[PATCHv9 07/14] efi/x86: Implement support for unaccepted memory

Kirill A. Shutemov
2023-03-30T11:50:31Z

UEFI Specification version 2.9 introduces the concept of memory
acceptance: Some Virtual Machine platforms, such as Intel TDX or AMD
SEV-SNP, requiring memory to be accepted before it can be used by the
guest. Accepting happens via a protocol specific for the Virtual
Machine platform.

Accepting memory is costly and it makes VMM allocate memory for the
accepted guest physical address range. It's better to postpone memory
acceptance until memory is needed. It lowers boot time and reduces
memory overhead.

The kernel needs to know what memory has been accepted. Firmware
communicates this information via memory map: a new memory type --
EFI_UNACCEPTED_MEMORY -- indicates such memory.

Range-based tracking works fine for firmware, but it gets bulky for
the kernel: e820 has to be modified on every page acceptance. It leads
to table fragmentation, but there's a limited number of entries in the
e820 table

Another option is to mark such memory as usable in e820 and track if the
range has been accepted in a bitmap. One bit in the bitmap represents
2MiB in the address space: one 4k page is enough to track 64GiB or
physical address space.

In the worst-case scenario -- a huge hole in the middle of the
address space -- It needs 256MiB to handle 4PiB of the address
space.

Any unaccepted memory that is not aligned to 2M gets accepted upfront.

The bitmap is allocated and constructed in the EFI stub and passed down
to the kernel via boot_params. allocate_e820() allocates the bitmap if
unaccepted memory is present, according to the maximum address in the
memory map.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
Documentation/x86/zero-page.rst | 1 +
arch/x86/boot/compressed/Makefile | 1 +
arch/x86/boot/compressed/mem.c | 73 ++++++++++++++++++++++++
arch/x86/include/asm/unaccepted_memory.h | 10 ++++
arch/x86/include/uapi/asm/bootparam.h | 2 +-
drivers/firmware/efi/Kconfig | 14 +++++
drivers/firmware/efi/efi.c | 1 +
drivers/firmware/efi/libstub/x86-stub.c | 65 +++++++++++++++++++++
include/linux/efi.h | 3 +-
9 files changed, 168 insertions(+), 2 deletions(-)
create mode 100644 arch/x86/boot/compressed/mem.c
create mode 100644 arch/x86/include/asm/unaccepted_memory.h

diff --git a/Documentation/x86/zero-page.rst b/Documentation/x86/zero-page.rst
index 45aa9cceb4f1..f21905e61ade 100644
--- a/Documentation/x86/zero-page.rst
+++ b/Documentation/x86/zero-page.rst
@@ -20,6 +20,7 @@ Offset/Size Proto Name Meaning
060/010 ALL ist_info Intel SpeedStep (IST) BIOS support information
(struct ist_info)
070/008 ALL acpi_rsdp_addr Physical address of ACPI RSDP table
+078/008 ALL unaccepted_memory Bitmap of unaccepted memory (1bit == 2M)
080/010 ALL hd0_info hd0 disk parameter, OBSOLETE!!
090/010 ALL hd1_info hd1 disk parameter, OBSOLETE!!
0A0/010 ALL sys_desc_table System description table (struct sys_desc_table),
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 6b6cfe607bdb..f62c02348f9a 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -107,6 +107,7 @@ endif

vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
vmlinux-objs-$(CONFIG_INTEL_TDX_GUEST) += $(obj)/tdx.o $(obj)/tdcall.o
+vmlinux-objs-$(CONFIG_UNACCEPTED_MEMORY) += $(obj)/bitmap.o $(obj)/mem.o

vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_mixed.o
diff --git a/arch/x86/boot/compressed/mem.c b/arch/x86/boot/compressed/mem.c
new file mode 100644
index 000000000000..6b15a0ed8b54
--- /dev/null
+++ b/arch/x86/boot/compressed/mem.c
@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include "../cpuflags.h"
+#include "bitmap.h"
+#include "error.h"
+#include "math.h"
+
+#define PMD_SHIFT 21
+#define PMD_SIZE (_AC(1, UL) << PMD_SHIFT)
+#define PMD_MASK (~(PMD_SIZE - 1))
+
+static inline void __accept_memory(phys_addr_t start, phys_addr_t end)
+{
+ /* Platform-specific memory-acceptance call goes here */
+ error("Cannot accept memory");
+}
+
+/*
+ * The accepted memory bitmap only works at PMD_SIZE granularity. Take
+ * unaligned start/end addresses and either:
+ * 1. Accepts the memory immediately and in its entirety
+ * 2. Accepts unaligned parts, and marks *some* aligned part unaccepted
+ *
+ * The function will never reach the bitmap_set() with zero bits to set.
+ */
+void process_unaccepted_memory(struct boot_params *params, u64 start, u64 end)
+{
+ /*
+ * Ensure that at least one bit will be set in the bitmap by
+ * immediately accepting all regions under 2*PMD_SIZE. This is
+ * imprecise and may immediately accept some areas that could
+ * have been represented in the bitmap. But, results in simpler
+ * code below
+ *
+ * Consider case like this:
+ *
+ * | 4k | 2044k | 2048k |
+ * ^ 0x0 ^ 2MB ^ 4MB
+ *
+ * Only the first 4k has been accepted. The 0MB->2MB region can not be
+ * represented in the bitmap. The 2MB->4MB region can be represented in
+ * the bitmap. But, the 0MB->4MB region is <2*PMD_SIZE and will be
+ * immediately accepted in its entirety.
+ */
+ if (end - start < 2 * PMD_SIZE) {
+ __accept_memory(start, end);
+ return;
+ }
+
+ /*
+ * No matter how the start and end are aligned, at least one unaccepted
+ * PMD_SIZE area will remain to be marked in the bitmap.
+ */
+
+ /* Immediately accept a <PMD_SIZE piece at the start: */
+ if (start & ~PMD_MASK) {
+ __accept_memory(start, round_up(start, PMD_SIZE));
+ start = round_up(start, PMD_SIZE);
+ }
+
+ /* Immediately accept a <PMD_SIZE piece at the end: */
+ if (end & ~PMD_MASK) {
+ __accept_memory(round_down(end, PMD_SIZE), end);
+ end = round_down(end, PMD_SIZE);
+ }
+
+ /*
+ * 'start' and 'end' are now both PMD-aligned.
+ * Record the range as being unaccepted:
+ */
+ bitmap_set((unsigned long *)params->unaccepted_memory,
+ start / PMD_SIZE, (end - start) / PMD_SIZE);
+}
diff --git a/arch/x86/include/asm/unaccepted_memory.h b/arch/x86/include/asm/unaccepted_memory.h
new file mode 100644
index 000000000000..df0736d32858
--- /dev/null
+++ b/arch/x86/include/asm/unaccepted_memory.h
@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (C) 2020 Intel Corporation */
+#ifndef _ASM_X86_UNACCEPTED_MEMORY_H
+#define _ASM_X86_UNACCEPTED_MEMORY_H
+
+struct boot_params;
+
+void process_unaccepted_memory(struct boot_params *params, u64 start, u64 num);
+
+#endif
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 01d19fc22346..630a54046af0 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -189,7 +189,7 @@ struct boot_params {
__u64 tboot_addr; /* 0x058 */
struct ist_info ist_info; /* 0x060 */
__u64 acpi_rsdp_addr; /* 0x070 */
- __u8 _pad3[8]; /* 0x078 */
+ __u64 unaccepted_memory; /* 0x078 */
__u8 hd0_info[16]; /* obsolete! */ /* 0x080 */
__u8 hd1_info[16]; /* obsolete! */ /* 0x090 */
struct sys_desc_table sys_desc_table; /* obsolete! */ /* 0x0a0 */
diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
index 043ca31c114e..231f1c70d1db 100644
--- a/drivers/firmware/efi/Kconfig
+++ b/drivers/firmware/efi/Kconfig
@@ -269,6 +269,20 @@ config EFI_COCO_SECRET
virt/coco/efi_secret module to access the secrets, which in turn
allows userspace programs to access the injected secrets.

+config UNACCEPTED_MEMORY
+ bool
+ depends on EFI_STUB
+ help
+ Some Virtual Machine platforms, such as Intel TDX, require
+ some memory to be "accepted" by the guest before it can be used.
+ This mechanism helps prevent malicious hosts from making changes
+ to guest memory.
+
+ UEFI specification v2.9 introduced EFI_UNACCEPTED_MEMORY memory type.
+
+ This option adds support for unaccepted memory and makes such memory
+ usable by the kernel.
+
config EFI_EMBEDDED_FIRMWARE
bool
select CRYPTO_LIB_SHA256
diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index abeff7dc0b58..7dce06e419c5 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -843,6 +843,7 @@ static __initdata char memory_type_name[][13] = {
"MMIO Port",
"PAL Code",
"Persistent",
+ "Unaccepted",
};

char * __init efi_md_typeattr_format(char *buf, size_t size,
diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index fff81843169c..1643ddbde249 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -15,6 +15,7 @@
#include <asm/setup.h>
#include <asm/desc.h>
#include <asm/boot.h>
+#include <asm/unaccepted_memory.h>

