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

Andy Lutomirski
2018-05-03T22:35:07Z

On Thu, May 3, 2018 at 3:31 PM Luis R. Rodriguez <mcgrof@kernel.org> 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.

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.

> 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. It seems unclear if we're
> all happy with this yet as well.

Fair enough.