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Subject: [PATCH v2 1/1] x86/hyperv: Use Hyper-V entropy to seed guest random number generator
Date: Thu,  7 Mar 2024 10:48:20 -0800
Message-Id: <20240307184820.70589-1-mhklinux@outlook.com>
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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 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://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>
---
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