linux-kernel - Re: Should we automatically generate a module signing key at all?

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Message-ID: <CALCETrUAfiaPOSV9hqjka+Yoq-aKGUxOUmy0vDCZaCvv3+E1NA@mail.gmail.com>
Date:	Tue, 19 May 2015 10:17:32 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Howells <dhowells@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sedat Dilek <sedat.dilek@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Jiri Kosina <jkosina@...e.cz>,
	LSM List <linux-security-module@...r.kernel.org>,
	Michal Marek <mmarek@...e.cz>, keyrings@...ux-nfs.org,
	Abelardo Ricart III <aricart@...nix.com>
Subject: Re: Should we automatically generate a module signing key at all?

On May 19, 2015 8:30 AM, "David Howells" <dhowells@...hat.com> wrote:
>
> Andy Lutomirski <luto@...capital.net> wrote:
>
> > I'll assume that everyone uses a 256-bit hash.
>
> UEFI makes it very likely that SHA256 is in use, at least on x86_64.
>
> > The public key is tiny, and the signature is 512 bytes per module.
> > (Actually, it's probably more because of PKCS garbage.
>
> There is metadata selecting the particular key to be checked against, so with
> a 512-byte signature, you get around 500 bytes of metadata and ASN.1
> wrappings.  We could probably trim that some more by removing PKCS#7 attribute
> sections.

You could trim even more by simply not using PKCS#7.  A raw PKCS#1
signature would be just fine.  (We should really be using PSS,
though.)

>
> We do have to allow people to load external modules.  Yes, you could argue
> that you should just disable all your security systems if you want to do
> that...
>

Or use signatures for external modules.  I'm not arguing for removal
of signature support; I'm arguing that the automatically generated key
mechanism be removed in favor of hashes.

> > This is a total of ~21kB of non-swappable storage and 2MB of disk space for
> > all the signatures.
>
> Disk space is a lot cheaper than RAM.
>
> > Ed25519
>
> Is it endorsed by various governmental authorities?  It's not entirely clear.

For users who care about cryptographic security instead of compliance
with poorly designed standards, this is completely irrelevant...

> And also the aforementioned authorities may mandate minimum key (eg. 2048) and
> digest sizes (eg. 256) which we need to deal with.

...and for users who need to comply with unfortunate standards,
there's ECDSA on P-256, which is blessed by NIST.

>
> > With the hash-based scheme I outlined, the kernel text needed is
> > nearly zero.
>
> What matters is kernel text *plus* kernel data.

The kernel data involved is 32 bytes.

>
> > What integrity stuff?  IIRC dm-verity doesn't use asymmetric crypto at
> > all.  IMA probably does, though.
>
> IMA.

I don't think that the needs of IMA users should affect normal people
who run 'make' on their kernel tree.

>
> > For firmware validation, there's no good reason it couldn't work exactly
> > like module signatures.
>
> That's really impractical.  It would mean that the kernel would have to be
> built with a hash, grand-hash, great-grand-hash or whatever that covers every
> possible firmware blob you might want to load.
>
> If a vendor releases a new firmware blob, this has to be added to the
> linux-firmware hash list, say, then the hash of that added to the kernel, say,
> and the kernel rebuilt and reissued before the firmware blob can be used.

Deterministic builds can't apply to firmware regardless, so users are
trusting a vendor one way or another.  And for Chromebook or
Atomic-like uses, hashes are fine.

>
> With a key-based approach, you just need to get a signature for the new
> firmware blob.  You can even sign it yourself and add your key to your UEFI
> database.

Agreed, although I don't understand why UEFI is a reasonable place for
firmware or module keys.  UEFI is a giant implementation detail,
whereas module and firmware validation is really a cross-architecture
thing.

>
> > For kexec, I think that the main use is for crash dumps
>
> We also want to be able to kexec new kernels on servers to avoid heavy duty
> hardware reboot cycles.  But you can't put the new kernel's hash in the old
> kernel.

That should use signatures, too.

AFAICS, none of the cases for which signatures seem better than hashes
make sense in the signing_key.priv model.  If distros want to
configure a public key, too, they're welcome to.  When I do "make &&
make install", I think we shouldn't generate a key pair.

--Andy
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