[PATCH] arm64: mm: account for hotplug memory when randomizing the linear region

[Ard Biesheuvel]

As a hardening measure, we currently randomize the placement of
physical memory inside the linear region when KASLR is in effect.
Since the random offset at which to place the available physical
memory inside the linear region is chosen early at boot, it is
based on the memblock description of memory, which does not cover
hotplug memory. The consequence of this is that the randomization
offset may be chosen such that any hotplugged memory located above
memblock_end_of_DRAM() that appears later is pushed off the end of
the linear region, where it cannot be accessed.

So let's limit this randomization of the linear region to ensure
that this can no longer happen, by using the CPU's addressable PA
range instead. As it is guaranteed that no hotpluggable memory will
appear that falls outside of that range, we can safely put this PA
range sized window anywhere in the linear region.

Cc: Anshuman Khandual <anshuman.khandual at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: Steven Price <steven.price at arm.com>
Cc: Robin Murphy <robin.murphy at arm.com>
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
Related to discussion here:
https://lore.kernel.org/linux-arm-kernel/1600332402-30123-1-git-send-email-anshuman.khandual@arm.com/

 arch/arm64/mm/init.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 0b393c275be0..af1b4ed2daa8 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -350,13 +350,16 @@ void __init arm64_memblock_init(void)
 
 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
 		extern u16 memstart_offset_seed;
-		u64 range = linear_region_size -
-			    (memblock_end_of_DRAM() - memblock_start_of_DRAM());
+		u64 mmfr0 = read_cpuid(ID_AA64MMFR0_EL1);
+		int parange = cpuid_feature_extract_unsigned_field(
+					mmfr0, ID_AA64MMFR0_PARANGE_SHIFT);
+		s64 range = linear_region_size -
+			    BIT(id_aa64mmfr0_parange_to_phys_shift(parange));
 
 		/*
 		 * If the size of the linear region exceeds, by a sufficient
-		 * margin, the size of the region that the available physical
-		 * memory spans, randomize the linear region as well.
+		 * margin, the size of the region that the physical memory can
+		 * span, randomize the linear region as well.
 		 */
 		if (memstart_offset_seed > 0 && range >= ARM64_MEMSTART_ALIGN) {
 			range /= ARM64_MEMSTART_ALIGN;
-- 
2.17.1