linux-kernel - Re: [PATCH] ALSA: core: Fix possible null-pointer dereferences in snd_timer_proc

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Message-ID: <s5hpnlz396j.wl-tiwai@suse.de>
Date:   Wed, 24 Jul 2019 14:32:04 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     "Jia-Ju Bai" <baijiaju1990@...il.com>
Cc:     <bhelgaas@...gle.com>, <tglx@...utronix.de>,
        <gregkh@...uxfoundation.org>, <kirr@...edi.com>, <perex@...ex.cz>,
        <rfontana@...hat.com>, <alsa-devel@...a-project.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ALSA: core: Fix possible null-pointer dereferences in snd_timer_proc_read()

On Wed, 24 Jul 2019 14:13:27 +0200,
Jia-Ju Bai wrote:
> 
> In snd_timer_proc_read(), there is an if statement on line 1204 to check
> whether timer->card is NULL:
>     if (timer->card && timer->card->shutdown)
> 
> When timer->card is NULL, it is used on lines 1212 and 1215:
>     timer->card->number
> 
> Thus, possible null-pointer dereferences may occur.
> 
> To fix these bugs, timer->card is checked before being used.

Both cases can (should) never happen.  The SNDRV_TIMER_CLASS_CARD is
always associated with a timer with a card object.  Ditto for
SNDRV_TIMER_CLASS_PCM, where a PCM is always tied with a card.

So, if any, this should be a WARN_ON().  But I doubt it being any
useful.


thanks,

Takashi

> 
> These bugs are found by a static analysis tool STCheck written by us.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
>  sound/core/timer.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/sound/core/timer.c b/sound/core/timer.c
> index 5c9fbf3f4340..967d06a3cdec 100644
> --- a/sound/core/timer.c
> +++ b/sound/core/timer.c
> @@ -1208,11 +1208,13 @@ static void snd_timer_proc_read(struct snd_info_entry *entry,
>  			snd_iprintf(buffer, "G%i: ", timer->tmr_device);
>  			break;
>  		case SNDRV_TIMER_CLASS_CARD:
> -			snd_iprintf(buffer, "C%i-%i: ",
> -				    timer->card->number, timer->tmr_device);
> +			snd_iprintf(buffer, "C%i-%i: ", 
> +					timer->card ? timer->card->number : -1, 
> +					timer->tmr_device);
>  			break;
>  		case SNDRV_TIMER_CLASS_PCM:
> -			snd_iprintf(buffer, "P%i-%i-%i: ", timer->card->number,
> +			snd_iprintf(buffer, "P%i-%i-%i: ",
> +					timer->card ? timer->card->number : -1,
>  				    timer->tmr_device, timer->tmr_subdevice);
>  			break;
>  		default:
> -- 
> 2.17.0
> 
>