linux-kernel - Re: [PATCH] virt: vbox: struct vmmdev_hgcm_pagelist: Replace 1-element array with flexible array

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Message-ID: <29e58a99-560a-4922-85b2-9ce71bc43ec4@embeddedor.com>
Date: Wed, 10 Jul 2024 17:21:49 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <kees@...nel.org>, Hans de Goede <hdegoede@...hat.com>
Cc: Arnd Bergmann <arnd@...db.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virt: vbox: struct vmmdev_hgcm_pagelist: Replace
 1-element array with flexible array



On 10/07/24 17:15, Kees Cook wrote:
> Replace the deprecated[1] use of a 1-element array in
> struct vmmdev_hgcm_pagelist with a modern flexible array. As this is
> UAPI, we cannot trivially change the size of the struct, so use a union
> to retain the old first element's size, but switch "pages" to a flexible
> array.
> 
> No binary differences are present after this conversion.
> 
> Link: https://github.com/KSPP/linux/issues/79 [1]
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Hans de Goede <hdegoede@...hat.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: linux-hardening@...r.kernel.org

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

Thanks
--
Gustavo

> ---
>   include/uapi/linux/vbox_vmmdev_types.h | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/vbox_vmmdev_types.h b/include/uapi/linux/vbox_vmmdev_types.h
> index f8a8d6b3c521..6073858d52a2 100644
> --- a/include/uapi/linux/vbox_vmmdev_types.h
> +++ b/include/uapi/linux/vbox_vmmdev_types.h
> @@ -282,7 +282,10 @@ struct vmmdev_hgcm_pagelist {
>   	__u32 flags;             /** VMMDEV_HGCM_F_PARM_*. */
>   	__u16 offset_first_page; /** Data offset in the first page. */
>   	__u16 page_count;        /** Number of pages. */
> -	__u64 pages[1];          /** Page addresses. */
> +	union {
> +		__u64 unused;	/** Deprecated place-holder for first "pages" entry. */
> +		__DECLARE_FLEX_ARRAY(__u64, pages); /** Page addresses. */
> +	};
>   };
>   VMMDEV_ASSERT_SIZE(vmmdev_hgcm_pagelist, 4 + 2 + 2 + 8);
>