RE: [PATCH v2 1/1] x86/hyperv: Use Hyper-V entropy to seed guest random number generator

[Long Li]

> 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
>