Re: [PATCH v4 4/5] LoongArch: Add support for kernel relocation

From: Huacai Chen
Date: Fri Feb 10 2023 - 04:07:10 EST

Hi, Youling,

On Fri, Feb 10, 2023 at 4:48 PM Youling Tang <tangyouling@xxxxxxxxxxx> wrote:
>
> This config allows to compile kernel as PIE and to relocate it at
> any virtual address at runtime: this paves the way to KASLR.
> Runtime relocation is possible since relocation metadata are embedded
> into the kernel.
>
> Signed-off-by: Youling Tang <tangyouling@xxxxxxxxxxx>
> Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx> # Use arch_initcall
> ---
> arch/loongarch/Kconfig | 15 ++++++
> arch/loongarch/Makefile | 5 ++
> arch/loongarch/kernel/Makefile | 2 +
> arch/loongarch/kernel/head.S | 5 ++
> arch/loongarch/kernel/relocate.c | 78 +++++++++++++++++++++++++++++
> arch/loongarch/kernel/vmlinux.lds.S | 11 +++-
> 6 files changed, 114 insertions(+), 2 deletions(-)
> create mode 100644 arch/loongarch/kernel/relocate.c
>
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index 9cc8b84f7eb0..089a4695b1b3 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -48,6 +48,7 @@ config LOONGARCH
> select ARCH_SUPPORTS_ATOMIC_RMW
> select ARCH_SUPPORTS_HUGETLBFS
> select ARCH_SUPPORTS_NUMA_BALANCING
> + select SYS_SUPPORTS_RELOCATABLE
We don't need such a Kconfig option since it is always true.

