linux-hardening - Re: Crash in VirtualBox virtual machines running kernel 6.5

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Date:   Wed, 19 Jul 2023 16:24:52 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        Azeem Shaikh <azeemshaikh38@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>
Subject: Re: Crash in VirtualBox virtual machines running kernel 6.5

On 7/19/23 10:36, Kees Cook wrote:
> Okay, please try:
> 
> diff --git a/fs/vboxsf/shfl_hostintf.h b/fs/vboxsf/shfl_hostintf.h
> index aca829062c12..902fe3224453 100644
> --- a/fs/vboxsf/shfl_hostintf.h
> +++ b/fs/vboxsf/shfl_hostintf.h
> @@ -68,12 +68,11 @@ struct shfl_string {
>   
>   	/** UTF-8 or UTF-16 string. Nul terminated. */
>   	union {
> -		u8 utf8[2];
> -		u16 utf16[1];
> -		u16 ucs2[1]; /* misnomer, use utf16. */
> +		DECLARE_FLEX_ARRAY(u8, utf8);
> +		DECLARE_FLEX_ARRAY(u16, utf16);
>   	} string;
>   };
> -VMMDEV_ASSERT_SIZE(shfl_string, 6);
> +VMMDEV_ASSERT_SIZE(shfl_string, 4);
>   
>   /* The size of shfl_string w/o the string part. */
>   #define SHFLSTRING_HEADER_SIZE  4
> 
> 
> The size assert doesn't seem to be used anywhere else, but I can do a
> more careful binary analysis later today...''

Kees,

The testing was harder than I expected. My standard kernel would not load the 
system disk on the VM, thus I had to build one using my distros configuration. 
It is really painful to wait for all those drivers to build, but I figured that 
might be faster than trying to find the incorrect parameter.

I can finally report that vboxsf no longer generated a BUG. It is getting 
farther, but I am not done yet. It now generates a line that says "Unknown 
parameter tag" and then hangs. I have not tracked that down yet.

I was not aware of the DECLARE_FLEX_ARRAY() macro. I had considered removing the 
union and creating a simple string[] declaration, but I do not mess with file 
systems, and decided to let the experts handle it.

I will let you know what I find about that unknown parameter. It probably is 
coming from VirtualBox.

Thanks,

Larry