Re: [PATCH v4] x86/coco: Require seeding RNG with RDRAND on CoCo systems

[Tom Lendacky <thomas.lendacky@amd.com>]

On 2/21/24 20:05, 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. Berrangé <berrange@redhat.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: stable@vger.kernel.org
> Reviewed-by: Elena Reshetova <elena.reshetova@intel.com>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> Changes v3->v4:
> - Add stable@ tag and reviewed-by lines.
> - Add comment for Dave explaining where the "32" comes from.
> 
>   arch/x86/coco/core.c        | 40 +++++++++++++++++++++++++++++++++++++
>   arch/x86/include/asm/coco.h |  2 ++
>   arch/x86/kernel/setup.c     |  2 ++
>   3 files changed, 44 insertions(+)
> 
> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index eeec9986570e..0e988bff4aec 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 +156,40 @@ __init void cc_set_mask(u64 mask)
>   {
>   	cc_mask = mask;
>   }
> +
> +__init void cc_random_init(void)
> +{
> +	/*
> +	 * The seed is 32 bytes (in units of longs), which is 256 bits, which
> +	 * is the security level that the RNG is targeting.
> +	 */
> +	unsigned long rng_seed[32 / sizeof(long)];
> +	size_t i, longs;
> +
> +	if (cc_vendor == CC_VENDOR_NONE)

I responded to an earlier version of this patch, adding that response here:

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
> +	 * CoCo guests have no other unobservable source of entropy, 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 doesn't have RDRAND
> +		 * or the CPU is physically broken, and in both cases that
> +		 * 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.
> +		 */
> +		if (longs == 0)
> +			panic("RDRAND is defective.");
> +	}
> +	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();