Re: [PATCH v8 00/22] add integrity and security to TPM2 transactions

James Bottomley
2024-04-30T19:23:46Z

On Tue, 2024-04-30 at 01:22 +0300, Jarkko Sakkinen wrote:
> On Mon Apr 29, 2024 at 11:27 PM EEST, James Bottomley wrote:
> > The interest in securing the TPM against interposers, both active
> > and
> > passive has risen to fever pitch with the demonstration of key
> > recovery against windows bitlocker:
> >
> > https://dolosgroup.io/blog/2021/7/9/from-stolen-laptop-to-inside-the-company-network
> >
> > And subsequently the same attack being successful against all the
> > Linux TPM based security solutions:
> >
> > https://www.secura.com/blog/tpm-sniffing-attacks-against-non-bitlocker-targets
> >
> > The attacks fall into two categories:
> >
> > 1. Passive Interposers, which sit on the bus and merely observe
> > 2. Active Interposers, which try to manipulate TPM transactions on
> > the
> >    bus using man in the middle and packet stealing to create TPM
> > state
> >    the interposer owner desires.
> >
> > Our broadest interposer target is the use of TPM_RS_PW for password
> > authorization which sends the actual password to the TPM without
> > any
> > obfuscation and effectively hands it to any interposer. The way to
> > fix
> > this is to use real sessions for HMAC capabilities to ensure
> > integrity
> > and to use parameter and response encryption to ensure
> > confidentiality
> > of the data flowing over the TPM bus.  HMAC sessions by agreeing a
> > challenge with the TPM and then giving a response which is a HMAC
> > of
> > the password and the challenge, so the application proves knowledge
> > of
> > the password to the TPM without ever transmitting the password
> > itself.
> > Using HMAC sessions when sending commands to the TPM also provides
> > some measure of protection against active interposers, since the
> > interposer can't interfere with or delete a HMAC'd command (because
> > they can't manufacture a response with the correct HMAC).
> >
> > To protect TPM transactions where there isn't a shared secret
> > (i.e. the command is something like a PCR extension which doesn't
> > involve a TPM object with a password) we have to do a bit more work
> > to
> > set up sessions with a passed in encrypted secret (called a salt)
> > to
> > act in place of the shared secret in the HMAC.  This secret salt is
> > effectively a random number encrypted to a public key of the TPM.
> > The
> > final piece of the puzzle is using parameter input and response
> > return
> > encryption, so any interposer can't see the data passing from the
> > application to the TPM and vice versa.
> >
> > The most insidious interposer attack of all is a reset attack:
> > since
> > the interposer has access to the TPM bus, it can assert the TPM
> > reset
> > line any time it wants.  When a TPM resets it mostly comes back in
> > the
> > same state except that all the PCRs are reset to their initial
> > values.
> > Controlling the reset line allows the interposer to change the PCR
> > state after the fact by resetting the TPM and then replaying PCR
> > extends to get the PCRs into a valid state to release secrets, so
> > even
> > if an attack event was recorded, the record is erased.  This reset
> > attack violates the fundamental princible of non-repudiability of
> > TPM
> > logs.  Defeating the reset attack involves tying all TPM operations
> > within the kernel to a property which will change detectably if the
> > TPM is reset.  For that reason, we tie all TPM sessions to the null
> > hierarchy we obtain at start of day and whose seed changes on every
> > reset.  If an active interposer asserts a TPM reset, the new null
> > primary won't match the kernel's stored one and all TPM operations
> > will start failing because of HMAC mismatches in the sessions.  So
> > if
> > the kernel TPM code keeps operating, it guarantees that a reset
> > hasn't
> > occurred.
> >
> > The final part of the puzzle is that the machine owner must have a
> > fixed idea of the EK of their TPM and should have certified this
> > with
> > the TPM manufacturer.  On every boot, the certified EK public key
> > should be used to do a make credential/activate credential
> > attestation
> > key insertion and then the null key certified with the attestation
> > key.  We can follow a trust on first use model where an OS
> > installation will extract and verify a public EK and save it to a
> > read
> > only file.
> >
> > This patch series adds a simple API which can ensure the above
> > properties as a layered addition to the existing TPM handling code.
> > This series now includes protections for PCR extend, getting random
> > numbers from the TPM and data sealing and unsealing.  It therefore
> > eliminates all uses of TPM2_RS_PW in the kernel and adds encryption
> > protection to sensitive data flowing into and out of the TPM.  The
> > first four patches add more sophisticated buffer handling to the
> > TPM
> > which is needed to build the more complex encryption and
> > authentication based commands.  Patch 6 adds all the generic
> > cryptography primitives and patches 7-9 use them in critical TPM
> > operations where we want to avoid or detect interposers.  Patch 10
> > exports the name of the null key we used for boot/run time
> > verification and patch 11 documents the security guarantees and
> > expectations.
> >
> > This was originally sent over four years ago, with the last
> > iteration
> > being:
> >
> > https://lore.kernel.org/linux-integrity/1568031515.6613.31.camel@HansenPartnership.com/
> >
> > I'm dusting it off now because various forces at Microsoft and
> > Google
> > via the Open Compute Platform are making a lot of noise about
> > interposers and we in the linux kernel look critically lacking in
> > that
> > regard, particularly for TPM trusted keys.
> >
> > ---
> > v2 fixes the problems smatch reported and adds more explanation
> > about
> > the code motion in the first few patches
> > v3 rebases the encryption to be against Ard's new library function,
> > the
> > aescfb addition of which appears as patch 1.
> > v4 refreshes Ard's patch, adds kernel doc (including a new patch to
> > add it to the moved tpm-buf functions) updates and rewords some
> > commit
> > logs
> > v5: update to proposed tpm-buf implementation (for ease of use all
> > precursor patches are part of this series, so the actual session
> > HMAC
> > and encryption begins at patch 10) and add review feedback
> > v6: split the original sessions patch into three and change the
> > config
> > variable name
> > v7: Collect reviews and add extra patch to check for and disable
> > the TPM on
> > detecting a reset attack.
> > v8: split KDF out, add tpm_ prefix + other cosmetic updates
> >
> > James
> >
> > ---
> >
> > Ard Biesheuvel (1):
> >   crypto: lib - implement library version of AES in CFB mode
> >
> > James Bottomley (14):
> >   tpm: Move buffer handling from static inlines to real functions
> >   tpm: add buffer function to point to returned parameters
> >   tpm: export the context save and load commands
> >   tpm: Add NULL primary creation
> >   tpm: Add TCG mandated Key Derivation Functions (KDFs)
> >   tpm: Add HMAC session start and end functions
> >   tpm: Add HMAC session name/handle append
> >   tpm: Add the rest of the session HMAC API
> >   tpm: add hmac checks to tpm2_pcr_extend()
> >   tpm: add session encryption protection to tpm2_get_random()
> >   KEYS: trusted: Add session encryption protection to the
> > seal/unseal
> >     path
> >   tpm: add the null key name as a sysfs export
> >   Documentation: add tpm-security.rst
> >   tpm: disable the TPM if NULL name changes
> >
> > Jarkko Sakkinen (7):
> >   tpm: Remove unused tpm_buf_tag()
> >   tpm: Remove tpm_send()
> >   tpm: Update struct tpm_buf documentation comments
> >   tpm: Store the length of the tpm_buf data separately.
> >   tpm: TPM2B formatted buffers
> >   tpm: Add tpm_buf_read_{u8,u16,u32}
> >   KEYS: trusted: tpm2: Use struct tpm_buf for sized buffers
> >
> >  Documentation/security/tpm/tpm-security.rst |  216 ++++
> >  drivers/char/tpm/Kconfig                    |   14 +
> >  drivers/char/tpm/Makefile                   |    2 +
> >  drivers/char/tpm/tpm-buf.c                  |  251 ++++
> >  drivers/char/tpm/tpm-chip.c                 |    6 +
> >  drivers/char/tpm/tpm-interface.c            |   26 +-
> >  drivers/char/tpm/tpm-sysfs.c                |   18 +
> >  drivers/char/tpm/tpm.h                      |   14 +
> >  drivers/char/tpm/tpm2-cmd.c                 |   53 +-
> >  drivers/char/tpm/tpm2-sessions.c            | 1280
> > +++++++++++++++++++
> >  drivers/char/tpm/tpm2-space.c               |   11 +-
> >  include/crypto/aes.h                        |    5 +
> >  include/keys/trusted_tpm.h                  |    2 -
> >  include/linux/tpm.h                         |  316 +++--
> >  lib/crypto/Kconfig                          |    5 +
> >  lib/crypto/Makefile                         |    3 +
> >  lib/crypto/aescfb.c                         |  257 ++++
> >  security/keys/trusted-keys/trusted_tpm1.c   |   23 +-
> >  security/keys/trusted-keys/trusted_tpm2.c   |  136 +-
> >  19 files changed, 2443 insertions(+), 195 deletions(-)
> >  create mode 100644 Documentation/security/tpm/tpm-security.rst
> >  create mode 100644 drivers/char/tpm/tpm-buf.c
> >  create mode 100644 drivers/char/tpm/tpm2-sessions.c
> >  create mode 100644 lib/crypto/aescfb.c
>
> Thanks for the update!
>
> I think I asked this already earlier but unfortunately could not
> find the corresponding email from lore.

