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

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Date:   Wed, 19 Jul 2023 15:37:02 -0700
From:   Kees Cook <kees@...nel.org>
To:     Larry Finger <Larry.Finger@...inger.net>,
        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 July 19, 2023 2:24:52 PM PDT, Larry Finger <Larry.Finger@...inger.net> wrote:
>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.

It's possible the size really needs to stay 6 bytes. In that case, try adding a "u8 legacy_padding[2]" to the union and restore the VMMDEV_ASSERT_SIZE?


-- 
Kees Cook