Re: [PATCH] riscv: Introduce CONFIG_RELOCATABLE

[Alex Ghiti <alex@ghiti.fr>]

Hi Zong,

On 1/29/20 11:13 PM, Zong Li wrote:
> On Fri, Jan 24, 2020 at 4:14 AM Alexandre Ghiti <alex@ghiti.fr> wrote:
>> This config allows to compile the kernel as PIE and to relocate it at any
>> virtual address at runtime: this paves the way to KASLR and to 4-level
>> page table folding at runtime. Runtime relocation is possible since
>> relocation metadata are embedded into the kernel.
>>
>> Note that relocating at runtime introduces an overhead even if the kernel
>> is loaded at the same address it was linked at and that the compiler
>> options are those used in arm64 which uses the same RELA relocation format.
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
>>   arch/riscv/Kconfig              | 11 ++++
>>   arch/riscv/Makefile             |  5 +-
>>   arch/riscv/boot/loader.lds.S    |  2 +-
>>   arch/riscv/include/asm/page.h   |  5 +-
>>   arch/riscv/kernel/head.S        |  3 +-
>>   arch/riscv/kernel/vmlinux.lds.S | 10 ++--
>>   arch/riscv/mm/Makefile          |  4 ++
>>   arch/riscv/mm/init.c            | 92 ++++++++++++++++++++++++++++-----
>>   8 files changed, 110 insertions(+), 22 deletions(-)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index fa7dc03459e7..c652b4b850ce 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -163,6 +163,17 @@ config PGTABLE_LEVELS
>>          default 3 if 64BIT
>>          default 2
>>
>> +config RELOCATABLE
>> +       bool
>> +       help
>> +          This builds a 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.
>> +          Since RISCV uses the RELA relocation format, this requires a
>> +          relocation pass at runtime even if the kernel is loaded at the
>> +          same address it was linked at.
>> +
>>   source "arch/riscv/Kconfig.socs"
>>
>>   menu "Platform type"
>> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
>> index b9009a2fbaf5..5a115cf6a9c1 100644
>> --- a/arch/riscv/Makefile
>> +++ b/arch/riscv/Makefile
>> @@ -9,7 +9,10 @@
>>   #
>>
>>   OBJCOPYFLAGS    := -O binary
>> -LDFLAGS_vmlinux :=
>> +ifeq ($(CONFIG_RELOCATABLE),y)
>> +LDFLAGS_vmlinux := -shared -Bsymbolic -z notext -z norelro
>> +KBUILD_CFLAGS += -fPIE
>> +endif
>>   ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
>>          LDFLAGS_vmlinux := --no-relax
>>   endif
>> diff --git a/arch/riscv/boot/loader.lds.S b/arch/riscv/boot/loader.lds.S
>> index 47a5003c2e28..a9ed218171aa 100644
>> --- a/arch/riscv/boot/loader.lds.S
>> +++ b/arch/riscv/boot/loader.lds.S
>> @@ -7,7 +7,7 @@ ENTRY(_start)
>>
>>   SECTIONS
>>   {
>> -       . = PAGE_OFFSET;
>> +       . = CONFIG_PAGE_OFFSET;
>>
>>          .payload : {
>>                  *(.payload)
>> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
>> index ac699246ae7e..27c95da68ecb 100644
>> --- a/arch/riscv/include/asm/page.h
>> +++ b/arch/riscv/include/asm/page.h
>> @@ -31,9 +31,9 @@
>>    * When not using MMU this corresponds to the first free page in
>>    * physical memory (aligned on a page boundary).
>>    */
>> -#define PAGE_OFFSET            _AC(CONFIG_PAGE_OFFSET, UL)
>> +#define PAGE_OFFSET            kernel_load_addr
>>
>> -#define KERN_VIRT_SIZE (-PAGE_OFFSET)
>> +#define KERN_VIRT_SIZE         (-_AC(CONFIG_PAGE_OFFSET, UL))
>>
>>   #ifndef __ASSEMBLY__
>>
>> @@ -97,6 +97,7 @@ extern unsigned long pfn_base;
>>   #define ARCH_PFN_OFFSET                (PAGE_OFFSET >> PAGE_SHIFT)
>>   #endif /* CONFIG_MMU */
>>
>> +extern unsigned long kernel_load_addr;
>>   extern unsigned long max_low_pfn;
>>   extern unsigned long min_low_pfn;
>>
>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>> index 2227db63f895..5042b2b48a06 100644
>> --- a/arch/riscv/kernel/head.S
>> +++ b/arch/riscv/kernel/head.S
>> @@ -126,7 +126,8 @@ clear_bss_done:
>>   #ifdef CONFIG_MMU
>>   relocate:
>>          /* Relocate return address */
>> -       li a1, PAGE_OFFSET
>> +       la a1, kernel_load_addr
>> +       REG_L a1, 0(a1)
>>          la a2, _start
>>          sub a1, a1, a2
>>          add ra, ra, a1
>> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
>> index 12f42f96d46e..5095aee7c37e 100644
>> --- a/arch/riscv/kernel/vmlinux.lds.S
>> +++ b/arch/riscv/kernel/vmlinux.lds.S
>> @@ -4,7 +4,7 @@
>>    * Copyright (C) 2017 SiFive
>>    */
>>
>> -#define LOAD_OFFSET PAGE_OFFSET
>> +#define LOAD_OFFSET CONFIG_PAGE_OFFSET
