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Date: Sun, 13 May 2018 12:05:39 +0100 From: Hans de Goede <hdegoede@...hat.com> To: "Luis R. Rodriguez" <mcgrof@...nel.org> Cc: Andy Lutomirski <luto@...nel.org>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Greg KH <gregkh@...uxfoundation.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Peter Jones <pjones@...hat.com>, dave@...aar.me, Will Deacon <will.deacon@....com>, Matt Fleming <matt@...eblueprint.co.uk>, David Howells <dhowells@...hat.com>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, Josh Triplett <josh@...htriplett.org>, Dmitry Torokhov <dmitry.torokhov@...il.com>, Martin Fuzzey <mfuzzey@...keon.com>, Kalle Valo <kvalo@...eaurora.org>, Arend Van Spriel <arend.vanspriel@...adcom.com>, Linus Torvalds <torvalds@...ux-foundation.org>, nbroeking@...com, Bjorn Andersson <bjorn.andersson@...aro.org>, duwe@...e.de, Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>, linux-efi <linux-efi@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, LSM List <linux-security-module@...r.kernel.org> Subject: Re: [PATCH v5 2/5] efi: Add embedded peripheral firmware support 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@...hat.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
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