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

Jarkko Sakkinen
2024-04-30T21:48:53Z

On Tue Apr 30, 2024 at 10:23 PM EEST, James Bottomley wrote:
> 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?

That definitely does help. I can point out my build to that repository,
(or actually clone of it).

As said the "valid flow" has been tested multiple times. I guess I can
hold v6.10 PR to next week so there is still time to barely squeeze this
to v6.10.

BR, Jarkko