Linux Fix Incoming For Intel Fallout After The Kernel Disabled Buggy AMD fTPM RNG

Written by Michael Larabel in Intel on 4 September 2023 at 07:58 PM EDT. 15 Comments
The decision last month for the Linux kernel to disable random number generation (RNG) for all AMD fTPMs ended up having some unintended consequences on Intel systems that ended up breaking S3 suspend behavior.

The patch to disable RNG on all AMD fTPMs ended up regressing some Intel systems that caused issues when triggering S3 suspend-to-RAM. A fix for that issue has now been submitted for the ongoing Linux 6.6 merge window and it's marked for back-porting to the stable kernel series as well in the coming days.

AMD Ryzen CPUs with fTPM RNG

Back in August this bug report by Intel Linux engineer Todd Brandt reported that S3 broke on five systems in their lab with a crash and reboot when initiating S3 suspend. That kernel regression was traced back to the AMD change for disabling RNG on all AMD fTPMs.

This patch came about to correct the AMD fTPM checking behavior to fix the regression on Intel hardware:
"The vendor check introduced by commit 554b841d4703 ("tpm: Disable RNG for all AMD fTPMs") doesn't work properly on a number of Intel fTPMs. On the reported systems the TPM doesn't reply at bootup and returns back the command code. This makes the TPM fail probe.

Since only Microsoft Pluton is the only known combination of AMD CPU and fTPM from other vendor, disable hwrng otherwise. In order to make sysadmin aware of this, print also info message to the klog."

That patch in turn was submitted today as part of the TPMDD changes for the Linux 6.6 merge window. This patch in turn is then marked for back-porting to the kernel stable versions once its picked up by mainline in the next day or two. That improved approach will fix the suspend behavior on affected Intel systems while still disabling RNG on all AMD fTPM systems.
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