Huacai
> select ARCH_USE_BUILTIN_BSWAP
> select ARCH_USE_CMPXCHG_LOCKREF
> select ARCH_USE_QUEUED_RWLOCKS
> @@ -229,6 +230,11 @@ config SCHED_OMIT_FRAME_POINTER
> config AS_HAS_EXPLICIT_RELOCS
> def_bool $(as-instr,x:pcalau12i \$t0$(comma)%pc_hi20(x))
>
> +config SYS_SUPPORTS_RELOCATABLE
> + bool
> + help
> + Selected if the platform supports relocating the kernel.
> +
> menu "Kernel type and options"
>
> source "kernel/Kconfig.hz"
> @@ -474,6 +480,15 @@ config PHYSICAL_START
> specified in the "crashkernel=YM@XM" command line boot parameter
> passed to the panic-ed kernel).
>
> +config RELOCATABLE
> + bool "Relocatable kernel"
> + depends on SYS_SUPPORTS_RELOCATABLE
> + help
> + This builds the kernel as a Position Independent Executable (PIE),
> + which retains all relocation metadata required to relocate the
> + kernel binary at runtime to a different virtual address than the
> + address it was linked at.
> +
> config SECCOMP
> bool "Enable seccomp to safely compute untrusted bytecode"
> depends on PROC_FS
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index 4402387d2755..27b5a70ff31c 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -71,6 +71,11 @@ KBUILD_AFLAGS_MODULE += -Wa,-mla-global-with-abs
> KBUILD_CFLAGS_MODULE += -fplt -Wa,-mla-global-with-abs,-mla-local-with-abs
> endif
>
> +ifeq ($(CONFIG_RELOCATABLE),y)
> +LDFLAGS_vmlinux += -static -pie --no-dynamic-linker -z notext
> +KBUILD_CFLAGS_KERNEL += -fPIE
> +endif
> +
> cflags-y += -ffreestanding
> cflags-y += $(call cc-option, -mno-check-zero-division)
>
> diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
> index c8cfbd562921..3341dd5f0926 100644
> --- a/arch/loongarch/kernel/Makefile
> +++ b/arch/loongarch/kernel/Makefile
> @@ -31,6 +31,8 @@ endif
> obj-$(CONFIG_MODULES) += module.o module-sections.o
> obj-$(CONFIG_STACKTRACE) += stacktrace.o
>
> +obj-$(CONFIG_RELOCATABLE) += relocate.o
> +
> obj-$(CONFIG_PROC_FS) += proc.o
>
> obj-$(CONFIG_SMP) += smp.o
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index d2ac26b5b22b..499edc80d8ab 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -86,6 +86,11 @@ SYM_CODE_START(kernel_entry) # kernel entry point
> PTR_ADD sp, sp, tp
> set_saved_sp sp, t0, t1
>
> +#ifdef CONFIG_RELOCATABLE
> + /* Apply the relocations */
> + bl relocate_kernel
> +#endif
> +
> bl start_kernel
> ASM_BUG()
>
> diff --git a/arch/loongarch/kernel/relocate.c b/arch/loongarch/kernel/relocate.c
> new file mode 100644
> index 000000000000..91ce92433bab
> --- /dev/null
> +++ b/arch/loongarch/kernel/relocate.c
> @@ -0,0 +1,78 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Support for Kernel relocation at boot time
> + *
> + * Copyright (C) 2023 Loongson Technology Corporation Limited
> + */
> +
> +#include <linux/elf.h>
> +#include <linux/kernel.h>
> +#include <linux/printk.h>
> +#include <linux/panic_notifier.h>
> +#include <asm/sections.h>
> +
> +#define RELOCATED(x) ((void *)((long)x + reloc_offset))
> +
> +extern long __rela_dyn_start;
> +extern long __rela_dyn_end;
> +
> +static unsigned long reloc_offset;
> +
> +void __init relocate_kernel(void)
> +{
> + reloc_offset = (unsigned long)_text - VMLINUX_LOAD_ADDRESS;
> +
> + if (reloc_offset) {
> + Elf64_Rela *rela, *rela_end;
> + rela = (Elf64_Rela *)&__rela_dyn_start;
> + rela_end = (Elf64_Rela *)&__rela_dyn_end;
> +
> + for ( ; rela < rela_end; rela++) {
> + Elf64_Addr addr = rela->r_offset;
> + Elf64_Addr relocated_addr = rela->r_addend;
> +
> + if (rela->r_info != R_LARCH_RELATIVE)
> + continue;
> +
> + if (relocated_addr >= VMLINUX_LOAD_ADDRESS)
> + relocated_addr =
> + (Elf64_Addr)RELOCATED(relocated_addr);
> +
> + *(Elf64_Addr *)RELOCATED(addr) = relocated_addr;
> + }
> + }
> +}
> +
> +/*
> + * Show relocation information on panic.
> + */
> +static void show_kernel_relocation(const char *level)
> +{
> + if (reloc_offset > 0) {
> + printk(level);
> + pr_cont("Kernel relocated offset @ 0x%lx\n", reloc_offset);
> + pr_cont(" .text @ 0x%lx\n", (unsigned long)&_text);
> + pr_cont(" .data @ 0x%lx\n", (unsigned long)&_sdata);
> + pr_cont(" .bss @ 0x%lx\n", (unsigned long)&__bss_start);
> + }
> +}
> +
> +static int kernel_location_notifier_fn(struct notifier_block *self,
> + unsigned long v, void *p)
> +{
> + show_kernel_relocation(KERN_EMERG);
> + return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block kernel_location_notifier = {
> + .notifier_call = kernel_location_notifier_fn
> +};
> +
> +static int __init register_kernel_offset_dumper(void)
> +{
> + atomic_notifier_chain_register(&panic_notifier_list,
> + &kernel_location_notifier);
> + return 0;
> +}
> +
> +arch_initcall(register_kernel_offset_dumper);
> diff --git a/arch/loongarch/kernel/vmlinux.lds.S b/arch/loongarch/kernel/vmlinux.lds.S
> index 733b16e8d55d..aec0b6567d24 100644
> --- a/arch/loongarch/kernel/vmlinux.lds.S
> +++ b/arch/loongarch/kernel/vmlinux.lds.S
> @@ -70,6 +70,8 @@ SECTIONS
> .plt : ALIGN(16) { *(.plt) }
> .got.plt : ALIGN(16) { *(.got.plt) }
>
> + .data.rel : { *(.data.rel*) }
> +
> . = ALIGN(PECOFF_SEGMENT_ALIGN);
> __init_begin = .;
> __inittext_begin = .;
> @@ -93,8 +95,6 @@ SECTIONS
> PERCPU_SECTION(1 << CONFIG_L1_CACHE_SHIFT)
> #endif
>
> - .rela.dyn : ALIGN(8) { *(.rela.dyn) *(.rela*) }
> -
> .init.bss : {
> *(.init.bss)
> }
> @@ -107,6 +107,12 @@ SECTIONS
> RO_DATA(4096)
> RW_DATA(1 << CONFIG_L1_CACHE_SHIFT, PAGE_SIZE, THREAD_SIZE)
>
> + .rela.dyn : ALIGN(8) {
> + __rela_dyn_start = .;
> + *(.rela.dyn) *(.rela*)
> + __rela_dyn_end = .;
> + }
> +
> .sdata : {
> *(.sdata)
> }
> @@ -133,6 +139,7 @@ SECTIONS
>
> DISCARDS
> /DISCARD/ : {
> + *(.dynamic .dynsym .dynstr .hash .gnu.hash)
> *(.gnu.attributes)
> *(.options)
> *(.eh_frame)
> --
> 2.37.3
>