We are exploring enabling hibernation in some new scenarios. However,
our security team has a few requirements, listed below:
1. The hibernate image must be encrypted with protection derived from
both the platform (eg TPM) and user authentication data (eg
password).
2. Hibernation must not be a vector by which a malicious userspace can
escalate to the kernel.
Requirement #1 can be achieved solely with uswsusp, however requirement
2 necessitates mechanisms in the kernel to guarantee integrity of the
hibernate image. The kernel needs a way to authenticate that it generated
the hibernate image being loaded, and that the image has not been tampered
with. Adding support for in-kernel AEAD encryption with a TPM-sealed key
allows us to achieve both requirements with a single computation pass.
Matthew Garrett published a series [1] that aligns closely with this
goal. His series utilized the fact that PCR23 is a resettable PCR that
can be blocked from access by usermode. The TPM can create a sealed key
tied to PCR23 in two ways. First, the TPM can attest to the value of
PCR23 when the key was created, which the kernel can use on resume to
verify that the kernel must have created the key (since it is the only
one capable of modifying PCR23). It can also create a policy that enforces
PCR23 be set to a specific value as a condition of unsealing the key,
preventing usermode from unsealing the key by talking directly to the
TPM.
This series adopts that primitive as a foundation, tweaking and building
on it a bit. Where Matthew's series used the TPM-backed key to encrypt a
hash of the image, this series uses the key directly as a gcm(aes)
encryption key, which the kernel uses to encrypt and decrypt the
hibernate image in chunks of 16 pages. This provides both encryption and
integrity, which turns out to be a noticeable performance improvement over
separate passes for encryption and hashing.
The series also introduces the concept of mixing user key material into
the encryption key. This allows usermode to introduce key material
based on unspecified external authentication data (in our case derived
from something like the user password or PIN), without requiring
usermode to do a separate encryption pass.
Matthew also documented issues his series had [2] related to generating
fake images by booting alternate kernels without the PCR23 limiting.
With access to PCR23 on the same machine, usermode can create fake
hibernate images that are indistinguishable to the new kernel from
genuine ones. His post outlines a solution that involves adding more
PCRs into the creation data and policy, with some gyrations to make this
work well on a standard PC.
Our approach would be similar: on our machines PCR 0 indicates whether
the system is booted in secure/verified mode or developer mode. By
adding PCR0 to the policy, we can reject hibernate images made in
developer mode while in verified mode (or vice versa).
Additionally, mixing in the user authentication data limits both
data exfiltration attacks (eg a stolen laptop) and forged hibernation
image attacks to attackers that already know the authentication data (eg
user's password). This, combined with our relatively sealed userspace
(dm-verity on the rootfs), and some judicious clearing of the hibernate
image (such as across an OS update) further reduce the risk of an online
attack. The remaining attack space of a forgery from someone with
physical access to the device and knowledge of the authentication data
is out of scope for us, given that flipping to developer mode or
reflashing RO firmware trivially achieves the same thing.
A couple of patches still need to be written on top of this series. The
generalized functionality to OR in additional PCRs via Kconfig (like PCR
0 or 5) still needs to be added. We'll also need a patch that disallows
unencrypted forms of resume from hibernation, to fully close the door
to malicious userspace. However, I wanted to get this series out first
and get reactions from upstream before continuing to add to it.
