUX);
>
> + /* Free up any link layer users */
> + if (dlci)
> + dlci->dead = 1;
> + for (i = 1; i < NUM_DLCI; i++)
> + if (gsm->dlci[i])
> + gsm_dlci_release(gsm->dlci[i]);
> +
> /* In theory disconnecting DLCI 0 is sufficient but for some
> modems this is apparently not the case. */
> if (dlci) {
> @@ -2041,15 +2048,11 @@ void gsm_cleanup_mux(struct gsm_mux *gsm)
> del_timer_sync(&gsm->t2_timer);
> /* Now we are sure T2 has stopped */
> if (dlci) {
> - dlci->dead = 1;
> gsm_dlci_begin_close(dlci);
> wait_event_interruptible(gsm->event,
> dlci->state == DLCI_CLOSED);
> + gsm_dlci_release(dlci);
> }
> - /* Free up any link layer users */
> - for (i = 0; i < NUM_DLCI; i++)
> - if (gsm->dlci[i])
> - gsm_dlci_release(gsm->dlci[i]);
> /* Now wipe the queues */
> list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)
> kfree(txq);
> @@ -2192,6 +2195,10 @@ EXPORT_SYMBOL_GPL(gsm_alloc_mux);
>
> static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
> {
> + if (gsm->tty == NULL) {
> + WARN(1, "NULL gsm->tty pointer in gsmld_output\n");
> + return -EINVAL;
> + }
> if (tty_write_room(gsm->tty) < len) {
> set_bit(TTY_DO_WRITE_WAKEUP, &gsm->tty->flags);
> return -ENOSPC;
> @@ -2323,7 +2330,7 @@ static void gsmld_flush_buffer(struct tty_struct *tty)
> * @tty: device
> *
> * Called from the terminal layer when this line discipline is
> - * being shut down, either because of a close or becsuse of a
> + * being shut down, either because of a close or because of a
> * discipline change. The function will not be called while other
> * ldisc methods are in progress.
> */
> @@ -2954,6 +2961,15 @@ static void gsmtty_close(struct tty_struct *tty, struct file *filp)
> gsm = dlci->gsm;
> if (tty_port_close_start(&dlci->port, tty, filp) == 0)
> goto out;
> + /* Should not happen because the DLCI ttys are hung up (which causes
> + tty_port_close_start to return 0) by gsm_dlci_release before setting
> + gsm->tty to NULL */
> + if (gsm->tty == NULL) {
> + WARN(1, "NULL gsm->tty pointer in gsmtty_close for %s\n",
> + tty->name);
> + goto out;
> + }
> +
> gsm_dlci_begin_close(dlci);
> tty_port_close_end(&dlci->port, tty);
> tty_port_tty_set(&dlci->port, NULL);
> @@ -2967,7 +2983,8 @@ static void gsmtty_hangup(struct tty_struct *tty)
> {
> struct gsm_dlci *dlci = tty->driver_data;
> tty_port_hangup(&dlci->port);
> - gsm_dlci_begin_close(dlci);
> + if (!dlci->gsm->dead)
> + gsm_dlci_begin_close(dlci);
> }
>
> static int gsmtty_write(struct tty_struct *tty, const unsigned char *buf,
> --
> 1.7.0.4
>
>
>
> #
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>
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Because of that footer, I am not allowed to accept this patch at all.
Please remove it and resend.
thanks,
greg k-h
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