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

[Hans de Goede]

Hi,

On 05/03/2018 11:35 PM, Andy Lutomirski wrote:
> On Thu, May 3, 2018 at 3:31 PM Luis R. Rodriguez <mcgrof@xxxxxxxxxx> 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@xxxxxxxxxx>
> 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.
>
> Nah, don't use the cryptoapi for this.  You'll probably regret it for any
> number of reasons.  My code is here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=crypto/sha256_bpf&id=e9e12f056f2abed50a30b762db9185799f5864e6
>
> and its two parents.  It needs a little bit of dusting and it needs
> checking that all combinations of modular and non-modular builds work.  Ard
> probably has further comments.

Looks good, I've cherry picked this into my personal tree and will make
the next version of the EFI embedded-firmware patches use SHA256.

As Luis already mentioned geting the EFI embedded-firmware patches
upstream is not something urgent, so it is probably best to just
wait for you to push these upstream I guess?

Regards,

Hans