Re: [PATCH] sound: usb: increase snd_card alloc size

[Takashi Iwai]

On Fri, 22 Sep 2023 02:51:53 +0200,
Ricardo B. Marliere wrote:
> 
> Syzbot reports a slab-out-of-bounds read of a snd_card object. When
> snd_usb_audio_create calls snd_card_new, it passes sizeof(*chip) as the
> extra_size argument, which is not enough in this case.
> 
> Relevant logs below:
> 
> BUG: KASAN: slab-out-of-bounds in imon_probe+0x2983/0x3910
> Read of size 1 at addr ffff8880436a2c71 by task kworker/1:2/777
> (...)
> The buggy address belongs to the object at ffff8880436a2000
>  which belongs to the cache kmalloc-4k of size 4096
> The buggy address is located 1 bytes to the right of
>  allocated 3184-byte region [ffff8880436a2000, ffff8880436a2c70)
> 
> Reported-by: syzbot+59875ffef5cb9c9b29e9@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/000000000000a838aa0603cc74d6@xxxxxxxxx/m
> Signed-off-by: Ricardo B. Marliere <ricardo@xxxxxxxxxxxx>
> ---
>  sound/usb/card.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/usb/card.c b/sound/usb/card.c
> index 1b2edc0fd2e9..6578326d33e8 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -619,7 +619,7 @@ static int snd_usb_audio_create(struct usb_interface *intf,
>  	}
>  
>  	err = snd_card_new(&intf->dev, index[idx], id[idx], THIS_MODULE,
> -			   sizeof(*chip), &card);
> +			   sizeof(*chip) + 2, &card);

Sorry, it's no-no.  We have to fix the cause of the OOB access instead
of papering over with a random number of increase.

Unfortunately, most important piece of information is trimmed in the
changelog, so I can't judge what's going on.  The only useful info
there is that it's something to do with imon driver, but it's
completely independent from USB-audio.  How does it access to the
external memory allocated by snd-usb-audio driver at all?

Before jumping to the solution, we must understand the problem.


thanks,

Takashi