On Wed, Feb 21, 2024 at 1:33 PM Jason A. Donenfeld Jason@zx2c4.com wrote:

There are few uses of CoCo that don't rely on working cryptography and hence a working RNG. Unfortunately, the CoCo threat model means that the VM host cannot be trusted and may actively work against guests to extract secrets or manipulate computation. Since a malicious host can modify or observe nearly all inputs to guests, the only remaining source of entropy for CoCo guests is RDRAND.

If RDRAND is broken -- due to CPU hardware fault -- the RNG as a whole is meant to gracefully continue on gathering entropy from other sources, but since there aren't other sources on CoCo, this is catastrophic. This is mostly a concern at boot time when initially seeding the RNG, as after that the consequences of a broken RDRAND are much more theoretical.

So, try at boot to seed the RNG using 256 bits of RDRAND output. If this fails, panic(). This will also trigger if the system is booted without RDRAND, as RDRAND is essential for a safe CoCo boot.

This patch is deliberately written to be "just a CoCo x86 driver feature" and not part of the RNG itself. Many device drivers and platforms have some desire to contribute something to the RNG, and add_device_randomness() is specifically meant for this purpose. Any driver can call this with seed data of any quality, or even garbage quality, and it can only possibly make the quality of the RNG better or have no effect, but can never make it worse. Rather than trying to build something into the core of the RNG, this patch interprets the particular CoCo issue as just a CoCo issue, and therefore separates this all out into driver (well, arch/platform) code.

Cc: Borislav Petkov bp@alien8.de Cc: Daniel P. Berrangé berrange@redhat.com Cc: Dave Hansen dave.hansen@linux.intel.com Cc: Elena Reshetova elena.reshetova@intel.com Cc: H. Peter Anvin hpa@zytor.com Cc: Ingo Molnar mingo@redhat.com Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Theodore Ts'o tytso@mit.edu Cc: Thomas Gleixner tglx@linutronix.de Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com

Also,

Cc: stable@vger.kernel.org

At least, I think that's probably what we want, though I don't know what version range is relevant for CoCo.