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

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Date:   Thu, 29 Jun 2023 13:22:09 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Jó Ágila Bitsch <jgilab@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] usb: ch9: Replace bmSublinkSpeedAttr 1-element array with
 flexible array



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@...en8.de>
> Closes: https://lore.kernel.org/lkml/2023062945-fencing-pebble-0411@gregkh/
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>

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

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