Re: [PATCH v2] tty: n_gsm: avoid call of sleeping functions from atomic context

[Pavel Machek <pavel@ucw.cz>]

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> spin_lock_irqsave(&gsm->tx_lock, flags) // taken a spinlock on TX data
>  con_write(...)
>   do_con_write(...)
>    console_lock()
>     might_sleep() // -> bug
> 
> As far as console_lock() might sleep it should not be called with
> spinlock held.
> 
> The patch replaces tx_lock spinlock with mutex in order to avoid the
> problem.
>

Do you have any hints why this might be correct?

Have you tested it?

Locking around line disciplines is spinlock_irqsave elsewhere in the
kernel.

Best regards,
								Pavel

> Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
> 
> Fixes: 32dd59f96924 ("tty: n_gsm: fix race condition in gsmld_write()")
> Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
> Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
> ---
> v2->v1: sorry, now adapted patch from 5.10 to upstream
> 
>  drivers/tty/n_gsm.c | 46 +++++++++++++++++++++------------------------
>  1 file changed, 21 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index caa5c14ed57f..be62c601058d 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -248,7 +248,7 @@ struct gsm_mux {
>  	bool constipated;		/* Asked by remote to shut up */
>  	bool has_devices;		/* Devices were registered */
>  
> -	spinlock_t tx_lock;
> +	struct mutex tx_mutex;
>  	unsigned int tx_bytes;		/* TX data outstanding */
>  #define TX_THRESH_HI		8192
>  #define TX_THRESH_LO		2048
> @@ -680,7 +680,6 @@ static int gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
>  	struct gsm_msg *msg;
>  	u8 *dp;
>  	int ocr;
> -	unsigned long flags;
>  
>  	msg = gsm_data_alloc(gsm, addr, 0, control);
>  	if (!msg)
> @@ -702,10 +701,10 @@ static int gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
>  
>  	gsm_print_packet("Q->", addr, cr, control, NULL, 0);
>  
> -	spin_lock_irqsave(&gsm->tx_lock, flags);
> +	mutex_lock(&gsm->tx_mutex);
>  	list_add_tail(&msg->list, &gsm->tx_ctrl_list);
>  	gsm->tx_bytes += msg->len;
> -	spin_unlock_irqrestore(&gsm->tx_lock, flags);
> +	mutex_unlock(&gsm->tx_mutex);
>  	gsmld_write_trigger(gsm);
>  
>  	return 0;
> @@ -730,7 +729,7 @@ static void gsm_dlci_clear_queues(struct gsm_mux *gsm, struct gsm_dlci *dlci)
>  	spin_unlock_irqrestore(&dlci->lock, flags);
>  
>  	/* Clear data packets in MUX write queue */
> -	spin_lock_irqsave(&gsm->tx_lock, flags);
> +	mutex_lock(&gsm->tx_mutex);
>  	list_for_each_entry_safe(msg, nmsg, &gsm->tx_data_list, list) {
>  		if (msg->addr != addr)
>  			continue;
> @@ -738,7 +737,7 @@ static void gsm_dlci_clear_queues(struct gsm_mux *gsm, struct gsm_dlci *dlci)
>  		list_del(&msg->list);
>  		kfree(msg);
>  	}
> -	spin_unlock_irqrestore(&gsm->tx_lock, flags);
> +	mutex_unlock(&gsm->tx_mutex);
>  }
>  
>  /**
> @@ -1024,10 +1023,9 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
>  
>  static void gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
>  {
> -	unsigned long flags;
> -	spin_lock_irqsave(&dlci->gsm->tx_lock, flags);
> +	mutex_lock(&dlci->gsm->tx_mutex);
>  	__gsm_data_queue(dlci, msg);
> -	spin_unlock_irqrestore(&dlci->gsm->tx_lock, flags);
> +	mutex_unlock(&dlci->gsm->tx_mutex);
>  }
>  
>  /**
> @@ -1283,13 +1281,12 @@ static int gsm_dlci_data_sweep(struct gsm_mux *gsm)
>  
>  static void gsm_dlci_data_kick(struct gsm_dlci *dlci)
>  {
> -	unsigned long flags;
>  	int sweep;
>  
>  	if (dlci->constipated)
>  		return;
>  
> -	spin_lock_irqsave(&dlci->gsm->tx_lock, flags);
> +	mutex_lock(&dlci->gsm->tx_mutex);
>  	/* If we have nothing running then we need to fire up */
>  	sweep = (dlci->gsm->tx_bytes < TX_THRESH_LO);
>  	if (dlci->gsm->tx_bytes == 0) {
> @@ -1300,7 +1297,7 @@ static void gsm_dlci_data_kick(struct gsm_dlci *dlci)