>>   #include <asm/vmlinux.lds.h>
>>   #include <asm/page.h>
>>   #include <asm/cache.h>
>> @@ -70,9 +70,11 @@ SECTIONS
>>
>>          EXCEPTION_TABLE(0x10)
>>
>> -       .rel.dyn : {
>> -               *(.rel.dyn*)
>> -       }
>> +        .rela.dyn : ALIGN(8) {
>> +               __rela_dyn_start = .;
>> +                *(.rela .rela*)
>> +               __rela_dyn_end = .;
>> +        }
>>
>>          _end = .;
>>
>> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
>> index a1bd95c8047a..dcd3d806243f 100644
>> --- a/arch/riscv/mm/Makefile
>> +++ b/arch/riscv/mm/Makefile
>> @@ -1,6 +1,10 @@
>>   # SPDX-License-Identifier: GPL-2.0-only
>>
>>   CFLAGS_init.o := -mcmodel=medany
>> +ifdef CONFIG_RELOCATABLE
>> +CFLAGS_init.o += -fno-pie
>> +endif
>> +
>>   ifdef CONFIG_FTRACE
>>   CFLAGS_REMOVE_init.o = -pg
>>   endif
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index 965a8cf4829c..ac9a9f69abc0 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -12,6 +12,9 @@
>>   #include <linux/sizes.h>
>>   #include <linux/of_fdt.h>
>>   #include <linux/libfdt.h>
>> +#ifdef CONFIG_RELOCATABLE
>> +#include <linux/elf.h>
>> +#endif
>>
>>   #include <asm/fixmap.h>
>>   #include <asm/tlbflush.h>
>> @@ -28,6 +31,9 @@ EXPORT_SYMBOL(empty_zero_page);
>>   extern char _start[];
>>   void *dtb_early_va;
>>
>> +unsigned long kernel_load_addr = _AC(CONFIG_PAGE_OFFSET, UL);
>> +EXPORT_SYMBOL(kernel_load_addr);
>> +
>>   static void __init zone_sizes_init(void)
>>   {
>>          unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
>> @@ -132,7 +138,8 @@ void __init setup_bootmem(void)
>>                  phys_addr_t end = reg->base + reg->size;
>>
>>                  if (reg->base <= vmlinux_end && vmlinux_end <= end) {
>> -                       mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
>> +                       mem_size = min(reg->size,
>> +                                      (phys_addr_t)-kernel_load_addr);
>>
>>                          /*
>>                           * Remove memblock from the end of usable area to the
>> @@ -269,7 +276,7 @@ static phys_addr_t __init alloc_pmd(uintptr_t va)
>>          if (mmu_enabled)
>>                  return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
>>
>> -       pmd_num = (va - PAGE_OFFSET) >> PGDIR_SHIFT;
>> +       pmd_num = (va - kernel_load_addr) >> PGDIR_SHIFT;
>>          BUG_ON(pmd_num >= NUM_EARLY_PMDS);
>>          return (uintptr_t)&early_pmd[pmd_num * PTRS_PER_PMD];
>>   }
>> @@ -370,6 +377,54 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
>>   #error "setup_vm() is called from head.S before relocate so it should not use absolute addressing."
>>   #endif
>>
>> +#ifdef CONFIG_RELOCATABLE
>> +extern unsigned long __rela_dyn_start, __rela_dyn_end;
>> +
>> +#ifdef CONFIG_64BIT
>> +#define Elf_Rela Elf64_Rela
>> +#define Elf_Addr Elf64_Addr
>> +#else
>> +#define Elf_Rela Elf32_Rela
>> +#define Elf_Addr Elf32_Addr
>> +#endif
>> +
>> +void __init relocate_kernel(uintptr_t load_pa)
>> +{
>> +       Elf_Rela *rela = (Elf_Rela *)&__rela_dyn_start;
>> +       uintptr_t link_addr = _AC(CONFIG_PAGE_OFFSET, UL);
>> +       /*
>> +        * This holds the offset between the linked virtual address and the
>> +        * relocated virtual address.
>> +        */
>> +       uintptr_t reloc_offset = kernel_load_addr - link_addr;
>> +       /*
>> +        * This holds the offset between linked virtual address and physical
>> +        * address whereas va_pa_offset holds the offset between relocated
>> +        * virtual address and physical address.
>> +        */
>> +       uintptr_t va_link_pa_offset = link_addr - load_pa;
>> +
>> +       for ( ; rela < (Elf_Rela *)&__rela_dyn_end; rela++) {
>> +               Elf_Addr addr = (rela->r_offset - va_link_pa_offset);
>> +               Elf_Addr relocated_addr = rela->r_addend;
>> +
>> +               if (rela->r_info != R_RISCV_RELATIVE)
>> +                       continue;
>> +
>> +               /*
>> +                * Make sure to not relocate vdso symbols like rt_sigreturn
>> +                * which are linked from the address 0 in vmlinux since
>> +                * vdso symbol addresses are actually used as an offset from
>> +                * mm->context.vdso in VDSO_OFFSET macro.
>> +                */
>> +               if (relocated_addr >= link_addr)
