On Mon, Jan 22, 2024 at 08:00:03AM -0800, mhkelley58@xxxxxxxxx wrote: > 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 boot using UEFI. Existing EFI code in Using -> use, I think. > 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. I think is a good idea. > > 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://download.microsoft.com/download/1/c/9/1c9813b8-089c-4fef-b2ad-ad80e79403ba/Whitepaper%20-%20The%20Windows%2010%20random%20number%20generation%20infrastructure.pdf > > Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx> > --- > arch/x86/kernel/cpu/mshyperv.c | 1 + > drivers/hv/hv_common.c | 62 ++++++++++++++++++++++++++++++++++ > include/asm-generic/mshyperv.h | 2 ++ > 3 files changed, 65 insertions(+) > > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c > index e6bba12c759c..c202a60ecc6c 100644 > --- a/arch/x86/kernel/cpu/mshyperv.c > +++ b/arch/x86/kernel/cpu/mshyperv.c > @@ -640,6 +640,7 @@ 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 ccad7bca3fd3..ebae19b708b4 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> > @@ -348,6 +350,66 @@ 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) { > + pr_info("Hyper-V: ACPI table OEM0 not found\n"); I would like this to be a pr_debug() instead of pr_info(), considering using the negative case may cause users to think not having this table can be problematic. Alternatively, we can remove this message here, and then ... > + return; > + } > + ... add a pr_debug() here to indicate that the table was found. pr_info("Hyper-V: Seeding randomness with data from ACPI table OEM0\n"); Dexuan, Saurabh, Haiyang and Long, can you give an ack or nack to this patch and help test it? Thanks, Wei. > + /* > + * 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); > + 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 >
