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intel / ccc-linux-guest-hardening

Linux Security Hardening for Confidential Compute
https://intel.github.io/ccc-linux-guest-hardening-docs
MIT License
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[Hardening aspect] Security of Linux RNG in a CoCo guest #140

Closed ereshetova closed 3 months ago

ereshetova commented 3 months ago

Linux RNG is one of the primary sources of cryptographically strong random numbers available for both kernel and userspace. The default sources of entropy for Linux RNG are timing and interrupts, which are both observable (at least in theory) by a host/VMM under a CoCo threat model. The only source that is not observable is CPU DRNG, which is on x86 can be accessed by RDRAND/RDSEED instructions. However, currently RDSEED can be made to fail if enough pressure is applied to it, and RDRAND can also fail in case of HW failure. In such cases we cannot allow a CoCo guest to proceed, since the Linux RNG won't be providing cryptographically secure random numbers.

ereshetova commented 3 months ago

This issue has been extensively discussed in lkml. Some references:

https://lkml.org/lkml/2024/1/30/352 https://lwn.net/Articles/961121/

The final solution has been agreed on: https://lore.kernel.org/lkml/20240224011921.2663985-1-Jason@zx2c4.com/

ereshetova commented 3 months ago

Merged commit to the mainline: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=99485c4c026f024e7cb82da84c7951dbe3deb584