Well, you did, but at that time I didn't have the null name change
detection so:

>
> Anyway, I've tested this series with QEMU i.e. to the point that
> I know that it does not break anything in the case when things are
> working as expected.
>
> What I would like to test is the negative case when the null key
> name changes and see what happens.
>
> I recall that you had some version of QEMU that had ability to test
> this and my latest question on that was what QEMU baseline it was
> expected to be applied over.

Yes, I added patches to qemu to make it talk directly to the mssim TPM
reference implementation

https://github.com/microsoft/ms-tpm-20-ref

so I could be sure I was testing against the reference implementation.
However, they also have the advantage that you can use wireshark to
dump the TPM transactions (ensuring encryption). You can also tamper
with the TPM state from the outside by connecting to the TPM socket.

For the case you want, you can simulate a reset by killing and
restarting the tpm server (you have to power it up and issue the
startup command manually). The next TPM command the kernel tries
should see the null name change and react accordingly.

It looks like the current qemu patches fail to apply again, so I just
reposted them against qemu git head:

https://lore.kernel.org/qemu-devel/20240430190855.2811-1-James.Bottomley@HansenPartnership.com/

> Since I could not find the email subthread I neither have the patch
> nor do know the baseline. So if you could help with these details
> then we can move forward.
>
> I can also work with QEMU Git fork if you have one and point out
> QEMU_OVERRIDE_SRCDIR to the clone.

I only have the patches in a local git repository, but I could push
qemu up onto kernel.org if it would help?

James