[1] https://patchwork.kernel.org/project/linux-pm/cover/[email protected]/
[2] https://mjg59.dreamwidth.org/58077.html
Evan Green (6):
security: keys: trusted: Verify creation data
PM: hibernate: Add kernel-based encryption
PM: hibernate: Use TPM-backed keys to encrypt image
PM: hibernate: Mix user key in encrypted hibernate
PM: hibernate: Verify the digest encryption key
PM: hibernate: seal the encryption key with a PCR policy
Matthew Garrett (4):
tpm: Add support for in-kernel resetting of PCRs
tpm: Allow PCR 23 to be restricted to kernel-only use
security: keys: trusted: Parse out individual components of the key
blob
security: keys: trusted: Allow storage of PCR values in creation data
Documentation/power/userland-swsusp.rst | 8 +
.../security/keys/trusted-encrypted.rst | 4 +
drivers/char/tpm/Kconfig | 10 +
drivers/char/tpm/tpm-dev-common.c | 8 +
drivers/char/tpm/tpm-interface.c | 28 +
drivers/char/tpm/tpm.h | 23 +
drivers/char/tpm/tpm1-cmd.c | 69 ++
drivers/char/tpm/tpm2-cmd.c | 58 +
drivers/char/tpm/tpm2-space.c | 2 +-
include/keys/trusted-type.h | 9 +
include/linux/tpm.h | 12 +
include/uapi/linux/suspend_ioctls.h | 28 +-
kernel/power/Kconfig | 15 +
kernel/power/Makefile | 1 +
kernel/power/power.h | 1 +
kernel/power/snapenc.c | 1076 +++++++++++++++++
kernel/power/snapshot.c | 5 +
kernel/power/user.c | 44 +-
kernel/power/user.h | 114 ++
security/keys/trusted-keys/trusted_tpm1.c | 9 +
security/keys/trusted-keys/trusted_tpm2.c | 164 ++-
21 files changed, 1670 insertions(+), 18 deletions(-)
create mode 100644 kernel/power/snapenc.c
create mode 100644 kernel/power/user.h
--
2.31.0
Hi Rafael,
On Tue, May 17, 2022 at 9:06 AM Rafael J. Wysocki <[email protected]> wrote:
>
> On Mon, May 9, 2022 at 6:44 PM Evan Green <[email protected]> wrote:
> >
> > On Fri, May 6, 2022 at 9:08 AM Pavel Machek <[email protected]> wrote:
> > >
> > > Hi!
> > >
> > > > We are exploring enabling hibernation in some new scenarios. However,
> > > > our security team has a few requirements, listed below:
> > > > 1. The hibernate image must be encrypted with protection derived from
> > > > both the platform (eg TPM) and user authentication data (eg
> > > > password).
> > > > 2. Hibernation must not be a vector by which a malicious userspace can
> > > > escalate to the kernel.
> > >
> > > Can you (or your security team) explain why requirement 2. is needed?
> > >
> > > On normal systems, trusted userspace handles kernel upgrades (for example),
> > > so it can escalate to kernel priviledges.
> > >
> >
> > Our systems are a little more sealed up than a normal distro, we use
> > Verified Boot [1]. To summarize, RO firmware with an embedded public
> > key verifies that the kernel+commandline was signed by Google. The
> > commandline includes the root hash of the rootfs as well (where the
> > modules live). So when an update is applied (A/B style, including the
> > whole rootfs), assuming the RO firmware stayed RO (which requires
> > physical measures to defeat), we can guarantee that the kernel,
> > commandline, and rootfs have not been tampered with.
> >
> > Verified boot gives us confidence that on each boot, we're at least
> > starting from known code. This makes it more challenging for an
> > attacker to persist an exploit across reboot. With the kernel and
> > modules verified, we try to make it non-trivial for someone who does
> > manage to gain root execution once from escalating to kernel
> > execution. Hibernation would be one obvious escalation route, so we're
> > hoping to find a way to enable it without handing out that easy
> > primitive.
> >
> > [1] https://www.chromium.org/chromium-os/chromiumos-design-docs/verified-boot/
>
> So I guess this really is an RFC.
Yes, I suppose it is.
>
> Honestly, I need more time to go through this and there are pieces of
> it that need to be looked at other people (like the TPM-related
> changes).
No problem, thanks for the reply to let me know. I expect some back
and forth in terms of what should be hidden behind abstractions and
where exactly things should live. But I wanted to get this out to
upstream as early as I could, just to get initial reactions on the
overall concept and design. Looking forward to hearing your thoughts
when you get a chance, and let me know if there are others I should be
adding that I've missed.
-Evan
>
> Thanks!
On Tue, May 17, 2022 at 10:34 AM Evan Green <[email protected]> wrote:
>
> Hi Rafael,
>
> On Tue, May 17, 2022 at 9:06 AM Rafael J. Wysocki <[email protected]> wrote:
> >
> > On Mon, May 9, 2022 at 6:44 PM Evan Green <[email protected]> wrote:
> > >
> > > On Fri, May 6, 2022 at 9:08 AM Pavel Machek <[email protected]> wrote:
> > > >
> > > > Hi!
