On 3/18/2024 8:54 AM, 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 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 provided in such VMs is an additional > source of entropy. > > 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. For > Generation 2 VMs, use the OEM0 table to provide additional entropy > beyond the EFI_RNG_PROTOCOL. 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 add it to 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. The zero'ing is done out of an abundance of caution to avoid > potential security risks to the rng. Also set the OEM0 data length > to zero so a kexec or other subsequent use of the table won't try > to use the zero'ed bits. > > [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> > --- > Changes in v3: > * Removed restriction to just Generation 1 VMs. Generation 2 VMs > now also use the additional entropy even though they also get > initial entropy via EFI_RNG_PROTOCOL [Jason Donenfeld] > * Process the OEM0 table on ARM64 systems in addition to x86/x64, > as a result of no longer excluding Generation 2 VM. > * Enlarge the range of entropy byte counts that are considered valid > in the OEM0 table. New range is 8 to 4K; previously the range was > 32 to 256. [Jason Donenfeld] > * After processing the entropy bits in OEM0, also set the OEM0 > table length to indicate that the entropy byte count is zero, > to prevent a subsequent kexec or other use of the table from > trying to use the zero'ed bits. [Jason Donenfeld] > > 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/arm64/hyperv/mshyperv.c | 2 + > arch/x86/kernel/cpu/mshyperv.c | 1 + > drivers/hv/hv_common.c | 69 ++++++++++++++++++++++++++++++++++ > include/asm-generic/mshyperv.h | 2 + > 4 files changed, 74 insertions(+) > > diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c > index f1b8a04ee9f2..c8193cec1b90 100644 > --- a/arch/arm64/hyperv/mshyperv.c > +++ b/arch/arm64/hyperv/mshyperv.c > @@ -74,6 +74,8 @@ static int __init hyperv_init(void) > return ret; > } > > + ms_hyperv_late_init(); > + > hyperv_initialized = true; > return 0; > } > 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..724de94d885f 100644 > --- a/drivers/hv/hv_common.c > +++ b/drivers/hv/hv_common.c > @@ -20,8 +20,11 @@ > #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/sizes.h> > #include <linux/slab.h> > #include <linux/dma-map-ops.h> > #include <linux/set_memory.h> > @@ -347,6 +350,72 @@ 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. > + */ > + if (!IS_ENABLED(CONFIG_ACPI)) > + 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 8 bytes and > + * no more than 4K bytes is somewhat arbitrary and just protects > + * against a malformed table. Hyper-V currently provides 64 bytes, > + * but allow for a change in a later version. > + */ > + if (header->length < sizeof(*header) + 8 || > + header->length > sizeof(*header) + SZ_4K> + 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. > + * The zero'ing is done out of an abundance of caution in avoiding > + * potential security risks to the rng. Similarly, reset the table > + * length to just the header size so that a subsequent kexec doesn't > + * try to use the zero'ed out random data. > + */ > + for (i = 0; i < length; i++) { > + header->checksum += randomdata[i]; > + randomdata[i] = 0; > + } > + > + for (i = 0; i < sizeof(header->length); i++) > + header->checksum += ((u8 *)&header->length)[i]; > + header->length = sizeof(*header); > + for (i = 0; i < sizeof(header->length); i++) > + header->checksum -= ((u8 *)&header->length)[i]; > + > +error: > + 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) > { This patch looks good to me. The code comments were very helpful in explaining what is going on. Nuno
