[vbox-dev] [PATCH] Fix broken SMBios checksum calculation

Martin Fleisz martin.fleisz at thincast.com
Tue Feb 5 16:57:44 GMT 2019 

Hi Michal,

when you look at the source of dmidecode that is not really the case.
You are right that dmidecode first looks for the _SM_ header. But then
it will check the _SM_ checksum, the _DMI_ signature and the _DMI_
checksum (see
https://github.com/mirror/dmidecode/blob/master/dmidecode.c#L5379). If
any  of these is wrong/missing it will fail with the error message from
my previous mail.
In both our cases the _DMI_ checksum is wrong (as you stated the code
obviously calculates the wrong checksum).
The _SM_ checksum is just wrong because of the incorrect value for the
_DMI_ checksum. If the _DMI_ checksum was calculated correctly, the _SM_
checksum is correct as it is now.
I hope that also makes sense ... maybe we even mean the same thing
anyways ... :-)

Cheers, Martin

On 05.02.2019 17:37, Michal Necasek wrote:
>
>    Hi Martin,
>
>  That's not what I'm saying. What I'm saying is that because my BIOS
> image was run through biossums, I didn't see the dmidecode problem
> because although the _DMI_ header checksum is wrong (incorrect
> calculation in DevPcBios.cpp), it does not matter in practice because
> dmidecode finds the _SM_ header first.
>
>  In other words, for you both _SM_ and _DMI_ headers had bad
> checksums, but for me only _DMI_ did. After your fix (which is
> correct), the _SM_ header you have probably still has a bad checksum
> but dmidecode does not care because it finds the valid _DMI_ header.
>
>  I hope that made sense.
>
>     - Michal
>
> On 2/5/2019 5:27 PM, Martin Fleisz wrote:
>> Hi Michal,
>>
>> I don't think that biossums is the way you can fix this issue because in
>> DevPcBios.cpp the _DMI_ table is patched dynamically (see
>> https://www.virtualbox.org/browser/vbox/trunk/src/VBox/Devices/PC/DevPcBios.cpp#L1594
>>
>> where the number and size of the DMI tables is updated) and therefore
>> the checksum must be re-calculated. The _SM_ table checksum is not
>> affected by these changes (as long as the _DMI_ checksum is correctly).
>>
>> Cheers, Martin
>>
>> On 05.02.2019 17:02, Michal Necasek wrote:
>>>
>>>     Hi Martin,
>>>
>>>   Now it's making sense. The '_DMI_' table header checksum code is
>>> wrong in DevPCBios.cpp. But dmidecode normally does not get that far,
>>> because it finds the '_SM_' marker first, and that one has the right
>>> checksum if the BIOS image got run through the 'biossums' tool. I
>>> suspect that's not happening for you, and that is something which is
>>> probably missing from our makefiles.
>>>
>>>   I can't see into your repository on github. But you're probably right
>>> and it's not doing any harm. Anyway, now I see how to reproduce the
>>> problem, thanks.
>>>
>>>       - Michal
>>>
>>> On 2/5/2019 4:09 PM, Martin Fleisz wrote:
>>>> Hi Michal,
>>>>
>>>> I compile the OSE version (with some very small modifications in
>>>> VBoxBiosAlternative386.asm
>>>> <https://github.com/thincast/VirtualBox-RDP-Server/commit/98e7959e484a0145e39619dc6dbc40d3b9282520#diff-9c7472aa47554e70acbb1a4298b673e4>
>>>>
>>>>
>>>> but that shouldn't do any harm to my understanding) and when running
>>>> dmidecode on Ubuntu 18.04 it reports "# No SMBIOS nor DMI entry point
>>>> found, sorry.". After applying the patch everything worked just fine.
>>>> I checked with VirtualBox 6.0.4 and it works just fine out of the box
>>>> ... so I guess either the checksum is somehow valid by accident or
>>>> there
>>>> is some code missing/different in the OSE repository.
>>>>
>>>> Best regards, Martin
>>>>
>>>> On 05.02.2019 15:48, Michal Necasek wrote:
>>>>>
>>>>>     Hi Martin,
>>>>>
>>>>>    The patch looks good, but I'm struggling to reproduce the problem.
>>>>> The dmidecode utility in Ubuntu 18.04.1 and 18.10 (dmidecode version
>>>>> 3.1 in both) does not generate any complaints. So what exactly does?
>>>>>
>>>>>        Regards,
>>>>>          Michal
>>>>>
>>>>> On 2/4/2019 12:16 PM, Martin Fleisz wrote:
>>>>>> When using legacy BIOS the Intermediate Checksum of the SMBIOS Entry
>>>>>> Point Structure is calculated incorrectly.
>>>>>>
>>>>>> The calculation starts at the wrong offset (at the beginning of
>>>>>> pu8PcBios rather than pu8PcBios[i]) and creates the sum over the
>>>>>> whole pu8PcBios data instead of just the next 15 bytes.
>>>>>>
>>>>>> The patch below fixes checksum calculation and makes applications
>>>>>> like dmidecode work again in the guest.
>>>>>>
>>>>>> The patch is contributed under the terms of the MIT license.
>>>>>>
>>>>>> Cheers, Martin
>>>>>>
>>>>>> diff --git src/VBox/Devices/PC/DevPcBios.cpp
>>>>>> src/VBox/Devices/PC/DevPcBios.cpp
>>>>>> index e967adff2f..7e56cc18fe 100644
>>>>>> --- src/VBox/Devices/PC/DevPcBios.cpp
>>>>>> +++ src/VBox/Devices/PC/DevPcBios.cpp
>>>>>> @@ -1594,9 +1594,9 @@ static DECLCALLBACK(int)
>>>>>> pcbiosConstruct(PPDMDEVINS pDevIns, int iInstance, PCF
>>>>>>                 *(uint16_t*)&pThis->pu8PcBios[i + 0x06] =
>>>>>> RT_H2LE_U16(cbDmiTables);
>>>>>>                 *(uint16_t*)&pThis->pu8PcBios[i + 0x0C] =
>>>>>> RT_H2LE_U16(cNumDmiTables);
>>>>>>                 uint8_t u8Sum = 0;
>>>>>> -            for (unsigned j = 0; j < pThis->cbPcBios; j++)
>>>>>> -                if (j != i + 0x05)
>>>>>> -                    u8Sum += pThis->pu8PcBios[j];
>>>>>> +            for (unsigned j = 0; j < 0x0F; j++)
>>>>>> +                if (j != 0x05)
>>>>>> +                    u8Sum += pThis->pu8PcBios[i + j];
>>>>>>                 pThis->pu8PcBios[i + 0x05] = -u8Sum;
>>>>>>                 break;
>>>>>>             }
>>>>>>
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