Re: [PATCH] riscv: Introduce CONFIG_RELOCATABLE

[Anup Patel <anup@brainfault.org>]

On Thu, Jan 30, 2020 at 9:44 AM Zong Li <zong.li@sifive.com> 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'm working on KASLR on top of this patch, it's work to me.
>
> Reviewed-by: Zong Li <zong.li@sifive.com>
> Tested-by: Zong Li <zong.li@sifive.com>

Thanks for testing. I will not test since you already tested it.

Regards,
Anup

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