arm64/mm: Remove randomization of the linear map

Since commit

  97d6786 ("arm64: mm: account for hotplug memory when randomizing the linear region")

the decision whether or not to randomize the placement of the system's
DRAM inside the linear map is based on the capabilities of the CPU
rather than how much memory is present at boot time. This change was
necessary because memory hotplug may result in DRAM appearing in places
that are not covered by the linear region at all (and therefore
unusable) if the decision is solely based on the memory map at boot.

In the Android GKI kernel, which requires support for memory hotplug,
and is built with a reduced virtual address space of only 39 bits wide,
randomization of the linear map never happens in practice as a result.
And even on arm64 kernels built with support for 48 bit virtual
addressing, the wider PArange of recent CPUs means that linear map
randomization is slowly becoming a feature that only works on systems
that will soon be obsolete.

So let's just remove this feature. We can always bring it back in an
improved form if there is a real need for it.

Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Kees Cook <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: ardbiesheuvel

arch/arm64/kernel/image-vars.h

@@ -52,7 +52,6 @@ PROVIDE(__pi_cavium_erratum_27456_cpus	= cavium_erratum_27456_cpus);
 PROVIDE(__pi_is_midr_in_range_list	= is_midr_in_range_list);
 #endif
 PROVIDE(__pi__ctype			= _ctype);
-PROVIDE(__pi_memstart_offset_seed	= memstart_offset_seed);
 
 PROVIDE(__pi_init_idmap_pg_dir		= init_idmap_pg_dir);
 PROVIDE(__pi_init_idmap_pg_end		= init_idmap_pg_end);

arch/arm64/kernel/kaslr.c

@@ -10,8 +10,6 @@
 #include <asm/cpufeature.h>
 #include <asm/memory.h>
 
-u16 __initdata memstart_offset_seed;
-
 bool __ro_after_init __kaslr_is_enabled = false;
 
 void __init kaslr_init(void)

arch/arm64/kernel/pi/kaslr_early.c

@@ -18,8 +18,6 @@
 
 #include "pi.h"
 
-extern u16 memstart_offset_seed;
-
 static u64 __init get_kaslr_seed(void *fdt, int node)
 {
 	static char const seed_str[] __initconst = "kaslr-seed";
@@ -53,8 +51,6 @@ u64 __init kaslr_early_init(void *fdt, int chosen)
 			return 0;
 	}
 
-	memstart_offset_seed = seed & U16_MAX;
-
 	/*
 	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 	 * kernel image offset from the seed. Let's place the kernel in the

arch/arm64/mm/init.c

@@ -275,26 +275,6 @@ void __init arm64_memblock_init(void)
 		}
 	}
 
-	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
-		extern u16 memstart_offset_seed;
-		u64 mmfr0 = read_cpuid(ID_AA64MMFR0_EL1);
-		int parange = cpuid_feature_extract_unsigned_field(
-					mmfr0, ID_AA64MMFR0_EL1_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 physical memory can
-		 * span, randomize the linear region as well.
-		 */
-		if (memstart_offset_seed > 0 && range >= (s64)ARM64_MEMSTART_ALIGN) {
-			range /= ARM64_MEMSTART_ALIGN;
-			memstart_addr -= ARM64_MEMSTART_ALIGN *
-					 ((range * memstart_offset_seed) >> 16);
-		}
-	}
-
 	/*
 	 * Register the kernel text, kernel data, initrd, and initial
 	 * pagetables with memblock.