LoongArch: Add support for kernel address space layout randomization (KASLR)

This patch adds support for relocating the kernel to a random address.

Entropy is derived from the banner, which will change every build and
random_get_entropy() which should provide additional runtime entropy.

The kernel is relocated by up to RANDOMIZE_BASE_MAX_OFFSET bytes from
its link address. Because relocation happens so early during the kernel
booting, the amount of physical memory has not yet been determined. This
means the only way to limit relocation within the available memory is
via Kconfig. So we limit the maximum value of RANDOMIZE_BASE_MAX_OFFSET
to 256M (0x10000000) because our memory layout has many holes.
Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Xi Ruoyao <xry111@xry111.site> # Fix compiler warnings
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -501,6 +501,31 @@ config RELOCATABLE
 	  the kernel binary at runtime to a different virtual address from
 	  its link address.
 
+config RANDOMIZE_BASE
+	bool "Randomize the address of the kernel (KASLR)"
+	depends on RELOCATABLE
+	help
+	   Randomizes the physical and virtual address at which the
+	   kernel image is loaded, as a security feature that
+	   deters exploit attempts relying on knowledge of the location
+	   of kernel internals.
+
+	   The kernel will be offset by up to RANDOMIZE_BASE_MAX_OFFSET.
+
+	   If unsure, say N.
+
+config RANDOMIZE_BASE_MAX_OFFSET
+	hex "Maximum KASLR offset" if EXPERT
+	depends on RANDOMIZE_BASE
+	range 0x0 0x10000000
+	default "0x01000000"
+	help
+	  When KASLR is active, this provides the maximum offset that will
+	  be applied to the kernel image. It should be set according to the
+	  amount of physical RAM available in the target system.
+
+	  This is limited by the size of the lower address memory, 256MB.
+
 config SECCOMP
 	bool "Enable seccomp to safely compute untrusted bytecode"
 	depends on PROC_FS

arch/loongarch/include/asm/setup.h

@@ -33,7 +33,7 @@ extern long __la_abs_end;
 extern long __rela_dyn_begin;
 extern long __rela_dyn_end;
 
-extern void __init relocate_kernel(void);
+extern void * __init relocate_kernel(void);
 
 #endif
 

arch/loongarch/kernel/head.S

@@ -87,7 +87,20 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
 	set_saved_sp	sp, t0, t1
 
 #ifdef CONFIG_RELOCATABLE
+
 	bl		relocate_kernel
+
+#ifdef CONFIG_RANDOMIZE_BASE
+	/* Repoint the sp into the new kernel */
+	PTR_LI		sp, (_THREAD_SIZE - PT_SIZE)
+	PTR_ADD		sp, sp, tp
+	set_saved_sp	sp, t0, t1
+#endif
+
+	/* relocate_kernel() returns the new kernel entry point */
+	jr		a0
+	ASM_BUG()
+
 #endif
 
 	bl		start_kernel

arch/loongarch/kernel/relocate.c

@@ -9,11 +9,15 @@
 #include <linux/kernel.h>
 #include <linux/printk.h>
 #include <linux/panic_notifier.h>
+#include <linux/start_kernel.h>
+#include <asm/bootinfo.h>
+#include <asm/early_ioremap.h>
 #include <asm/inst.h>
 #include <asm/sections.h>
 #include <asm/setup.h>
 
 #define RELOCATED(x) ((void *)((long)x + reloc_offset))
+#define RELOCATED_KASLR(x) ((void *)((long)x + random_offset))
 
 static unsigned long reloc_offset;
 
@@ -37,13 +41,13 @@ static inline void __init relocate_relative(void)
 	}
 }
 
-static inline void __init relocate_absolute(void)
+static inline void __init relocate_absolute(long random_offset)
 {
 	void *begin, *end;
 	struct rela_la_abs *p;
 
-	begin = &__la_abs_begin;
-	end   = &__la_abs_end;
+	begin = RELOCATED_KASLR(&__la_abs_begin);
+	end   = RELOCATED_KASLR(&__la_abs_end);
 
 	for (p = begin; (void *)p < end; p++) {
 		long v = p->symvalue;
@@ -62,14 +66,145 @@ static inline void __init relocate_absolute(void)
 	}
 }
 
