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Date: Wed, 21 Feb 2024 10:51:57 -0500
From: "Theodore Ts'o" <tytso@mit.edu>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: x86@kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org,
        Borislav Petkov <bp@alien8.de>,
        Daniel P =?iso-8859-1?Q?=2E_Berrang=E9?= <berrange@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v3] x86/coco: Require seeding RNG with RDRAND on CoCo
 systems
Message-ID: <20240221155157.GB594407@mit.edu>
References: <20240221123248.25570-1-Jason@zx2c4.com>
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In-Reply-To: <20240221123248.25570-1-Jason@zx2c4.com>

On Wed, Feb 21, 2024 at 01:32:40PM +0100, 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@alien8.de>
> Cc: Daniel P. Berrang? <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>

Reviewed-by: Theodore Ts'o <tytso@mit.edu>

						- Ted