> > > >
> > > > > We are exploring enabling hibernation in some new scenarios. However,
> > > > > our security team has a few requirements, listed below:
> > > > > 1. The hibernate image must be encrypted with protection derived from
> > > > > both the platform (eg TPM) and user authentication data (eg
> > > > > password).
> > > > > 2. Hibernation must not be a vector by which a malicious userspace can
> > > > > escalate to the kernel.
> > > >
> > > > Can you (or your security team) explain why requirement 2. is needed?
> > > >
> > > > On normal systems, trusted userspace handles kernel upgrades (for example),
> > > > so it can escalate to kernel priviledges.
> > > >
> > >
> > > Our systems are a little more sealed up than a normal distro, we use
> > > Verified Boot [1]. To summarize, RO firmware with an embedded public
> > > key verifies that the kernel+commandline was signed by Google. The
> > > commandline includes the root hash of the rootfs as well (where the
> > > modules live). So when an update is applied (A/B style, including the
> > > whole rootfs), assuming the RO firmware stayed RO (which requires
> > > physical measures to defeat), we can guarantee that the kernel,
> > > commandline, and rootfs have not been tampered with.
> > >
> > > Verified boot gives us confidence that on each boot, we're at least
> > > starting from known code. This makes it more challenging for an
> > > attacker to persist an exploit across reboot. With the kernel and
> > > modules verified, we try to make it non-trivial for someone who does
> > > manage to gain root execution once from escalating to kernel
> > > execution. Hibernation would be one obvious escalation route, so we're
> > > hoping to find a way to enable it without handing out that easy
> > > primitive.
> > >
> > > [1] https://www.chromium.org/chromium-os/chromiumos-design-docs/verified-boot/
> >
> > So I guess this really is an RFC.
>
> Yes, I suppose it is.
>
> >
> > Honestly, I need more time to go through this and there are pieces of
> > it that need to be looked at other people (like the TPM-related
> > changes).
>
> No problem, thanks for the reply to let me know. I expect some back
> and forth in terms of what should be hidden behind abstractions and
> where exactly things should live. But I wanted to get this out to
> upstream as early as I could, just to get initial reactions on the
> overall concept and design. Looking forward to hearing your thoughts
> when you get a chance, and let me know if there are others I should be
> adding that I've missed.
Gentle bump in case this dropped off of radars, I'd still appreciate
any feedback folks had on this series.
-Evan
>
> -Evan
>
> >
> > Thanks!
On Thu, Jun 16, 2022 at 8:42 AM Evan Green <[email protected]> wrote:
>
> On Tue, May 17, 2022 at 10:34 AM Evan Green <[email protected]> wrote:
> >
> > Hi Rafael,
> >
> > On Tue, May 17, 2022 at 9:06 AM Rafael J. Wysocki <[email protected]> wrote:
> > >
> > > On Mon, May 9, 2022 at 6:44 PM Evan Green <[email protected]> wrote:
> > > >
> > > > On Fri, May 6, 2022 at 9:08 AM Pavel Machek <[email protected]> wrote:
> > > > >
> > > > > Hi!
> > > > >
> > > > > > We are exploring enabling hibernation in some new scenarios. However,
> > > > > > our security team has a few requirements, listed below:
> > > > > > 1. The hibernate image must be encrypted with protection derived from
> > > > > > both the platform (eg TPM) and user authentication data (eg
> > > > > > password).
> > > > > > 2. Hibernation must not be a vector by which a malicious userspace can
> > > > > > escalate to the kernel.
> > > > >
> > > > > Can you (or your security team) explain why requirement 2. is needed?
> > > > >
> > > > > On normal systems, trusted userspace handles kernel upgrades (for example),
> > > > > so it can escalate to kernel priviledges.
