Received-SPF: pass (google.com: domain of linux-kernel+bounces-79234-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Message-ID: <9cec487f-ce7d-809d-b4a7-9b7a8a6c9c93@amd.com>
Date: Fri, 23 Feb 2024 16:05:21 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.15.1
Subject: Re: [PATCH v4] x86/coco: Require seeding RNG with RDRAND on CoCo
 systems
Content-Language: en-US
To: "Jason A. Donenfeld" <Jason@zx2c4.com>, x86@kernel.org,
 linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>, =?UTF-8?Q?Daniel_P_=2e_Berrang=c3=a9?=
 <berrange@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>,
 "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
 Thomas Gleixner <tglx@linutronix.de>, stable@vger.kernel.org,
 Elena Reshetova <elena.reshetova@intel.com>,
 "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
 Theodore Ts'o <tytso@mit.edu>
References: <5648f43d-76e4-4396-b626-411d60657c93@intel.com>
 <20240222020616.2315199-1-Jason@zx2c4.com>
From: Tom Lendacky <thomas.lendacky@amd.com>
In-Reply-To: <20240222020616.2315199-1-Jason@zx2c4.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0:
	=?utf-8?B?c3Q4NStMV2NTTGlzYVRnZFFXQ1d2MDlVdFVXbytuNUl3Z3NPc2tEZmNjRlV1?=
 =?utf-8?B?SCszL0NxcFZGNG80RTdXYzNxTkloclRid0J0UUdrd1BlYmNmTTJRN2F1N2Yy?=
 =?utf-8?B?UkxhWDdtekpDdm1aNENQSzJoYVZLdHNUOTlXcU81eVN6UlF2bVMrVmxBcHdB?=
 =?utf-8?B?M2VydDRnUDQvbDdKWnJtV2U1MDBGd0JmV1pIcERKQjlaVDQ4eUg2UXVZTW1L?=
 =?utf-8?B?cDdXSFJwT3pBUEpjWkFEeXhmdmg1R3RaSVR3QWdFeWQ3b0lyVnRodjFmVG01?=
 =?utf-8?B?aVdDdEI1Qk5DZW12N3YxRDRPM1hIUmhmNW5ncHN1cVhvbW1hYlN3Sy96MTc1?=
 =?utf-8?B?a1p6QW96VGJVWDlmRkg2dEVNRWxWM1BzdWxvWm5OekZmYmoyZkZUazhSNHZX?=
 =?utf-8?B?S0lPT0tKMGNXeEcxSmpEcW5nVWQvV0dtcUtrNHJOZGlzSWR5dFlhY3IxR2NL?=
 =?utf-8?B?TUxXamlnZDRjMWpwamFnS2trZmVqeEkwNVFYL2NWaExxamRrZzVQYzNzL0Iw?=
 =?utf-8?B?cEN3elQ5R1RRQ0oyNjJFNzR0TnFMaVdzWTdDWkppYUd4YTRVQnRnRHhMajRL?=
 =?utf-8?B?c0RVR1d3VmZzeGFmSlJzem1jc29tNWEweXpJRGVIN2pBQ0ZCZDE3Q0hPQ0tV?=
 =?utf-8?B?cDhZN3hCeDJjcUM5Mkc5ZHBIOTNGOU1CZzh4Qjg5M3h4Z1VkeDkxWXFkTldm?=
 =?utf-8?B?cE1rWHY0endBR0gwZkoxV3BUcTN4dGtMaVYyaEdHd0N3bjdZSnBCc0NNTGlV?=
 =?utf-8?B?SnAxWUIrT09pZ0VENDFrRTBWcStNbjQ2UG53T2NlTkIrdGdscjRwZmgyWC9n?=