>> +                       relocated_addr += reloc_offset;
>> +
>> +               *(Elf_Addr *)addr = relocated_addr;
>> +       }
>> +}
>> +#endif
>> +
>>   asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>   {
>>          uintptr_t va, end_va;
>> @@ -377,9 +432,20 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>          uintptr_t load_sz = (uintptr_t)(&_end) - load_pa;
>>          uintptr_t map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);
>>
>> -       va_pa_offset = PAGE_OFFSET - load_pa;
>> +       va_pa_offset = kernel_load_addr - load_pa;
>>          pfn_base = PFN_DOWN(load_pa);
>>
>> +#ifdef CONFIG_RELOCATABLE
>> +       /*
>> +        * Early page table uses only one PGDIR, which makes it possible
>> +        * to map 1GB aligned on 1GB: if the relocation offset makes the kernel
>> +        * cross over a 1G boundary, raise a bug since a part of the kernel
>> +        * would not get mapped.
>> +        */
>> +       BUG_ON(SZ_1G - (kernel_load_addr & (SZ_1G - 1)) < load_sz);
>> +       relocate_kernel(load_pa);
>> +#endif
>> +
>>          /*
>>           * Enforce boot alignment requirements of RV32 and
>>           * RV64 by only allowing PMD or PGD mappings.
>> @@ -387,7 +453,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>          BUG_ON(map_size == PAGE_SIZE);
>>
>>          /* Sanity check alignment and size */
>> -       BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
>> +       BUILD_BUG_ON((_AC(CONFIG_PAGE_OFFSET, UL) % PGDIR_SIZE) != 0);
>>          BUG_ON((load_pa % map_size) != 0);
>>          BUG_ON(load_sz > MAX_EARLY_MAPPING_SIZE);
>>
>> @@ -400,13 +466,13 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>          create_pmd_mapping(fixmap_pmd, FIXADDR_START,
>>                             (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
>>          /* Setup trampoline PGD and PMD */
>> -       create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
>> +       create_pgd_mapping(trampoline_pg_dir, kernel_load_addr,
>>                             (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
>> -       create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
>> +       create_pmd_mapping(trampoline_pmd, kernel_load_addr,
>>                             load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
>>   #else
>>          /* Setup trampoline PGD */
>> -       create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
>> +       create_pgd_mapping(trampoline_pg_dir, kernel_load_addr,
>>                             load_pa, PGDIR_SIZE, PAGE_KERNEL_EXEC);
>>   #endif
>>
>> @@ -415,10 +481,10 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>           * us to reach paging_init(). We map all memory banks later
>>           * in setup_vm_final() below.
>>           */
>> -       end_va = PAGE_OFFSET + load_sz;
>> -       for (va = PAGE_OFFSET; va < end_va; va += map_size)
>> +       end_va = kernel_load_addr + load_sz;
>> +       for (va = kernel_load_addr; va < end_va; va += map_size)
>>                  create_pgd_mapping(early_pg_dir, va,
>> -                                  load_pa + (va - PAGE_OFFSET),
>> +                                  load_pa + (va - kernel_load_addr),
>>                                     map_size, PAGE_KERNEL_EXEC);
>>
>>          /* Create fixed mapping for early FDT parsing */
>> @@ -457,9 +523,9 @@ static void __init setup_vm_final(void)
>>                          break;
>>                  if (memblock_is_nomap(reg))
>>                          continue;
>> -               if (start <= __pa(PAGE_OFFSET) &&
>> -                   __pa(PAGE_OFFSET) < end)
>> -                       start = __pa(PAGE_OFFSET);
>> +               if (start <= __pa(kernel_load_addr) &&
>> +                   __pa(kernel_load_addr) < end)
>> +                       start = __pa(kernel_load_addr);
> Here should use __pa_symbol() instead of __pa() for kernel symbols.


I'll add it to the v2 too, thanks.


> I'm working on KASLR on top of this patch, it's work to me.


If I can help, don't hesitate, even if it is simply for testing :)


>
> Reviewed-by: Zong Li <zong.li@sifive.com>
> Tested-by: Zong Li <zong.li@sifive.com>


Thanks for that,

Alex


>
>>                  map_size = best_map_size(start, end - start);
>>                  for (pa = start; pa < end; pa += map_size) {
>> --
>> 2.20.1
>>