> > > > >
> > > >
> > > > Our systems are a little more sealed up than a normal distro, we use
> > > > Verified Boot [1]. To summarize, RO firmware with an embedded public
> > > > key verifies that the kernel+commandline was signed by Google. The
> > > > commandline includes the root hash of the rootfs as well (where the
> > > > modules live). So when an update is applied (A/B style, including the
> > > > whole rootfs), assuming the RO firmware stayed RO (which requires
> > > > physical measures to defeat), we can guarantee that the kernel,
> > > > commandline, and rootfs have not been tampered with.
> > > >
> > > > Verified boot gives us confidence that on each boot, we're at least
> > > > starting from known code. This makes it more challenging for an
> > > > attacker to persist an exploit across reboot. With the kernel and
> > > > modules verified, we try to make it non-trivial for someone who does
> > > > manage to gain root execution once from escalating to kernel
> > > > execution. Hibernation would be one obvious escalation route, so we're
> > > > hoping to find a way to enable it without handing out that easy
> > > > primitive.
> > > >
> > > > [1] https://www.chromium.org/chromium-os/chromiumos-design-docs/verified-boot/
> > >
> > > So I guess this really is an RFC.
> >
> > Yes, I suppose it is.
> >
> > >
> > > Honestly, I need more time to go through this and there are pieces of
> > > it that need to be looked at other people (like the TPM-related
> > > changes).
> >
> > No problem, thanks for the reply to let me know. I expect some back
> > and forth in terms of what should be hidden behind abstractions and
> > where exactly things should live. But I wanted to get this out to
> > upstream as early as I could, just to get initial reactions on the
> > overall concept and design. Looking forward to hearing your thoughts
> > when you get a chance, and let me know if there are others I should be
> > adding that I've missed.
>
> Gentle bump in case this dropped off of radars, I'd still appreciate
> any feedback folks had on this series.
One more bump here, as we'd really love to get encrypted hibernation
to a form upstream would accept if at all possible. We were
considering landing this in our Chrome OS tree for now, then coming
back in a couple months with a "we've been baking this ourselves and
it's going so great, oooh yeah". I'm not sure if upstream would find
that compelling or not. But in any case, some guidance towards making
this more upstream friendly would be well appreciated.
One thing I realized in attempting to pick this myself is that the
trusted key blob format has moved to ASN.1. So I should really move
the creation ticket to the new ASN.1 format (if I can figure out the
right OID for that piece), which would allow me to drop a lot of the
ugly stuff in tpm2_unpack_blob(). Maybe if I get no other comments
I'll work on that and resend.
-Evan
On Mon, Aug 1, 2022 at 3:33 PM Evan Green <[email protected]> wrote:
> One more bump here, as we'd really love to get encrypted hibernation
> to a form upstream would accept if at all possible. We were
> considering landing this in our Chrome OS tree for now, then coming
> back in a couple months with a "we've been baking this ourselves and
> it's going so great, oooh yeah". I'm not sure if upstream would find
> that compelling or not. But in any case, some guidance towards making
> this more upstream friendly would be well appreciated.
>
> One thing I realized in attempting to pick this myself is that the
> trusted key blob format has moved to ASN.1. So I should really move
> the creation ticket to the new ASN.1 format (if I can figure out the
> right OID for that piece), which would allow me to drop a lot of the
> ugly stuff in tpm2_unpack_blob(). Maybe if I get no other comments
> I'll work on that and resend.
I've been revamping my TPM-backed verified hibernation implementation
based on this work, so I'd definitely be enthusiastic about it being
mergeable.
On Tue, Aug 02, 2022 at 11:36:43AM -0700, Matthew Garrett wrote:
> On Mon, Aug 1, 2022 at 3:33 PM Evan Green <[email protected]> wrote:
>
> > One more bump here, as we'd really love to get encrypted hibernation
> > to a form upstream would accept if at all possible. We were
> > considering landing this in our Chrome OS tree for now, then coming
> > back in a couple months with a "we've been baking this ourselves and
> > it's going so great, oooh yeah". I'm not sure if upstream would find
> > that compelling or not. But in any case, some guidance towards making
> > this more upstream friendly would be well appreciated.
