Re: [PATCH] usb: ch9: Replace bmSublinkSpeedAttr 1-element array with flexible array

[Gustavo A. R. Silva]

On 6/29/23 13:09, Kees Cook wrote:
> Since commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"),
> UBSAN_BOUNDS no longer pretends 1-element arrays are unbounded. Walking
> bmSublinkSpeedAttr will trigger a warning, so make it a proper flexible
> array. Add a union to keep the struct size identical for userspace in
> case anything was depending on the old size.
>
> False positive warning was:
>
> UBSAN: array-index-out-of-bounds in drivers/usb/host/xhci-hub.c:231:31 index 1 is out of range for type '__le32 [1]'
>
> for this line of code:
>
> 	ssp_cap->bmSublinkSpeedAttr[offset++] = cpu_to_le32(attr);
>
> Reported-by: Borislav Petkov <bp@xxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/2023062945-fencing-pebble-0411@gregkh/
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Reviewed-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Thanks!
--
Gustavo

> ---
>   include/uapi/linux/usb/ch9.h | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/usb/ch9.h b/include/uapi/linux/usb/ch9.h
> index b17e3a21b15f..3ff98c7ba7e3 100644
> --- a/include/uapi/linux/usb/ch9.h
> +++ b/include/uapi/linux/usb/ch9.h
> @@ -981,7 +981,11 @@ struct usb_ssp_cap_descriptor {
>   #define USB_SSP_MIN_RX_LANE_COUNT		(0xf << 8)
>   #define USB_SSP_MIN_TX_LANE_COUNT		(0xf << 12)
>   	__le16 wReserved;
> -	__le32 bmSublinkSpeedAttr[1]; /* list of sublink speed attrib entries */
> +	union {
> +		__le32 legacy_padding;
> +		/* list of sublink speed attrib entries */
> +		__DECLARE_FLEX_ARRAY(__le32, bmSublinkSpeedAttr);
> +	};
>   #define USB_SSP_SUBLINK_SPEED_SSID	(0xf)		/* sublink speed ID */
>   #define USB_SSP_SUBLINK_SPEED_LSE	(0x3 << 4)	/* Lanespeed exponent */
>   #define USB_SSP_SUBLINK_SPEED_LSE_BPS		0