Re: [PATCH 0/5 v2][RFC] Encryption and authentication for hibernate snapshot image

Andy Lutomirski
2019-01-11T01:10:03Z

On Thu, Jan 10, 2019 at 7:13 AM joeyli <jlee@suse.com> wrote:
>
> On Wed, Jan 09, 2019 at 10:47:42AM -0800, Andy Lutomirski wrote:
> > On Wed, Jan 9, 2019 at 8:40 AM joeyli <jlee@suse.com> wrote:
> > >
> > > Hi Andy,
> > >
> > > Thanks for your review!
> > >
> > > On Tue, Jan 08, 2019 at 01:41:48PM -0800, Andy Lutomirski wrote:
> > > > > On Jan 7, 2019, at 9:37 AM, joeyli <jlee@suse.com> wrote:
> > > > >
> > > > > Hi Pavel,
> > > > >
> > > > > Thanks for your review!
> > > > >
> > > > >> On Sun, Jan 06, 2019 at 07:10:27PM +0100, Pavel Machek wrote:
> > > > >> Hi!
> > > > >>
> > > > >>> This patchset is the implementation of encryption and authentication
> > > > >>> for hibernate snapshot image. The image will be encrypted by AES and
> > > > >>> authenticated by HMAC.
> > > > >>
> > > > >> Ok, so you encrypt.
> > > > >
> > > > > Yes, encryption and authentication.
> > > > >
> > > > >>> The hibernate function can be used to snapshot memory pages to an image,
> > > > >>> then kernel restores the image to memory space in a appropriate time.
> > > > >>> There have secrets in snapshot image and cracker may modifies it for
> > > > >>> hacking system. Encryption and authentication of snapshot image can protect
> > > > >>> the system.
> > > > >>>
> > > > >>> Hibernate function requests the master key through key retention service.
> > > > >>> The snapshot master key can be a trusted key or a user defined key. The
> > > > >>> name of snapshot master key is fixed to "swsusp-kmk". User should loads
> > > > >>> swsusp-kmk to kernel by keyctl tool before the hibernation resume.
> > > > >>> e.g. The swsusp-kmk must be loaded before systemd-hibernate-resume
> > > > >>
> > > > >> But if userspace has a key, encryption is useless against root.
> > > > >
> > > > > Yes, but this concern is not only for hibernation encryption. This patch
> > > > > set does not provide solution against this concern.
> > > > >
> > > > > The purpose of this patch set is to encrypt and authenticate hibernate
> > > > > snapshot image in kernel space. It also requests key through keyring
> > > > > mechanism. Which means that we can easy to adapt to new key type from
> > > > > keyring in the future.
> > > > >
> > > > > Currently TPM trusted key or user defined key types are not against
> > > > > root. Even using the TPM trusted key, it still can be unsealed by root
> > > > > before the PCRs be capped (unless we capped PCRs in kernel).
> > > > >
> > > > > My solution for keeping the secret by kernel is the EFI secure key type:
> > > > > https://lkml.org/lkml/2018/8/5/31
> > > > >
> > > > > But the EFI boot variable doesn't design for keeping secret, so Windows
> > > > > and OEM/ODM do not use boot variable to keep secret. So this idea can
> > > > > not be accepted. We must think other key type against root.
> > > > >
> > > > >>> The TPM trusted key type is preferred to be the master key. But user
> > > > >>> defined key can also be used for testing or when the platform doesn't
> > > > >>> have TPM. User must be aware that the security of user key relies on
> > > > >>> user space. If the root account be compromised, then the user key will
> > > > >>> easy to be grabbed.
> > > > >>
> > > > >> In the TPM case, does userland have access to the key?
> > > > >
> > > > > In the TPM case, userland can only touch the sealed key blob. So userland
> > > > > doesn't know the real secret. But it has risk that root unseals the key
> > > > > before PCRs be capped.
> > > > >
> > > > >> Please explain your security goals.
> > > > >
> > > > > My security goals:
> > > > >
> > > > > - Encrypt and authicate hibernate snapshot image in kernel space. Userspace
> > > > > can only touch an encrypted and signed snapshot image.
> > > >
> > > > Signed?
> > > >
> > > > I’m not entirely convinced that the keyring mechanism is what you
> > > > want. ISTM that there are two goals here:
> > > >
> > > > a) Encryption: it should be as hard as can reasonably be arranged to
> > > > extract secrets from a hibernation image.
> > > >
> > > > b) Authentication part 1: it should not be possible for someone in
> > > > possession of a turned-off machine to tamper with the hibernation
> > > > image such that the image, when booted, will leak its secrets. This
> > > > should protect against attackers who don’t know the encryption key.
> > > >
> > > > c) Authentication part 2: it should be to verify, to the extent
> > > > practical, that the image came from the same machine and was really
> > > > created using hibernation. Or maybe by the same user.
> > > >
> > > > For (a) and (b), using an AE mode where the key is protected in some
> > > > reasonable way. Joey, why are you using HMAC? Please tell me you’re
> > > > at least doing encrypt-then-MAC. But why not use a real AE mode like
> > > > AES-GCM?
> > >
> > > The reason for using HMAC is the history for development. My first patch
> > > set is only for hibernate authentication. Then I added encryption code on
> > > top of my authentication patches in last version.
> > >
> > > I am doing encrypt-then-MAC. My code ecrypts each page by AES then HMAC
> > > whole snapshot image. I feed encrypted data pages one by one to
> > > crypto_shash_update() API for calculating the hash for whole image.
> >
> > ...
> >
> > I think you should write down a clear description of the data format.
>
> Hibernation allocates free pages for building snapshot image. Those free
> pages are scattered in memory. So kernel marks those page numbers on a
> bitmap to locate them. Because this image buffer is discontinuous, so I
> use update mode hashing whole image.
>
> > A general problem with crypto is that the fact that it appears to work
> > doesn't mean it's secure at all, and it's very hard to follow the
> > code. Especially in Linux using the crypto API -- code using the
> > crypto API tends to be mostly boilerplate.
> >
>
> hm... Do you mean that the implementation of HMAC in crypto cannot be
> trusted? I hope at least that I can trust the update mode for normal
> hash like SHA256 or SHA512?

No, I fully expect the crypto code to do what it says. What I mean is
that you can easily create utterly broken things that involve crypto,
but they still work fine when you use then non-maliciously.

> > >
> > > Sorry for I didn't capture the meaning of "acceptable usage". The trusted
> > > key already be unsealed by TPM when the key be enrolled by keyctl tool.
> > > So my code just extract the unsealed key data (the random number) for
> > > encryption.
> >
> > If someone creates a trusted key that is used for asymmetric crypto or
> > perhaps a trusted key that is intended to be used for, say, an HMAC
> > key, you should not also use it to derive hibernation keys. This is
> > what I mean by "acceptable usage".
> >
>
> When keyring is producing encrypted key, the trusted key be used to
> derive the encrypt key and authenticate key to encrypt and hmac sign
> a encrypted key. So trusted key can be used in symmetric crypto.

Can it?

David, you actually understand the overall kernel keyring design. Do
keys in the keyring have usage constraints?

But I think you need to take a big step back here. We already have
kernel/power/user.c. It seems to me that you can do a better job of
what you're trying to do with less code by using it. Why do you need
new kernel code *at all* to accomplish your goals?