Re: [PATCH v5 2/5] efi: Add embedded peripheral firmware support

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 05/03/2018 11:31 PM, Luis R. Rodriguez wrote:
> On Wed, May 02, 2018 at 04:49:53PM +0200, Hans de Goede wrote:
>> Hi,
>>
>> On 05/01/2018 09:29 PM, Andy Lutomirski wrote:
>>> On Sun, Apr 29, 2018 at 2:36 AM Hans de Goede <hdegoede@redhat.com> wrote:
>>>> +The EFI embedded-fw code works by scanning all EFI_BOOT_SERVICES_CODE
>>> memory
>>>> +segments for an eight byte sequence matching prefix, if the prefix is
>>> found it
>>>> +then does a crc32 over length bytes and if that matches makes a copy of
>>> length
>>>> +bytes and adds that to its list with found firmwares.
>>>> +
>>>
>>> Eww, gross.  Is there really no better way to do this?
>>
>> I'm afraid not.
>>
>>>   Is the issue that
>>> the EFI code does not intend to pass the firmware to the OS but that it has
>>> a copy for its own purposes and that Linux is just going to hijack EFI's
>>> copy?  If so, that's brilliant and terrible at the same time.
>>
>> Yes that is exactly the issue / what it happening here.
>>
>>>
>>>> +       for (i = 0; i < size; i += 8) {
>>>> +               if (*((u64 *)(mem + i)) != *((u64 *)desc->prefix))
>>>> +                       continue;
>>>> +
>>>> +               /* Seed with ~0, invert to match crc32 userspace utility
>>> */
>>>> +               crc = ~crc32(~0, mem + i, desc->length);
>>>> +               if (crc == desc->crc)
>>>> +                       break;
>>>> +       }
>>>
>>> I hate to play the security card, but this stinks a bit.  The kernel
>>> obviously needs to trust the EFI boot services code since the EFI boot
>>> services code is free to modify the kernel image.  But your patch is not
>>> actually getting this firmware blob from the boot services code via any
>>> defined interface -- you're literally snarfing up the blob from a range of
>>> memory.  I fully expect there to be any number of ways for untrustworthy
>>> entities to inject malicious blobs into this memory range on quite a few
>>> implementations.  For example, there are probably unauthenticated EFI
>>> variables and even parts of USB sticks and such that get read into boot
>>> services memory, and I see no reason at all to expect that nothing in the
>>> so-called "boot services code" range is actually just plain old boot
>>> services *heap*.
>>>
>>> Fortunately, given your design, this is very easy to fix.  Just replace
>>> CRC32 with SHA-256 or similar.  If you find the crypto api too ugly for
>>> this purpose, I have patches that only need a small amount of dusting off
>>> to give an entirely reasonable SHA-256 API in the kernel.
>>
>> My main reason for going with crc32 is that the scanning happens before
>> the kernel is fully up and running (it happens just before the rest_init()
>> call in start_kernel() (from init/main.c) I'm open to using the
>> crypto api, but I was not sure if that is ready for use at that time.
> 
> Not being sure is different than being certain. As Andy noted, if that does
> not work please poke Andy about the SHA-256 API he has which would enable
> its use in kernel.
> 
> Right now this is just a crazy hack for *2* drivers. Its a lot of hacks for
> just that, so no need to rush this in just yet.

I agree that there is no rush to get this in. I will rebase this on top
of the "[PATCH v7 00/14] firmware_loader changes for v4.18" series you recently
send as well as try to address all the remarks made sofar. I'm not entirely
sure when I will get around to this.

Regards,

Hans