LoongArch: Add support for kernel relocation

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 code
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

8 files changed, 173 insertions, 2 deletions

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 0c1c6063cc66..32ab90dd76e5 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -493,6 +493,14 @@ 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, so as to relocate
+	  the kernel binary at runtime to a different virtual address from
+	  its link address.
+
 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 6e1c931a8507..768592998b39 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)
+KBUILD_CFLAGS_KERNEL		+= -fPIE
+LDFLAGS_vmlinux			+= -static -pie --no-dynamic-linker -z notext
+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 cdc9935d5554..c51a1b43acb4 100644
--- a/arch/loongarch/include/asm/asmmacro.h
+++ b/arch/loongarch/include/asm/asmmacro.h
@@ -275,7 +275,20 @@
 .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..27d968655b4b 100644
--- a/arch/loongarch/include/asm/setup.h
+++ b/arch/loongarch/include/asm/setup.h
@@ -21,4 +21,20 @@ 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 __la_abs_begin;
+extern long __la_abs_end;
+extern long __rela_dyn_begin;
+extern long __rela_dyn_end;
+
+extern void __init relocate_kernel(void);
+
+#endif
+
 #endif /* __SETUP_H */
diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
index df5dbabfe7a6..94d33dbd4912 100644
--- a/arch/loongarch/kernel/Makefile
+++ b/arch/loongarch/kernel/Makefile
@@ -41,6 +41,8 @@ obj-$(CONFIG_NUMA)		+= numa.o
 
 obj-$(CONFIG_MAGIC_SYSRQ)	+= sysrq.o
 
+obj-$(CONFIG_RELOCATABLE)	+= relocate.o
+
 obj-$(CONFIG_KEXEC)		+= machine_kexec.o relocate_kernel.o
 obj-$(CONFIG_CRASH_DUMP)	+= crash_dump.o
 
diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
index d2ac26b5b22b..c5c3ec2b819a 100644
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -86,6 +86,10 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
 	PTR_ADD		sp, sp, tp
 	set_saved_sp	sp, t0, t1
 
+#ifdef CONFIG_RELOCATABLE
+	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..879c40372fba
--- /dev/null
+++ b/arch/loongarch/kernel/relocate.c
@@ -0,0 +1,107 @@
+// 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 void __init relocate_relative(void)
+{
+	Elf64_Rela *rela, *rela_end;
+	rela = (Elf64_Rela *)&__rela_dyn_begin;
+	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_absolute(void)
+{
+	void *begin, *end;
+	struct rela_la_abs *p;
+
+	begin = &__la_abs_begin;
+	end   = &__la_abs_end;
+
+	for (p = begin; (void *)p < end; p++) {
+		long v = p->symvalue;
+		uint32_t lu12iw, ori, lu32id, lu52id;
+		union loongarch_instruction *insn = (void *)p - p->offset;
+
+		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_absolute();
+}
+
+/*
+ * Show relocation information on panic.
+ */
+static void show_kernel_relocation(const char *level)
+{
+	if (reloc_offset > 0) {
+		printk(level);
+		pr_cont("Kernel relocated by 0x%lx\n", reloc_offset);
+		pr_cont(" .text @ 0x%px\n", _text);
+		pr_cont(" .data @ 0x%px\n", _sdata);
+		pr_cont(" .bss  @ 0x%px\n", __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..ad2ce1a0908e 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_begin = .;
+		 *(.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)