> >
> > One thing I realized in attempting to pick this myself is that the
> > trusted key blob format has moved to ASN.1. So I should really move
> > the creation ticket to the new ASN.1 format (if I can figure out the
> > right OID for that piece), which would allow me to drop a lot of the
> > ugly stuff in tpm2_unpack_blob(). Maybe if I get no other comments
> > I'll work on that and resend.
>
> I've been revamping my TPM-backed verified hibernation implementation
> based on this work, so I'd definitely be enthusiastic about it being
> mergeable.
BTW, is it tested with QEMU + swtpm?
BR, Jarkko
On Wed, Aug 3, 2022 at 5:59 PM Jarkko Sakkinen <[email protected]> wrote:
>
> On Tue, Aug 02, 2022 at 11:36:43AM -0700, Matthew Garrett wrote:
> > On Mon, Aug 1, 2022 at 3:33 PM Evan Green <[email protected]> wrote:
> >
> > > One more bump here, as we'd really love to get encrypted hibernation
> > > to a form upstream would accept if at all possible. We were
> > > considering landing this in our Chrome OS tree for now, then coming
> > > back in a couple months with a "we've been baking this ourselves and
> > > it's going so great, oooh yeah". I'm not sure if upstream would find
> > > that compelling or not. But in any case, some guidance towards making
> > > this more upstream friendly would be well appreciated.
> > >
> > > One thing I realized in attempting to pick this myself is that the
> > > trusted key blob format has moved to ASN.1. So I should really move
> > > the creation ticket to the new ASN.1 format (if I can figure out the
> > > right OID for that piece), which would allow me to drop a lot of the
> > > ugly stuff in tpm2_unpack_blob(). Maybe if I get no other comments
> > > I'll work on that and resend.
> >
> > I've been revamping my TPM-backed verified hibernation implementation
> > based on this work, so I'd definitely be enthusiastic about it being
> > mergeable.
>
> BTW, is it tested with QEMU + swtpm?
For myself, so far I've been testing on a recent Intel Chromebook. The
H1 (aka cr50) security chip on modern chromebooks implements a subset
[1] of TPM2.0, and is exposed through the standard TPM APIs in the
kernel. I can make sure to test on Qemu as well, is there anything in
particular I should look out for?
-Evan
[1] https://chromium-review.googlesource.com/c/chromiumos/third_party/tpm2/+/3373466
>
> BR, Jarkko
On Thu, Aug 04, 2022 at 02:55:35PM -0700, Evan Green wrote:
> On Wed, Aug 3, 2022 at 5:59 PM Jarkko Sakkinen <[email protected]> wrote:
> >
> > On Tue, Aug 02, 2022 at 11:36:43AM -0700, Matthew Garrett wrote:
> > > On Mon, Aug 1, 2022 at 3:33 PM Evan Green <[email protected]> wrote:
> > >
> > > > One more bump here, as we'd really love to get encrypted hibernation
> > > > to a form upstream would accept if at all possible. We were
> > > > considering landing this in our Chrome OS tree for now, then coming
> > > > back in a couple months with a "we've been baking this ourselves and
> > > > it's going so great, oooh yeah". I'm not sure if upstream would find
> > > > that compelling or not. But in any case, some guidance towards making
> > > > this more upstream friendly would be well appreciated.
> > > >
> > > > One thing I realized in attempting to pick this myself is that the
> > > > trusted key blob format has moved to ASN.1. So I should really move
> > > > the creation ticket to the new ASN.1 format (if I can figure out the
> > > > right OID for that piece), which would allow me to drop a lot of the
> > > > ugly stuff in tpm2_unpack_blob(). Maybe if I get no other comments
> > > > I'll work on that and resend.
> > >
> > > I've been revamping my TPM-backed verified hibernation implementation
> > > based on this work, so I'd definitely be enthusiastic about it being
> > > mergeable.
> >
> > BTW, is it tested with QEMU + swtpm?
>
> For myself, so far I've been testing on a recent Intel Chromebook. The
> H1 (aka cr50) security chip on modern chromebooks implements a subset
> [1] of TPM2.0, and is exposed through the standard TPM APIs in the
> kernel. I can make sure to test on Qemu as well, is there anything in
> particular I should look out for?
I was just thinking what I could use for testing
BR, Jarkko