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

Tom Lendacky
2024-02-23T19:33:44Z

On 2/9/24 10:49, 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.
>
> Unfortunately, RDRAND itself can be rendered unreliable by the host,
> since the host controls guest scheduling and can starve RDRAND's
> generation. A malicious host could also choose to simply terminate or
> not boot a CoCo guest. So, tie the starvation of RDRAND to a BUG_ON at
> boot time.
>
> Specifically, try at boot to seed the RNG using 256 bits of RDRAND
> output. If these fail, BUG(). This doesn't handle the more complicated
> case of reseeding later in boot, but that's fraught with its own
> difficulties, such as a malicious userspace starving the kernel. For
> now, simply make sure the RNG is initially seeded securely during boot,
> avoiding the worst of potential pitfalls.
>
> 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>
> ---
> Probably this shouldn't be merged until Dave/Elena and others get back
> with regards to the full picture, with information from inside Intel.
> But I have a feeling this patch, or something like it, is ultimately
> what we'll wind up with, so I'm posting it now.
>
> I don't have a functional CoCo setup, so this patch has only been very
> lightly tested.
>
> arch/x86/coco/core.c | 36 ++++++++++++++++++++++++++++++++++++
> arch/x86/include/asm/coco.h | 2 ++
> arch/x86/kernel/setup.c | 2 ++
> 3 files changed, 40 insertions(+)
>
> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index eeec9986570e..4e3b1cfe0063 100644
> --- a/arch/x86/coco/core.c
> +++ b/arch/x86/coco/core.c
> @@ -3,13 +3,16 @@
> * Confidential Computing Platform Capability checks
> *
> * Copyright (C) 2021 Advanced Micro Devices, Inc.
> + * Copyright (C) 2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
> *
> * Author: Tom Lendacky <thomas.lendacky@amd.com>
> */
>
> #include <linux/export.h>
> #include <linux/cc_platform.h>
> +#include <linux/random.h>
>
> +#include <asm/archrandom.h>
> #include <asm/coco.h>
> #include <asm/processor.h>
>
> @@ -153,3 +156,36 @@ __init void cc_set_mask(u64 mask)
> {
> cc_mask = mask;
> }
> +
> +__init void cc_random_init(void)
> +{
> + unsigned long rng_seed[32 / sizeof(long)];
> + size_t i, longs;
> +
> + if (cc_vendor == CC_VENDOR_NONE)

You probably want to use:

if (!cc_platform_has(CC_GUEST_MEM_ENCRYPT))
return;

Otherwise, you can hit the bare-metal case where AMD SME is active and
then cc_vendor will not be CC_VENDOR_NONE.

Thanks,
Tom

> + return;
> +
> + /*
> + * Since the CoCo threat model includes the host, the only reliable
> + * source of entropy that can be neither observed nor manipulated is
> + * RDRAND. Usually, RDRAND failure is considered tolerable, but since a
> + * host can possibly induce failures consistently, it's important to at
> + * least ensure the RNG gets some initial random seeds.
> + */
> + for (i = 0; i < ARRAY_SIZE(rng_seed); i += longs) {
> + longs = arch_get_random_longs(&rng_seed[i], ARRAY_SIZE(rng_seed) - i);
> +
> + /*
> + * A zero return value means that the guest is under attack,
> + * the hardware is broken, or some other mishap has occurred
> + * that means the RNG cannot be properly rng_seeded, which also
> + * likely means most crypto inside of the CoCo instance will be
> + * broken, defeating the purpose of CoCo in the first place. So
> + * just panic here because it's absolutely unsafe to continue
> + * executing.
> + */
> + BUG_ON(longs == 0);
> + }
> + add_device_randomness(rng_seed, sizeof(rng_seed));
> + memzero_explicit(rng_seed, sizeof(rng_seed));
> +}
> diff --git a/arch/x86/include/asm/coco.h b/arch/x86/include/asm/coco.h
> index 76c310b19b11..e9d059449885 100644
> --- a/arch/x86/include/asm/coco.h
> +++ b/arch/x86/include/asm/coco.h
> @@ -15,6 +15,7 @@ extern enum cc_vendor cc_vendor;
> void cc_set_mask(u64 mask);
> u64 cc_mkenc(u64 val);
> u64 cc_mkdec(u64 val);
> +void cc_random_init(void);
> #else
> #define cc_vendor (CC_VENDOR_NONE)
>
> @@ -27,6 +28,7 @@ static inline u64 cc_mkdec(u64 val)
> {
> return val;
> }
> +static inline void cc_random_init(void) { }
> #endif
>
> #endif /* _ASM_X86_COCO_H */
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 84201071dfac..30a653cfc7d2 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -36,6 +36,7 @@
> #include <asm/bios_ebda.h>
> #include <asm/bugs.h>
> #include <asm/cacheinfo.h>
> +#include <asm/coco.h>
> #include <asm/cpu.h>
> #include <asm/efi.h>
> #include <asm/gart.h>
> @@ -994,6 +995,7 @@ void __init setup_arch(char **cmdline_p)
> * memory size.
> */
> mem_encrypt_setup_arch();
> + cc_random_init();
>
> efi_fake_memmap();
> efi_find_mirror();