Re: [PATCH v6] x86/coco: Require seeding RNG with RDRAND on CoCo systems

[Tom Lendacky]

On 3/26/24 11:07, Jason A. Donenfeld 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@xxxxxxxxx>
> Cc: Daniel P. Berrangé <berrange@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reviewed-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reviewed-by: Theodore Ts'o <tytso@xxxxxxx>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>

Tested-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
> Changes v5->v6:
> - Rebase on tip/master.
> - Add string.h include.
>
>   arch/x86/coco/core.c        | 41 +++++++++++++++++++++++++++++++++++++
>   arch/x86/include/asm/coco.h |  2 ++
>   arch/x86/kernel/setup.c     |  2 ++
>   3 files changed, 45 insertions(+)