 =?utf-8?B?bmJSbWFoK3FXUVhxMjZKdEJhZWdhTis0cUdCaUt5dVRPTWl0eldKNkw3QU12?=
 =?utf-8?B?aGpQcFdFTDhBVzdzL1ZlbU9RMkx2MDNXQ21MWWlWSmw0cHp0SjQ0cXRTR2Ix?=
 =?utf-8?B?NWUwRWRvRDc5Z2I0QXgxQ1l3NVMydFBOUjcxQ1cySmJqN2d5Zm9VQ0w4Z3Rl?=
 =?utf-8?B?K2lRbk9ka2tFd2d6RlRCOVJEN1ZIRWJnUzY5dDFFTDZZZG5razFkb2E5NTYr?=
 =?utf-8?B?RVdTVzE3YU9JczJYTHNSSDV2aWo0MU16TkVWWjZGQmxxTm9RR2lnemFXN1Rx?=
 =?utf-8?B?K05GbE04NGJvc1o2ZFZQQTgzbXdVVnh2OURRWi9XdkRDSTFCaGlGcVBTK1ha?=
 =?utf-8?B?MTBjUW4wbGNGb0FoejBPR3JtdnVPd2pITGkyeU80S3RvT1VUeWMvbENkVTVs?=
 =?utf-8?B?WHFRUjJ6eEZPTVp6Q1c1dkJjTG1hR3NzbWsvcnUzMm9lZ0haY3J0MStYV1l0?=
 =?utf-8?B?SjN1M3llc0FOaUNxWHBFRVgyQkFzWStSTTFJa3hkZ1F3cDJOTElvVEpFT1Zv?=
 =?utf-8?B?MVV6YVUzMDNWNUR6UXdreEMzNS90blBaMW5vck1kRXZmWE1sQlpvU0Z3Y1lJ?=
 =?utf-8?B?UlE1UHhDc05yNzZrS29iWVJiVTRHQ3B5WGNONVBpRmtHSkhuOGt1UVRRZ0kx?=
 =?utf-8?B?Tjhjdms1SjhlWVlON25iRTdCNnVJaDNDSEdQaFRQSDZoZ1R0dC93YVJvcDF2?=
 =?utf-8?B?eHFSdE96L0pIbW1oQnhNWTkwT2pkdjAvRFc2QkZLOGVhbVhMNVRxYzlQR1lL?=
 =?utf-8?B?RW1MZ21yMWtISzZobUk0c1FPUHFkNVdOZW4rK1daTzFoTEVDYitGb0grR1FP?=
 =?utf-8?B?VUh3dTB5eTNQR0pFWWIxYkVuakVMUUtjamR6ejV5VitlZFB5NDdYWHc1Vkk4?=
 =?utf-8?B?Nno1eFlWZjR2dDRSR1p4WGtDMU5PRzQ0V1FwalRmZThEOWNJVGpEWmgwamtC?=
 =?utf-8?B?TnR6dmsyWENqbGY0b29ZOVVjZnhacTgwNHZYUGMxK1l1RUZzaEVxaHg0VVl5?=
 =?utf-8?B?RlF6Z3RLUGFXTk9NZUU1VjFZT2oyelJLMlRuNXhoUGxqUW9iMVBBWEgyTExM?=
 =?utf-8?B?SUxVaWxjb0J5T0FlT051MUlQRllLUmMzR3R3RHdRVzdFaG53Tmp4UjBXdXRp?=
 =?utf-8?B?Ly92YUNLdlZTNG5sc2Q4a1lId0lFeHR5Tlg4MjQ5RnBIaDEyeUhYTHJ6VXow?=
 =?utf-8?B?WFlmWEVFRWxaRXJtSWZpNkV1RnVWMEE3dllFN28yZVJyQXpaTW9neElseXQx?=
 =?utf-8?Q?by5XzJJZ40C+QAlW4Nsxmpz1k?=
X-OriginatorOrg: amd.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 945ea2cf-d864-4ff1-4f32-08dc34bb8927
X-MS-Exchange-CrossTenant-AuthSource: BL1PR12MB5732.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Feb 2024 22:05:24.5396
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: ufhyvtMTRAofhFwn/jdG1adCYA6RLz7EDMwPxOSVgzLa3O+WdQb8Bhs1STZPj1ckq5DTcWuTenIhRk1drdUJNg==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR12MB4496

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();