Re: [PATCH][next] ALSA: 6fire: Fix undefined behavior bug in struct comm_runtime

[Takashi Iwai <tiwai@suse.de>]

On Fri, 29 Sep 2023 17:59:22 +0200,
Gustavo A. R. Silva wrote:
> 
> `struct urb` is a flexible structure, which means that it contains a
> flexible-array member at the bottom. This could potentially lead to an
> overwrite of the objects following `receiver` in `struct comm_runtime`,
> among them some function pointers.
> 
> Fix this by placing the declaration of object `receiver` at the end of
> `struct comm_runtime`.
> 
> Fixes: ddb6b5a96437 ("ALSA: 6fire: fix DMA issues with URB transfer_buffer usage")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Sorry for the late reply, as I've been (still) off since the last
week.

Through a quick glance, I don't mind much to apply this, but I still
wonder how this "fixes" anything.  Does it silence compiler warnings
or such?

Certainly struct urb *may* have flex array, but in this case, it's
clearly not used, so it's fixed-size.  And, even if we shuffle the
member to put to the last, it doesn't fix anything automagically
alone.  If a flex array were used, it still leads to memory corruption
unless we implement the allocation properly.  So I find the patch
description is somehow misleading.


thanks,

Takashi

> ---
>  sound/usb/6fire/comm.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/usb/6fire/comm.h b/sound/usb/6fire/comm.h
> index 2447d7ecf179..ee81572a4eec 100644
> --- a/sound/usb/6fire/comm.h
> +++ b/sound/usb/6fire/comm.h
> @@ -19,7 +19,6 @@ enum /* settings for comm */
>  struct comm_runtime {
>  	struct sfire_chip *chip;
>  
> -	struct urb receiver;
>  	u8 *receiver_buffer;
>  
>  	u8 serial; /* urb serial */
> @@ -30,6 +29,7 @@ struct comm_runtime {
>  	int (*write8)(struct comm_runtime *rt, u8 request, u8 reg, u8 value);
>  	int (*write16)(struct comm_runtime *rt, u8 request, u8 reg,
>  			u8 vh, u8 vl);
> +	struct urb receiver;
>  };
>  
>  int usb6fire_comm_init(struct sfire_chip *chip);
> -- 
> 2.34.1
>