linux-hardening - Re: [PATCH] vboxsf: Use flexible arrays for trailing string member

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Date:   Tue, 25 Jul 2023 11:00:01 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] vboxsf: Use flexible arrays for trailing string member

Hi Kees, Larry,

On 7/20/23 17:15, Kees Cook wrote:
> The declaration of struct shfl_string used trailing fake flexible arrays
> for the string member. This was tripping FORTIFY_SOURCE since commit
> df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"). Replace the
> utf8 and utf16 members with actual flexible arrays, drop the unused ucs2
> member, and retriain a 2 byte padding to keep the structure size the same.
> 
> Reported-by: Larry Finger <Larry.Finger@...inger.net>
> Closes: https://lore.kernel.org/lkml/ab3a70e9-60ed-0f13-e3d4-8866eaccc8c1@lwfinger.net/
> Tested-by: Larry Finger <Larry.Finger@...inger.net>
> Signed-off-by: Kees Cook <keescook@...omium.org>

Kees, Larry thank you for fixing this while I was on vacation.

The patch looks good to me:

Reviewed-by: Hans de Goede <hdegoede@...hat.com>

Kees, I'm the vboxsf maintainer and it would be easiest for
me if you can include this in a future 6.5 fixes pull-request
to Linus if that is possible ?

Regards,

Hans



> ---
>  fs/vboxsf/shfl_hostintf.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/vboxsf/shfl_hostintf.h b/fs/vboxsf/shfl_hostintf.h
> index aca829062c12..069a019c9247 100644
> --- a/fs/vboxsf/shfl_hostintf.h
> +++ b/fs/vboxsf/shfl_hostintf.h
> @@ -68,9 +68,9 @@ struct shfl_string {
>  
>  	/** UTF-8 or UTF-16 string. Nul terminated. */
>  	union {
> -		u8 utf8[2];
> -		u16 utf16[1];
> -		u16 ucs2[1]; /* misnomer, use utf16. */
> +		u8 legacy_padding[2];
> +		DECLARE_FLEX_ARRAY(u8, utf8);
> +		DECLARE_FLEX_ARRAY(u16, utf16);
>  	} string;
>  };
>  VMMDEV_ASSERT_SIZE(shfl_string, 6);