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

[Martin Fleisz]

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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>>>>
>>>
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>>
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