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Subject[PATCH v6 4/5] LoongArch: Add support for kernel relocation
Date
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@loongson.cn>
Signed-off-by: Xi Ruoyao <xry111@xry111.site> # Use arch_initcall
Signed-off-by: Jinyang He <hejinyang@loongson.cn> # Provide la_abs relocation draft code
---
arch/loongarch/Kconfig | 8 ++
arch/loongarch/Makefile | 5 ++
arch/loongarch/include/asm/asmmacro.h | 13 ++++
arch/loongarch/include/asm/setup.h | 14 ++++
arch/loongarch/kernel/Makefile | 2 +
arch/loongarch/kernel/head.S | 5 ++
arch/loongarch/kernel/relocate.c | 108 ++++++++++++++++++++++++++
arch/loongarch/kernel/vmlinux.lds.S | 20 ++++-
8 files changed, 173 insertions(+), 2 deletions(-)
create mode 100644 arch/loongarch/kernel/relocate.c

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 54bd3dbde1f2..406a28758a52 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -522,6 +522,14 @@ config PHYSICAL_START
specified in the "crashkernel=YM@XM" command line boot parameter
passed to the panic-ed kernel).

+config RELOCATABLE
+ bool "Relocatable kernel"
+ 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 ccfb52700237..2aba6ff30436 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/include/asm/asmmacro.h b/arch/loongarch/include/asm/asmmacro.h
index adc92464afa6..ba99908d7d10 100644
--- a/arch/loongarch/include/asm/asmmacro.h
+++ b/arch/loongarch/include/asm/asmmacro.h
@@ -668,7 +668,20 @@
.endm

.macro la_abs reg, sym
+#ifndef CONFIG_RELOCATABLE
la.abs \reg, \sym
+#else
+766:
+ lu12i.w \reg, 0
+ ori \reg, \reg, 0
+ lu32i.d \reg, 0
+ lu52i.d \reg, \reg, 0
+ .pushsection ".la_abs", "aw", %progbits
+768:
+ .dword 768b-766b
+ .dword \sym
+ .popsection
+#endif
.endm

#endif /* _ASM_ASMMACRO_H */
diff --git a/arch/loongarch/include/asm/setup.h b/arch/loongarch/include/asm/setup.h
index 72ead58039f3..68b5b5a1fe8a 100644
--- a/arch/loongarch/include/asm/setup.h
+++ b/arch/loongarch/include/asm/setup.h
@@ -21,4 +21,18 @@ extern void per_cpu_trap_init(int cpu);
extern void set_handler(unsigned long offset, void *addr, unsigned long len);
extern void set_merr_handler(unsigned long offset, void *addr, unsigned long len);

+#ifdef CONFIG_RELOCATABLE
+struct rela_la_abs {
+ long offset;
+ long symvalue;
+};
+
+extern long __rela_dyn_start;
+extern long __rela_dyn_end;
+extern void *__la_abs_begin;
+extern void *__la_abs_end;
+
+extern void __init relocate_kernel(void);
+#endif
+
#endif /* __SETUP_H */
diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
index 45c78aea63ce..3288854dbe36 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..feeba0730875
--- /dev/null
+++ b/arch/loongarch/kernel/relocate.c
@@ -0,0 +1,108 @@
+// 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/inst.h>
+#include <asm/sections.h>
+#include <asm/setup.h>
+
+#define RELOCATED(x) ((void *)((long)x + reloc_offset))
+
+static unsigned long reloc_offset;
+
+static inline __init void relocate_relative(void)
+{
+ 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;
+ }
+}
+
+static inline void __init relocate_la_abs(void)
+{
+ struct rela_la_abs *p;
+ void *start, *end;
+
+ start = &__la_abs_begin;
+ end = &__la_abs_end;
+
+ for (p = start; (void *)p < end; p++) {
+ long v = p->symvalue;
+ union loongarch_instruction *insn = (void *)p - p->offset;
+ u32 lu12iw, ori, lu32id, lu52id;
+
+ lu12iw = (v >> 12) & 0xfffff;
+ ori = v & 0xfff;
+ lu32id = (v >> 32) & 0xfffff;
+ lu52id = v >> 52;
+
+ insn[0].reg1i20_format.immediate = lu12iw;
+ insn[1].reg2i12_format.immediate = ori;
+ insn[2].reg1i20_format.immediate = lu32id;
+ insn[3].reg2i12_format.immediate = lu52id;
+ }
+}
+
+void __init relocate_kernel(void)
+{
+ reloc_offset = (unsigned long)_text - VMLINUX_LOAD_ADDRESS;
+
+ if (reloc_offset)
+ relocate_relative();
+
+ relocate_la_abs();
+}
+
+/*
+ * 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..5ec0c008f01d 100644
--- a/arch/loongarch/kernel/vmlinux.lds.S
+++ b/arch/loongarch/kernel/vmlinux.lds.S
@@ -66,10 +66,21 @@ SECTIONS
__alt_instructions_end = .;
}

+#ifdef CONFIG_RELOCATABLE
+ . = ALIGN(8);
+ .la_abs : AT(ADDR(.la_abs) - LOAD_OFFSET) {
+ __la_abs_begin = .;
+ *(.la_abs)
+ __la_abs_end = .;
+ }
+#endif
+
.got : ALIGN(16) { *(.got) }
.plt : ALIGN(16) { *(.plt) }
.got.plt : ALIGN(16) { *(.got.plt) }

+ .data.rel : { *(.data.rel*) }
+
. = ALIGN(PECOFF_SEGMENT_ALIGN);
__init_begin = .;
__inittext_begin = .;
@@ -93,8 +104,6 @@ SECTIONS
PERCPU_SECTION(1 << CONFIG_L1_CACHE_SHIFT)
#endif

- .rela.dyn : ALIGN(8) { *(.rela.dyn) *(.rela*) }
-
.init.bss : {
*(.init.bss)
}
@@ -107,6 +116,12 @@ 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 +148,7 @@ SECTIONS

DISCARDS
/DISCARD/ : {
+ *(.dynamic .dynsym .dynstr .hash .gnu.hash)
*(.gnu.attributes)
*(.options)
*(.eh_frame)
--
2.37.3
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