#include "efistub.h"

@@ -613,6 +614,16 @@ setup_e820(struct boot_params *params, struct setup_data *e820ext, u32 e820ext_s
e820_type = E820_TYPE_PMEM;
break;

+ case EFI_UNACCEPTED_MEMORY:
+ if (!IS_ENABLED(CONFIG_UNACCEPTED_MEMORY)) {
+ efi_warn_once(
+"The system has unaccepted memory, but kernel does not support it\nConsider enabling CONFIG_UNACCEPTED_MEMORY\n");
+ continue;
+ }
+ e820_type = E820_TYPE_RAM;
+ process_unaccepted_memory(params, d->phys_addr,
+ d->phys_addr + PAGE_SIZE * d->num_pages);
+ break;
default:
continue;
}
@@ -677,6 +688,57 @@ static efi_status_t alloc_e820ext(u32 nr_desc, struct setup_data **e820ext,
return status;
}

+static efi_status_t allocate_unaccepted_bitmap(struct boot_params *params,
+ __u32 nr_desc,
+ struct efi_boot_memmap *map)
+{
+ unsigned long *mem = NULL;
+ u64 size, max_addr = 0;
+ efi_status_t status;
+ bool found = false;
+ int i;
+
+ /* Check if there's any unaccepted memory and find the max address */
+ for (i = 0; i < nr_desc; i++) {
+ efi_memory_desc_t *d;
+ unsigned long m = (unsigned long)map->map;
+
+ d = efi_early_memdesc_ptr(m, map->desc_size, i);
+ if (d->type == EFI_UNACCEPTED_MEMORY)
+ found = true;
+ if (d->phys_addr + d->num_pages * PAGE_SIZE > max_addr)
+ max_addr = d->phys_addr + d->num_pages * PAGE_SIZE;
+ }
+
+ if (!found) {
+ params->unaccepted_memory = 0;
+ return EFI_SUCCESS;
+ }
+
+ /*
+ * If unaccepted memory is present, allocate a bitmap to track what
+ * memory has to be accepted before access.
+ *
+ * One bit in the bitmap represents 2MiB in the address space:
+ * A 4k bitmap can track 64GiB of physical address space.
+ *
+ * In the worst case scenario -- a huge hole in the middle of the
+ * address space -- It needs 256MiB to handle 4PiB of the address
+ * space.
+ *
+ * The bitmap will be populated in setup_e820() according to the memory
+ * map after efi_exit_boot_services().
+ */
+ size = DIV_ROUND_UP(max_addr, PMD_SIZE * BITS_PER_BYTE);
+ status = efi_allocate_pages(size, (unsigned long *)&mem, ULONG_MAX);
+ if (status == EFI_SUCCESS) {
+ memset(mem, 0, size);
+ params->unaccepted_memory = (unsigned long)mem;
+ }
+
+ return status;
+}
+
static efi_status_t allocate_e820(struct boot_params *params,
struct setup_data **e820ext,
u32 *e820ext_size)
@@ -697,6 +759,9 @@ static efi_status_t allocate_e820(struct boot_params *params,
status = alloc_e820ext(nr_e820ext, e820ext, e820ext_size);
}

+ if (IS_ENABLED(CONFIG_UNACCEPTED_MEMORY) && status == EFI_SUCCESS)
+ status = allocate_unaccepted_bitmap(params, nr_desc, map);
+
efi_bs_call(free_pool, map);
return status;
}
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 04a733f0ba95..1d4f0343c710 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -108,7 +108,8 @@ typedef struct {
#define EFI_MEMORY_MAPPED_IO_PORT_SPACE 12
#define EFI_PAL_CODE 13
#define EFI_PERSISTENT_MEMORY 14
-#define EFI_MAX_MEMORY_TYPE 15
+#define EFI_UNACCEPTED_MEMORY 15
+#define EFI_MAX_MEMORY_TYPE 16

/* Attribute values: */
#define EFI_MEMORY_UC ((u64)0x0000000000000001ULL) /* uncached */
--
2.39.2