>  	}
>  	if (sweep)
>  		gsm_dlci_data_sweep(dlci->gsm);
> -	spin_unlock_irqrestore(&dlci->gsm->tx_lock, flags);
> +	mutex_unlock(&dlci->gsm->tx_mutex);
>  }
>  
>  /*
> @@ -1994,14 +1991,13 @@ static void gsm_dlci_command(struct gsm_dlci *dlci, const u8 *data, int len)
>  static void gsm_kick_timer(struct timer_list *t)
>  {
>  	struct gsm_mux *gsm = from_timer(gsm, t, kick_timer);
> -	unsigned long flags;
>  	int sent = 0;
>  
> -	spin_lock_irqsave(&gsm->tx_lock, flags);
> +	mutex_lock(&gsm->tx_mutex);
>  	/* If we have nothing running then we need to fire up */
>  	if (gsm->tx_bytes < TX_THRESH_LO)
>  		sent = gsm_dlci_data_sweep(gsm);
> -	spin_unlock_irqrestore(&gsm->tx_lock, flags);
> +	mutex_unlock(&gsm->tx_mutex);
>  
>  	if (sent && debug & 4)
>  		pr_info("%s TX queue stalled\n", __func__);
> @@ -2506,7 +2502,7 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
>  	INIT_WORK(&gsm->tx_work, gsmld_write_task);
>  	init_waitqueue_head(&gsm->event);
>  	spin_lock_init(&gsm->control_lock);
> -	spin_lock_init(&gsm->tx_lock);
> +	mutex_init(&gsm->tx_mutex);
>  
>  	if (gsm->encoding == 0)
>  		gsm->receive = gsm0_receive;
> @@ -2538,6 +2534,7 @@ static void gsm_free_mux(struct gsm_mux *gsm)
>  			break;
>  		}
>  	}
> +	mutex_destroy(&gsm->tx_mutex);
>  	mutex_destroy(&gsm->mutex);
>  	kfree(gsm->txframe);
>  	kfree(gsm->buf);
> @@ -2609,6 +2606,7 @@ static struct gsm_mux *gsm_alloc_mux(void)
>  	}
>  	spin_lock_init(&gsm->lock);
>  	mutex_init(&gsm->mutex);
> +	mutex_init(&gsm->tx_mutex);
>  	kref_init(&gsm->ref);
>  	INIT_LIST_HEAD(&gsm->tx_ctrl_list);
>  	INIT_LIST_HEAD(&gsm->tx_data_list);
> @@ -2636,6 +2634,7 @@ static struct gsm_mux *gsm_alloc_mux(void)
>  	}
>  	spin_unlock(&gsm_mux_lock);
>  	if (i == MAX_MUX) {
> +		mutex_destroy(&gsm->tx_mutex);
>  		mutex_destroy(&gsm->mutex);
>  		kfree(gsm->txframe);
>  		kfree(gsm->buf);
> @@ -2791,17 +2790,16 @@ static void gsmld_write_trigger(struct gsm_mux *gsm)
>  static void gsmld_write_task(struct work_struct *work)
>  {
>  	struct gsm_mux *gsm = container_of(work, struct gsm_mux, tx_work);
> -	unsigned long flags;
>  	int i, ret;
>  
>  	/* All outstanding control channel and control messages and one data
>  	 * frame is sent.
>  	 */
>  	ret = -ENODEV;
> -	spin_lock_irqsave(&gsm->tx_lock, flags);
> +	mutex_lock(&gsm->tx_mutex);
>  	if (gsm->tty)
>  		ret = gsm_data_kick(gsm);
> -	spin_unlock_irqrestore(&gsm->tx_lock, flags);
> +	mutex_unlock(&gsm->tx_mutex);
>  
>  	if (ret >= 0)
>  		for (i = 0; i < NUM_DLCI; i++)
> @@ -3012,7 +3010,6 @@ static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
>  			   const unsigned char *buf, size_t nr)
>  {
>  	struct gsm_mux *gsm = tty->disc_data;
> -	unsigned long flags;
>  	int space;
>  	int ret;
>  
> @@ -3020,13 +3017,13 @@ static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
>  		return -ENODEV;
>  
>  	ret = -ENOBUFS;
> -	spin_lock_irqsave(&gsm->tx_lock, flags);
> +	mutex_lock(&gsm->tx_mutex);
>  	space = tty_write_room(tty);
>  	if (space >= nr)
>  		ret = tty->ops->write(tty, buf, nr);
>  	else
>  		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
> -	spin_unlock_irqrestore(&gsm->tx_lock, flags);
> +	mutex_unlock(&gsm->tx_mutex);
>  
>  	return ret;
>  }
> @@ -3323,14 +3320,13 @@ static struct tty_ldisc_ops tty_ldisc_packet = {
>  static void gsm_modem_upd_via_data(struct gsm_dlci *dlci, u8 brk)
>  {
>  	struct gsm_mux *gsm = dlci->gsm;
> -	unsigned long flags;
>  
>  	if (dlci->state != DLCI_OPEN || dlci->adaption != 2)
>  		return;
>  
> -	spin_lock_irqsave(&gsm->tx_lock, flags);
> +	mutex_lock(&gsm->tx_mutex);
>  	gsm_dlci_modem_output(gsm, dlci, brk);
> -	spin_unlock_irqrestore(&gsm->tx_lock, flags);
> +	mutex_unlock(&gsm->tx_mutex);
>  }
>  
>  /**
> -- 
> 2.25.1

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