-void __init relocate_kernel(void)
+#ifdef CONFIG_RANDOMIZE_BASE
+static inline __init unsigned long rotate_xor(unsigned long hash,
+					      const void *area, size_t size)
 {
+	size_t i, diff;
+	const typeof(hash) *ptr = PTR_ALIGN(area, sizeof(hash));
+
+	diff = (void *)ptr - area;
+	if (size < diff + sizeof(hash))
+		return hash;
+
+	size = ALIGN_DOWN(size - diff, sizeof(hash));
+
+	for (i = 0; i < size / sizeof(hash); i++) {
+		/* Rotate by odd number of bits and XOR. */
+		hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7);
+		hash ^= ptr[i];
+	}
+
+	return hash;
+}
+
+static inline __init unsigned long get_random_boot(void)
+{
+	unsigned long hash = 0;
+	unsigned long entropy = random_get_entropy();
+
+	/* Attempt to create a simple but unpredictable starting entropy. */
+	hash = rotate_xor(hash, linux_banner, strlen(linux_banner));
+
+	/* Add in any runtime entropy we can get */
+	hash = rotate_xor(hash, &entropy, sizeof(entropy));
+
+	return hash;
+}
+
+static inline __init bool kaslr_disabled(void)
+{
+	char *str;
+	const char *builtin_cmdline = CONFIG_CMDLINE;
+
+	str = strstr(builtin_cmdline, "nokaslr");
+	if (str == builtin_cmdline || (str > builtin_cmdline && *(str - 1) == ' '))
+		return true;
+
+	str = strstr(boot_command_line, "nokaslr");
+	if (str == boot_command_line || (str > boot_command_line && *(str - 1) == ' '))
+		return true;
+
+	return false;
+}
+
+/* Choose a new address for the kernel */
+static inline void __init *determine_relocation_address(void)
+{
+	unsigned long kernel_length;
+	unsigned long random_offset;
+	void *destination = _text;
+
+	if (kaslr_disabled())
+		return destination;
+
+	kernel_length = (long)_end - (long)_text;
+
+	random_offset = get_random_boot() << 16;
+	random_offset &= (CONFIG_RANDOMIZE_BASE_MAX_OFFSET - 1);
+	if (random_offset < kernel_length)
+		random_offset += ALIGN(kernel_length, 0xffff);
+
+	return RELOCATED_KASLR(destination);
+}
+
+static inline int __init relocation_addr_valid(void *location_new)
+{
+	if ((unsigned long)location_new & 0x00000ffff)
+		return 0; /* Inappropriately aligned new location */
+
+	if ((unsigned long)location_new < (unsigned long)_end)
+		return 0; /* New location overlaps original kernel */
+
+	return 1;
+}
+#endif
+
+static inline void __init update_reloc_offset(unsigned long *addr, long random_offset)
+{
+	unsigned long *new_addr = (unsigned long *)RELOCATED_KASLR(addr);
+
+	*new_addr = (unsigned long)reloc_offset;
+}
+
+void * __init relocate_kernel(void)
+{
+	unsigned long kernel_length;
+	unsigned long random_offset = 0;
+	void *location_new = _text; /* Default to original kernel start */
+	void *kernel_entry = start_kernel; /* Default to original kernel entry point */
+	char *cmdline = early_ioremap(fw_arg1, COMMAND_LINE_SIZE); /* Boot command line is passed in fw_arg1 */
+
+	strscpy(boot_command_line, cmdline, COMMAND_LINE_SIZE);
+
+#ifdef CONFIG_RANDOMIZE_BASE
+	location_new = determine_relocation_address();
+
+	/* Sanity check relocation address */
+	if (relocation_addr_valid(location_new))
+		random_offset = (unsigned long)location_new - (unsigned long)(_text);
+#endif
 	reloc_offset = (unsigned long)_text - VMLINUX_LOAD_ADDRESS;
 
+	if (random_offset) {
+		kernel_length = (long)(_end) - (long)(_text);
+
+		/* Copy the kernel to it's new location */
+		memcpy(location_new, _text, kernel_length);
+
+		/* Sync the caches ready for execution of new kernel */
+		__asm__ __volatile__ (
+			"ibar 0 \t\n"
+			"dbar 0 \t\n"
+			::: "memory");
+
+		reloc_offset += random_offset;
+
+		/* Return the new kernel's entry point */
+		kernel_entry = RELOCATED_KASLR(start_kernel);
+
+		/* The current thread is now within the relocated kernel */
+		__current_thread_info = RELOCATED_KASLR(__current_thread_info);
+
+		update_reloc_offset(&reloc_offset, random_offset);
+	}
+
 	if (reloc_offset)
 		relocate_relative();
 
-	relocate_absolute();
+	relocate_absolute(random_offset);
+
+	return kernel_entry;
 }
 
 /*