From: mhkelley58@gmail.com
To: haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com,
tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
dave.hansen@linux.intel.com, hpa@zytor.com, arnd@arndb.de,
tytso@mit.edu, Jason@zx2c4.com, x86@kernel.org,
linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
linux-arch@vger.kernel.org
Subject: [PATCH 1/1] x86/hyperv: Use Hyper-V entropy to seed guest random number generator
Date: Mon, 22 Jan 2024 08:00:03 -0800 [thread overview]
Message-ID: <20240122160003.348521-1-mhklinux@outlook.com> (raw)
From: Michael Kelley <mhklinux@outlook.com>
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
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://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@outlook.com>
---
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");
+ 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);
+ 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
next reply other threads:[~2024-01-22 16:00 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-22 16:00 mhkelley58 [this message]
2024-03-04 6:57 ` [PATCH 1/1] x86/hyperv: Use Hyper-V entropy to seed guest random number generator Wei Liu
2024-03-06 17:43 ` Michael Kelley
2024-03-08 23:53 ` Wei Liu
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