[SLOF] [PATCH 0/4] Block write support for SCSI and virtio-block disks

Alexey Kardashevskiy aik at ozlabs.ru
Wed Nov 16 12:53:44 AEDT 2016

On 15/11/16 19:13, Nikunj A Dadhania wrote:
> Thomas Huth <thuth at redhat.com> writes:
>> On 15.11.2016 02:47, Alexey Kardashevskiy wrote:
>>> On 14/11/16 23:03, Thomas Huth wrote:
>>>> On 14.11.2016 08:53, Nikunj A Dadhania wrote:
>>>>> Thomas Huth <thuth at redhat.com> writes:
>>>>>> On 14.11.2016 07:32, Nikunj A Dadhania wrote:
>>>> [...]
>>>>>>> My only worry here is that it would open up a way to write to the
>>>>>>> critical section of the disk image from the SLOF prompt. Is there a way
>>>>>>> we can prevent this?
>>>>>> Good idea, I also felt a little bit uneasy to have write support in the
>>>>>> firmware, but since GRUB needs it, we likely can't ignore this.
>>>>>> So with critical section, you mean the MBR, I assume?
>>>>> Yes.
>>>>>> That should be feasible, I think I could add a check that refuses
>>>>>> writes to the first 512 bytes (or a little bit more to also protect
>>>>>> the GPT? Suggestions welcome!).
>>>>> Correct. For MBR 1st sector. For GPT (34 sectors in the beginning and 33
>>>>> at the end) please refer to the following link for more details
>>>>> https://en.wikipedia.org/wiki/GUID_Partition_Table
>>>> I just had a look at it, but adding code for checking whether the GPT is
>>>> available or not (or using the checks from disk-label.fs) would render
>>>> the whole checking mechanism quite complicated, as far as I can see...
>>>> What about simply refusing write accesses to the first 4 sectors or so?
>>>> Would that be OK? I think GRUB should never try to write to them - with
>>>> MBR + partition header + file system superblock etc. the grubenv file
>>>> should never be located below sector 4.
>>> Can SLOF remember what partition it loaded grub from and restrict writes
>>> just to that partition? The "grub" which SLOF loads is the one at
>>> /boot/grub/grub, right?
> The grub that SLOF loads is from the PReP partition. Rest of the loading
> is then done by grub itself using client interface.

Can SLOF load grub from /boot/grub/grub at all, not from PReP?

> Are you recommending to remember the partition when client interface is
> invoked.
>> Good idea - I think such a check could be done at the disk-label.fs
>> level. I'll have a try...


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