Re: [PATCH] riscv: Introduce CONFIG_RELOCATABLE

[Zong Li <zong.li@sifive.com>]

On Fri, Jan 31, 2020 at 4:02 AM Alex Ghiti <alex@ghiti.fr> wrote:
>
> 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
> >>

Hi all,

I find that there is a risk when building kernel as PIE. The problem
is that hart_lottery will be put in .bss section rather than .sdata
section, so the value of hart_lottery will be cleared by main hart. I
post a patch to mark hart_lottery as sdata section by using gcc
attribution.

Please refer to https://patchwork.kernel.org/patch/11364457/.

Feel free to give any feedbacks and comments. Thanks.