> Subject: [PATCH v2 1/1] x86/hyperv: Use Hyper-V entropy to seed guest random > number generator > > [You don't often get email from mhkelley58@xxxxxxxxx. Learn why this is > important at https://aka.ms/LearnAboutSenderIdentification ] > > From: Michael Kelley <mhklinux@xxxxxxxxxxx> > > A Hyper-V host provides its guest VMs with entropy in a custom ACPI table > named "OEM0". The entropy bits are updated each time Hyper-V boots the VM, > and are suitable for seeding the Linux guest random number generator (rng). See > a brief description of OEM0 in [1]. > > Generation 2 VMs on Hyper-V use UEFI to boot. Existing EFI code in Linux seeds > the rng with entropy bits from the EFI_RNG_PROTOCOL. > Via this path, the rng is seeded very early during boot with good entropy. The ACPI > OEM0 table is still provided in such VMs, though it isn't needed. > > But Generation 1 VMs on Hyper-V boot from BIOS. For these VMs, Linux doesn't > currently get any entropy from the Hyper-V host. While this is not fundamentally > broken because Linux can generate its own entropy, using the Hyper-V host > provided entropy would get the rng off to a better start and would do so earlier in > the boot process. > > Improve the rng seeding for Generation 1 VMs by having Hyper-V specific code in > Linux take advantage of the OEM0 table to seed the rng. Because the OEM0 > table is custom to Hyper-V, parse it directly in the Hyper-V code in the Linux > kernel and use add_bootloader_randomness() to seed the rng. Once the entropy > bits are read from OEM0, zero them out in the table so they don't appear in > /sys/firmware/acpi/tables/OEM0 in the running VM. > > An equivalent change is *not* made for Linux VMs on Hyper-V for ARM64. Such > VMs are always Generation 2 and the rng is seeded with entropy obtained via the > EFI_RNG_PROTOCOL as described above. > > [1] > https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdownload > .microsoft.com%2Fdownload%2F1%2Fc%2F9%2F1c9813b8-089c-4fef-b2ad- > ad80e79403ba%2FWhitepaper%2520- > %2520The%2520Windows%252010%2520random%2520number%2520generatio > n%2520infrastructure.pdf&data=05%7C02%7Clongli%40microsoft.com%7C9ecf1 > 997333f4461292108dc3ed7371b%7C72f988bf86f141af91ab2d7cd011db47%7C1 > %7C0%7C638454341537898325%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4 > wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C > %7C&sdata=jluLa8BFJ0wxQ1m0jcuKQO9t%2FdFVFSsfHiiSLoJviAo%3D&reserved= > 0 > > Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx> Reviewed-by: Long Li <longli@xxxxxxxxxxxxx> > --- > Changes in v2: > * Tweaked commit message [Wei Liu] > * Removed message when OEM0 table isn't found. Added debug-level > message when OEM0 is successfully used to add randomness. [Wei Liu] > > arch/x86/kernel/cpu/mshyperv.c | 1 + > drivers/hv/hv_common.c | 64 ++++++++++++++++++++++++++++++++++ > include/asm-generic/mshyperv.h | 2 ++ > 3 files changed, 67 insertions(+) > > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c > index 303fef824167..65c9cbdd2282 100644 > --- a/arch/x86/kernel/cpu/mshyperv.c > +++ b/arch/x86/kernel/cpu/mshyperv.c > @@ -648,6 +648,7 @@ const __initconst struct hypervisor_x86 > x86_hyper_ms_hyperv = { > .init.x2apic_available = ms_hyperv_x2apic_available, > .init.msi_ext_dest_id = ms_hyperv_msi_ext_dest_id, > .init.init_platform = ms_hyperv_init_platform, > + .init.guest_late_init = ms_hyperv_late_init, > #ifdef CONFIG_AMD_MEM_ENCRYPT > .runtime.sev_es_hcall_prepare = hv_sev_es_hcall_prepare, > .runtime.sev_es_hcall_finish = hv_sev_es_hcall_finish, diff --git > a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c index > 0285a74363b3..219c4371314d 100644 > --- a/drivers/hv/hv_common.c > +++ b/drivers/hv/hv_common.c > @@ -20,6 +20,8 @@ > #include <linux/sched/task_stack.h> > #include <linux/panic_notifier.h> > #include <linux/ptrace.h> > +#include <linux/random.h> > +#include <linux/efi.h> > #include <linux/kdebug.h> > #include <linux/kmsg_dump.h> > #include <linux/slab.h> > @@ -347,6 +349,68 @@ int __init hv_common_init(void) > return 0; > } > > +void __init ms_hyperv_late_init(void) > +{ > + struct acpi_table_header *header; > + acpi_status status; > + u8 *randomdata; > + u32 length, i; > + > + /* > + * Seed the Linux random number generator with entropy provided by > + * the Hyper-V host in ACPI table OEM0. It would be nice to do this > + * even earlier in ms_hyperv_init_platform(), but the ACPI subsystem > + * isn't set up at that point. Skip if booted via EFI as generic EFI > + * code has already done some seeding using the EFI RNG protocol. > + */ > + if (!IS_ENABLED(CONFIG_ACPI) || efi_enabled(EFI_BOOT)) > + return; > + > + status = acpi_get_table("OEM0", 0, &header); > + if (ACPI_FAILURE(status) || !header) > + return; > + > + /* > + * Since the "OEM0" table name is for OEM specific usage, verify > + * that what we're seeing purports to be from Microsoft. > + */ > + if (strncmp(header->oem_table_id, "MICROSFT", 8)) > + goto error; > + > + /* > + * Ensure the length is reasonable. Requiring at least 32 bytes and > + * no more than 256 bytes is somewhat arbitrary. Hyper-V currently > + * provides 64 bytes, but allow for a change in a later version. > + */ > + if (header->length < sizeof(*header) + 32 || > + header->length > sizeof(*header) + 256) > + goto error; > + > + length = header->length - sizeof(*header); > + randomdata = (u8 *)(header + 1); > + > + pr_debug("Hyper-V: Seeding rng with %d random bytes from ACPI table > OEM0\n", > + length); > + > + add_bootloader_randomness(randomdata, length); > + > + /* > + * To prevent the seed data from being visible in /sys/firmware/acpi, > + * zero out the random data in the ACPI table and fixup the checksum. > + */ > + for (i = 0; i < length; i++) { > + header->checksum += randomdata[i]; > + randomdata[i] = 0; > + } > + > + acpi_put_table(header); > + return; > + > +error: > + pr_info("Hyper-V: Ignoring malformed ACPI table OEM0\n"); > + acpi_put_table(header); > +} > + > /* > * Hyper-V specific initialization and die code for > * individual CPUs that is common across all architectures. > diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h > index 430f0ae0dde2..e861223093df 100644 > --- a/include/asm-generic/mshyperv.h > +++ b/include/asm-generic/mshyperv.h > @@ -193,6 +193,7 @@ extern u64 (*hv_read_reference_counter)(void); > > int __init hv_common_init(void); > void __init hv_common_free(void); > +void __init ms_hyperv_late_init(void); > int hv_common_cpu_init(unsigned int cpu); int hv_common_cpu_die(unsigned > int cpu); > > @@ -290,6 +291,7 @@ void hv_setup_dma_ops(struct device *dev, bool > coherent); static inline bool hv_is_hyperv_initialized(void) { return false; } static > inline bool hv_is_hibernation_supported(void) { return false; } static inline void > hyperv_cleanup(void) {} > +static inline void ms_hyperv_late_init(void) {} > static inline bool hv_is_isolation_supported(void) { return false; } static inline > enum hv_isolation_type hv_get_isolation_type(void) { > -- > 2.25.1 >
