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Date: Fri, 23 Feb 2024 13:33:39 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: "Jason A. Donenfeld" <Jason@...c4.com>, x86@...nel.org,
 linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: Borislav Petkov <bp@...en8.de>, Daniel P . Berrangé
 <berrange@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Elena Reshetova <elena.reshetova@...el.com>, "H . Peter Anvin"
 <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Theodore Ts'o <tytso@....edu>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/coco: Require seeding RNG with RDRAND on CoCo systems

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@...en8.de>
> Cc: Daniel P. Berrangé <berrange@...hat.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Elena Reshetova <elena.reshetova@...el.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: Thomas Gleixner <tglx@...utronix.de>,
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.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@...c4.com>. All Rights Reserved.
>    *
>    * Author: Tom Lendacky <thomas.lendacky@....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();

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