[PATCH v7 1/3] riscv: Introduce CONFIG_RELOCATABLE
Palmer Dabbelt
palmer at dabbelt.com
Wed Oct 27 08:29:17 AEDT 2021
On Sat, 09 Oct 2021 10:20:20 PDT (-0700), alex at ghiti.fr wrote:
> Arf, I have sent this patchset with the wrong email address. @Palmer
> tell me if you want me to resend it correctly.
Sorry for being kind of slow here. It's fine: there's a "From:" in the
patch, and git picks those up so it'll match the signed-off-by line. I
send pretty much all my patches that way, as I never managed to get my
Google address working correctly.
>
> Thanks,
>
> Alex
>
> On 10/9/21 7:12 PM, Alexandre Ghiti wrote:
>> From: Alexandre Ghiti <alex at ghiti.fr>
>>
>> This config allows to compile 64b 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.
IMO this should really be user selectable, at a bare minimum so it's testable.
I just sent along a patch to do that (my power's off at home, so email is a bit
wacky right now).
I haven't put this on for-next yet as I'm not sure if you had a fix for the
kasan issue (which IIUC would conflict with this).
>> 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 at ghiti.fr>
>> ---
>> arch/riscv/Kconfig | 12 ++++++++
>> arch/riscv/Makefile | 7 +++--
>> arch/riscv/kernel/vmlinux.lds.S | 6 ++++
>> arch/riscv/mm/Makefile | 4 +++
>> arch/riscv/mm/init.c | 54 ++++++++++++++++++++++++++++++++-
>> 5 files changed, 80 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index ea16fa2dd768..043ba92559fa 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -213,6 +213,18 @@ config PGTABLE_LEVELS
>> config LOCKDEP_SUPPORT
>> def_bool y
>>
>> +config RELOCATABLE
>> + bool
>> + depends on MMU && 64BIT && !XIP_KERNEL
>> + 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"
>> source "arch/riscv/Kconfig.erratas"
>>
>> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
>> index 0eb4568fbd29..2f509915f246 100644
>> --- a/arch/riscv/Makefile
>> +++ b/arch/riscv/Makefile
>> @@ -9,9 +9,12 @@
>> #
>>
>> 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
>> + LDFLAGS_vmlinux += --no-relax
>> KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
>> CC_FLAGS_FTRACE := -fpatchable-function-entry=8
>> endif
>> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
>> index 5104f3a871e3..862a8c09723c 100644
>> --- a/arch/riscv/kernel/vmlinux.lds.S
>> +++ b/arch/riscv/kernel/vmlinux.lds.S
>> @@ -133,6 +133,12 @@ SECTIONS
>>
>> BSS_SECTION(PAGE_SIZE, PAGE_SIZE, 0)
>>
>> + .rela.dyn : ALIGN(8) {
>> + __rela_dyn_start = .;
>> + *(.rela .rela*)
>> + __rela_dyn_end = .;
>> + }
>> +
>> #ifdef CONFIG_EFI
>> . = ALIGN(PECOFF_SECTION_ALIGNMENT);
>> __pecoff_data_virt_size = ABSOLUTE(. - __pecoff_text_end);
>> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
>> index 7ebaef10ea1b..2d33ec574bbb 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 = $(CC_FLAGS_FTRACE)
>> CFLAGS_REMOVE_cacheflush.o = $(CC_FLAGS_FTRACE)
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index c0cddf0fc22d..42041c12d496 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -20,6 +20,9 @@
>> #include <linux/dma-map-ops.h>
>> #include <linux/crash_dump.h>
>> #include <linux/hugetlb.h>
>> +#ifdef CONFIG_RELOCATABLE
>> +#include <linux/elf.h>
>> +#endif
>>
>> #include <asm/fixmap.h>
>> #include <asm/tlbflush.h>
>> @@ -103,7 +106,7 @@ static void __init print_vm_layout(void)
>> print_mlm("lowmem", (unsigned long)PAGE_OFFSET,
>> (unsigned long)high_memory);
>> #ifdef CONFIG_64BIT
>> - print_mlm("kernel", (unsigned long)KERNEL_LINK_ADDR,
>> + print_mlm("kernel", (unsigned long)kernel_map.virt_addr,
>> (unsigned long)ADDRESS_SPACE_END);
>> #endif
>> }
>> @@ -518,6 +521,44 @@ static __init pgprot_t pgprot_from_va(uintptr_t va)
>> #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;
>> +
>> +static void __init relocate_kernel(void)
>> +{
>> + Elf64_Rela *rela = (Elf64_Rela *)&__rela_dyn_start;
>> + /*
>> + * This holds the offset between the linked virtual address and the
>> + * relocated virtual address.
>> + */
>> + uintptr_t reloc_offset = kernel_map.virt_addr - KERNEL_LINK_ADDR;
>> + /*
>> + * This holds the offset between kernel linked virtual address and
>> + * physical address.
>> + */
>> + uintptr_t va_kernel_link_pa_offset = KERNEL_LINK_ADDR - kernel_map.phys_addr;
>> +
>> + for ( ; rela < (Elf64_Rela *)&__rela_dyn_end; rela++) {
>> + Elf64_Addr addr = (rela->r_offset - va_kernel_link_pa_offset);
>> + Elf64_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 >= KERNEL_LINK_ADDR)
>> + relocated_addr += reloc_offset;
>> +
>> + *(Elf64_Addr *)addr = relocated_addr;
>> + }
>> +}
>> +#endif /* CONFIG_RELOCATABLE */
>> +
>> #ifdef CONFIG_XIP_KERNEL
>> static void __init create_kernel_page_table(pgd_t *pgdir,
>> __always_unused bool early)
>> @@ -625,6 +666,17 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>> BUG_ON((kernel_map.virt_addr + kernel_map.size) > ADDRESS_SPACE_END - SZ_4K);
>> #endif
>>
>> +#ifdef CONFIG_RELOCATABLE
>> + /*
>> + * Early page table uses only one PGDIR, which makes it possible
>> + * to map PGDIR_SIZE aligned on PGDIR_SIZE: if the relocation offset
>> + * makes the kernel cross over a PGDIR_SIZE boundary, raise a bug
>> + * since a part of the kernel would not get mapped.
>> + */
>> + BUG_ON(PGDIR_SIZE - (kernel_map.virt_addr & (PGDIR_SIZE - 1)) < kernel_map.size);
>> + relocate_kernel();
>> +#endif
>> +
>> pt_ops.alloc_pte = alloc_pte_early;
>> pt_ops.get_pte_virt = get_pte_virt_early;
>> #ifndef __PAGETABLE_PMD_FOLDED
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