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: Tom Lendacky thomas.lendacky@amd.com 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 v4->v5: - Use `cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)` instead of checking cc_vendor, per Tom's recommendation.
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..b419a29ef0e5 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_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) + 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();