From: Ard Biesheuvel Date: Tue, 18 Mar 2025 13:49:50 +0000 (+0100) Subject: arm64/mm: Remove randomization of the linear map X-Git-Tag: v6.16-rc1~133^2~5^2~19 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1db780bafa4cedd20f040c3fce616e47aa7c0c47;p=thirdparty%2Fkernel%2Flinux.git arm64/mm: Remove randomization of the linear map Since commit 97d6786e0669 ("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 Cc: Will Deacon Cc: Ryan Roberts Cc: Mark Rutland Cc: Anshuman Khandual Cc: Kees Cook Signed-off-by: Ard Biesheuvel Link: https://lore.kernel.org/r/20250318134949.3194334-2-ardb+git@google.com Signed-off-by: Will Deacon --- diff --git a/arch/arm64/kernel/image-vars.h b/arch/arm64/kernel/image-vars.h index 5e3c4b58f2790..0450b47c64ca5 100644 --- a/arch/arm64/kernel/image-vars.h +++ b/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); diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c index 1da3e25f9d9e5..c9503ed45a6c2 100644 --- a/arch/arm64/kernel/kaslr.c +++ b/arch/arm64/kernel/kaslr.c @@ -10,8 +10,6 @@ #include #include -u16 __initdata memstart_offset_seed; - bool __ro_after_init __kaslr_is_enabled = false; void __init kaslr_init(void) diff --git a/arch/arm64/kernel/pi/kaslr_early.c b/arch/arm64/kernel/pi/kaslr_early.c index 0257b43819db8..e0e018046a46a 100644 --- a/arch/arm64/kernel/pi/kaslr_early.c +++ b/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 diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c index b99bf3980fc68..0c8c35dd645e4 100644 --- a/arch/arm64/mm/init.c +++ b/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.