[PATCH 3/3] LoongArch: Add support for kernel address space layout randomization (KASLR)

[Youling Tang]

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 in the kernel boot,
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. Limit the maximum value of RANDOMIZE_BASE_MAX_OFFSET to
256M(0x10000000) because our memory layout has many holes.

KERNELOFFSET (kaslr_offset) is added to vmcoreinfo in the future, for
crash --kaslr support.

Signed-off-by: Youling Tang <tangyouling@xxxxxxxxxxx>
---
 arch/loongarch/Kconfig            |  22 ++++++
 arch/loongarch/include/asm/page.h |   6 ++
 arch/loongarch/kernel/relocate.c  | 114 ++++++++++++++++++++++++++++++
 arch/loongarch/kernel/setup.c     |   3 +
 4 files changed, 145 insertions(+)

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 271221d6036d..e2588ab281bc 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -449,6 +449,28 @@ config RELOCATION_TABLE_SIZE
 
 	  If unsure, leave at the default value.
 
+config RANDOMIZE_BASE
+	bool "Randomize the address of the kernel image (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 if 64BIT
+	default "0x01000000"
+	help
+	  When KASLR is active, this provides the maximum offset that will
+	  be applied to the kernel image.
+
 config SECCOMP
 	bool "Enable seccomp to safely compute untrusted bytecode"
 	depends on PROC_FS
diff --git a/arch/loongarch/include/asm/page.h b/arch/loongarch/include/asm/page.h
index 53f284a96182..6dda0d6271ca 100644
--- a/arch/loongarch/include/asm/page.h
+++ b/arch/loongarch/include/asm/page.h
@@ -106,6 +106,12 @@ extern int __virt_addr_valid(volatile void *kaddr);
 	 ((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | \
 	 VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
 
+extern unsigned long __kaslr_offset;
+static inline unsigned long kaslr_offset(void)
+{
+	return __kaslr_offset;
+}
+
 #include <asm-generic/memory_model.h>
 #include <asm-generic/getorder.h>
 
diff --git a/arch/loongarch/kernel/relocate.c b/arch/loongarch/kernel/relocate.c
index 492c833322a4..58e28b8f5d12 100644
--- a/arch/loongarch/kernel/relocate.c
+++ b/arch/loongarch/kernel/relocate.c
@@ -11,9 +11,11 @@
 #include <linux/elf.h>
 #include <linux/kernel.h>
 #include <linux/start_kernel.h>
+#include <linux/of_fdt.h>
 #include <linux/printk.h>
 #include <linux/panic_notifier.h>
 #include <asm/bootinfo.h>
+#include <asm/early_ioremap.h>
 #include <asm/inst.h>
 #include <asm/sections.h>
 
@@ -146,6 +148,70 @@ static void __init relocate_got_table(long offset)
 		*got += offset;
 }
 
+#ifdef CONFIG_RANDOMIZE_BASE
+
+static inline __init unsigned long rotate_xor(unsigned long hash,
+					      const void *area, size_t size)
+{
+	size_t i;
+	unsigned long *ptr = (unsigned long *)area;
+
+	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 entropy = random_get_entropy();
+	unsigned long hash = 0;
+
+	/* 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;
+
+	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;
+	void *dest = _text;
+	unsigned long offset;
+
+	if (kaslr_disabled())
+		return dest;
+
+	kernel_length = (long)_end - (long)_text;
+
+	offset = get_random_boot() << 16;
+	offset &= (CONFIG_RANDOMIZE_BASE_MAX_OFFSET - 1);
+	if (offset < kernel_length)
+		offset += ALIGN(kernel_length, 0xffff);
+
+	return RELOCATED(dest);
+}
+
+#else
+
 /*
  * Choose a new address for the kernel, for now we'll hard
  * code the destination.
@@ -155,6 +221,8 @@ static inline void __init *determine_relocation_address(void)
 	return (void *)(CACHE_BASE + 0x02000000);
 }
 
+#endif
+
 static inline int __init relocation_addr_valid(void *loc_new)
 {
 	if ((unsigned long)loc_new & 0x0000ffff) {
@@ -168,6 +236,13 @@ static inline int __init relocation_addr_valid(void *loc_new)
 	return 1;
 }
 
+static inline void __init update_kaslr_offset(unsigned long *addr, long offset)
+{
+	unsigned long *new_addr = (unsigned long *)RELOCATED(addr);
+
+	*new_addr = (unsigned long)offset;
+}
+
 void *__init relocate_kernel(void)
 {
 	void *loc_new;
@@ -177,6 +252,9 @@ void *__init relocate_kernel(void)
 	/* Default to original kernel entry point */
 	void *kernel_entry = start_kernel;
 
+	/* Boot command line was passed in FDT */
+	early_init_dt_scan(early_ioremap(fw_arg1, SZ_64K));
+
 	kernel_length = (long)(_end) - (long)(_text);
 
 	loc_new = determine_relocation_address();
@@ -208,7 +286,43 @@ void *__init relocate_kernel(void)
 
 		/* Return the new kernel's entry point */
 		kernel_entry = RELOCATED(start_kernel);
+
+		/* Error may occur before, so keep it at last */
+		update_kaslr_offset(&__kaslr_offset, offset);
 	}
 out:
 	return kernel_entry;
 }
+
+/*
+ * Show relocation information on panic.
+ */
+static void show_kernel_relocation(const char *level)
+{
+	if (__kaslr_offset > 0) {
+		printk(level);
+		pr_cont("Kernel relocated offset @ 0x%lx\n", __kaslr_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;
+}
+__initcall(register_kernel_offset_dumper);
diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
index 058c232c11e8..a176d7973ac7 100644
--- a/arch/loongarch/kernel/setup.c
+++ b/arch/loongarch/kernel/setup.c
@@ -73,6 +73,9 @@ static struct resource code_resource = { .name = "Kernel code", };
 static struct resource data_resource = { .name = "Kernel data", };
 static struct resource bss_resource  = { .name = "Kernel bss", };
 
+unsigned long __kaslr_offset __ro_after_init;
+EXPORT_SYMBOL(__kaslr_offset);
+
 const char *get_system_type(void)
 {
 	return "generic-loongson-machine";
-- 
2.36.1