Pierluigi Paganini January 06, 2018

Google expert discovered a new stack-based overflow vulnerability in AMD CPUs that could be exploited via crafted EK certificates,

Chip manufacturers are in the tempest, while media are continues sharing news about the Meltdown and Spectre attacks, the security researcher at Google’s cloud security team Cfir Cohen disclosed a stack-based overflow vulnerability in the fTMP of AMD’s Platform Security Processor (PSP).

The vulnerability affects 64-bit x86 processors, the AMD PSP provides administrative functions similar to the Intel Management Engine.

The fTMP is the firmware implementation of the Trusted Platform Module that is an international standard for a secure cryptoprocessor, The TPM is a dedicated microcontroller designed to secure hardware by integrating cryptographic keys into devices.

Cohen revealed that he reported the vulnerability to AMD in September, the manufacturer apparently had developed a patch by December 7. After the 90-day disclosure window, Google decided to publicly disclose the details of the vulnerability because AMD did not take any action to solve the problem.

“Through manual static analysis, we’ve 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:” reads the security advisory.

“A firmware update emerged for some AMD chips in mid-December, with an option to at least partially disable the PSP. However, a spokesperson for the tech giant said on Friday this week that the above fTMP issue will be addressed in an update due out this month, January 2018.”

Cohen explained that missing bounds checks while managing a TLV (type-length-value) structure are the root cause of a stack overflow.

The vulnerability requests the physical access as a prerequisite, the expert noted that the PSP doesn’t implement common exploit mitigation techniques such as stack cookies, No-eXecute stack, or ASLR.

The flaw is very hard to exploit as confirmed by an AMD spokesperson to The Register.

“an attacker would first have to gain access to the motherboard and then modify SPI-Flash before the issue could be exploited. But given those conditions, the attacker would have access to the information protected by the TPM, such as cryptographic keys.” said the AMD spokesperson.

AMD plans to address the vulnerability for a limited number of firmware versions, the security updates will be available later this month.

Pierluigi Paganini

(Security Affairs – AMD, stack-based overflow)

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