Messages in this thread Patch in this message | | | From | "Reshetova, Elena" <> | Subject | RE: [PATCH v3] x86/coco: Require seeding RNG with RDRAND on CoCo systems | Date | Wed, 21 Feb 2024 14:34:57 +0000 |
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> 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: 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>
Reviewed-by: Elena Reshetova <elena.reshetova@intel.com>
> --- > Changes v2->v3: > - Remove patch that handled generic RDRAND failures, because that > doesn't really have any implication for the RNG, since it's supposed > to run fine on systems without RDRAND anyway, and CoCo is a weird > special case. If people still want an extra generic RDRAND failure > handler, that's standalone anyway, so we can do that disconnected from > this patch. No need to make it a series. > - Update comments and commit message to reflect this. > > Changes v1->v2: > - panic() instead of BUG_ON(), as suggested by Andi Kleen. > - Update comments, now that we have info from AMD and Intel. > > 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..0a5d59966d6d 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) > + 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(); > -- > 2.43.0 >
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