[PATCH] arm64: permit ACPI core to map kernel memory used for table overrides

Ard Biesheuvel ardb at kernel.org
Tue Sep 29 09:25:22 EDT 2020

Jonathan reports that the strict policy for memory mapped by the
ACPI core breaks the use case of passing ACPI table overrides via
initramfs. This is due to the fact that the memory type used for
loading the initramfs in memory is not recognized as a memory type
that is typically used by firmware to pass firmware tables.

Since the purpose of the strict policy is to ensure that no AML or
other ACPI code can manipulate any memory that is used by the kernel
to keep its internal state or the state of user tasks, we can relax
the permission check, and allow mappings of memory that is reserved
and marked as NOMAP via memblock, and therefore not covered by the
linear mapping to begin with.

Cc: Sudeep Holla <sudeep.holla at arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
Fixes: 1583052d111f ("arm64/acpi: disallow AML memory opregions to access kernel memory")
Fixes: 325f5585ec36 ("arm64/acpi: disallow writeable AML opregion mapping for EFI code regions")
Reported-by: Jonathan Cameron <Jonathan.Cameron at huawei.com>
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
 arch/arm64/kernel/acpi.c | 22 ++++++++++++++++++--
 include/linux/acpi.h     |  2 +-
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
index a85174d05473..cada0b816c8a 100644
--- a/arch/arm64/kernel/acpi.c
+++ b/arch/arm64/kernel/acpi.c
@@ -298,8 +298,21 @@ void __iomem *acpi_os_ioremap(acpi_physical_address phys, acpi_size size)
-			pr_warn(FW_BUG "requested region covers kernel memory @ %pa\n", &phys);
-			return NULL;
+			if (memblock_is_map_memory(phys) ||
+			    !memblock_is_region_memory(phys, size)) {
+				pr_warn(FW_BUG "requested region covers kernel memory @ %pa\n", &phys);
+				return NULL;
+			}
+			/*
+			 * Mapping kernel memory is permitted if the region in
+			 * question is covered by a single memblock with the
+			 * NOMAP attribute set: this enables the use of ACPI
+			 * table overrides passed via initramfs, which are
+			 * reserved in memory using arch_reserve_mem_area()
+			 * below. As this particular use case only requires
+			 * read access, fall through to the R/O mapping case.
+			 */
+			fallthrough;
@@ -388,3 +401,8 @@ int apei_claim_sea(struct pt_regs *regs)
 	return err;
+void arch_reserve_mem_area(acpi_physical_address addr, size_t size)
+	memblock_mark_nomap(addr, size);
diff --git a/include/linux/acpi.h b/include/linux/acpi.h
index 1e4cdc6c7ae2..64ae25c59d55 100644
--- a/include/linux/acpi.h
+++ b/include/linux/acpi.h
@@ -958,7 +958,7 @@ void acpi_os_set_prepare_extended_sleep(int (*func)(u8 sleep_state,
 acpi_status acpi_os_prepare_extended_sleep(u8 sleep_state,
 					   u32 val_a, u32 val_b);
-#ifdef CONFIG_X86
+#ifndef CONFIG_IA64
 void arch_reserve_mem_area(acpi_physical_address addr, size_t size);
 static inline void arch_reserve_mem_area(acpi_physical_address addr,

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