[RFC PATCH] arm64/acpi: disallow AML memory opregions to access kernel memory

[Ard Biesheuvel]

ACPI provides support for SystemMemory opregions, to allow AML methods
to access MMIO registers of, e.g., GPIO controllers, or access reserved
regions of memory that are owned by the firmware.

Currently, we also permit AML methods to access memory that is owned by
the kernel and mapped via the linear region, which does not seem to be
supported by a valid use case, and exposes the kernel's internal state
to AML methods that may be buggy and exploitable.

So close the door on this, and simply reject AML remapping requests for
any memory that has a valid mapping in the linear region.

Reported-by: Jason A. Donenfeld <Jason at zx2c4.com>
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/include/asm/acpi.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/acpi.h b/arch/arm64/include/asm/acpi.h
index a45366c3909b..18dcef4e6764 100644
--- a/arch/arm64/include/asm/acpi.h
+++ b/arch/arm64/include/asm/acpi.h
@@ -50,9 +50,9 @@ pgprot_t __acpi_get_mem_attribute(phys_addr_t addr);
 static inline void __iomem *acpi_os_ioremap(acpi_physical_address phys,
 					    acpi_size size)
 {
-	/* For normal memory we already have a cacheable mapping. */
+	/* Don't allow access to kernel memory from AML code */
 	if (memblock_is_map_memory(phys))
-		return (void __iomem *)__phys_to_virt(phys);
+		return NULL;
 
 	/*
 	 * We should still honor the memory's attribute here because
-- 
2.27.0