Date:   Fri, 10 Feb 2023 17:06:05 +0800
From:   Youling Tang <tangyouling@...ngson.cn>
To:     Huacai Chen <chenhuacai@...nel.org>
Cc:     Xi Ruoyao <xry111@...111.site>, Jinyang He <hejinyang@...ngson.cn>,
        Xuerui Wang <kernel@...0n.name>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/5] LoongArch: Add support for kernel address space
 layout randomization (KASLR)



On 02/10/2023 04:47 PM, Youling Tang wrote:
> 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.
>
> Signed-off-by: Youling Tang <tangyouling@...ngson.cn>
> Signed-off-by: Xi Ruoyao <xry111@...111.site> # Fix compiler warnings
> ---
>  arch/loongarch/Kconfig           |  23 +++++
>  arch/loongarch/kernel/head.S     |  14 ++-
>  arch/loongarch/kernel/relocate.c | 143 ++++++++++++++++++++++++++++++-
>  3 files changed, 176 insertions(+), 4 deletions(-)
>
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index 089a4695b1b3..f0a070bd7254 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -489,6 +489,29 @@ config RELOCATABLE
>  	  kernel binary at runtime to a different virtual address than the
>  	  address it was linked at.
>
> +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/kernel/head.S b/arch/loongarch/kernel/head.S
> index 499edc80d8ab..b12f459ad73a 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -87,10 +87,22 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
>  	set_saved_sp	sp, t0, t1
>
>  #ifdef CONFIG_RELOCATABLE
> +#ifdef CONFIG_RANDOMIZE_BASE
> +	bl		do_kaslr
> +
> +	/* Repoint the sp into the new kernel image */
> +	PTR_LI		sp, (_THREAD_SIZE - PT_SIZE)
> +	PTR_ADD		sp, sp, tp
> +	set_saved_sp	sp, t0, t1
> +
> +	/* do_kaslr returns the new kernel image entry point */
> +	jr		a0
> +	ASM_BUG()
> +#else
>  	/* Apply the relocations */
>  	bl		relocate_kernel
>  #endif
> -
> +#endif
>  	bl		start_kernel
>  	ASM_BUG()
>
> diff --git a/arch/loongarch/kernel/relocate.c b/arch/loongarch/kernel/relocate.c
> index 91ce92433bab..5266f23a3006 100644
> --- a/arch/loongarch/kernel/relocate.c
> +++ b/arch/loongarch/kernel/relocate.c
> @@ -9,19 +9,21 @@
>  #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/sections.h>
>
>  #define RELOCATED(x) ((void *)((long)x + reloc_offset))
> +#define RELOCATED_KASLR(x) ((void *)((long)x + offset))
>
>  extern long __rela_dyn_start;
>  extern long __rela_dyn_end;
>
>  static unsigned long reloc_offset;
>
> -void __init relocate_kernel(void)
> +static inline __init void relocate_relative(void)
>  {
> -	reloc_offset = (unsigned long)_text - VMLINUX_LOAD_ADDRESS;
> -
>  	if (reloc_offset) {
>  		Elf64_Rela *rela, *rela_end;
>  		rela = (Elf64_Rela *)&__rela_dyn_start;
> @@ -43,6 +45,141 @@ 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;
> +	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_KASLR(dest);
> +}
> +
> +static inline int __init relocation_addr_valid(void *loc_new)
> +{
> +	if ((unsigned long)loc_new & 0x00000ffff) {
> +		/* Inappropriately aligned new location */
> +		return 0;
> +	}
> +	if ((unsigned long)loc_new < (unsigned long)_end) {
> +		/* New location overlaps original kernel */
> +		return 0;
> +	}
> +	return 1;
> +}
> +
> +static inline void __init update_reloc_offset(unsigned long *addr, long offset)
> +{
> +	unsigned long *new_addr = (unsigned long *)RELOCATED_KASLR(addr);
> +
> +	*new_addr = (unsigned long)offset;
> +}
> +
> +void *__init do_kaslr(void)
> +{
> +	void *loc_new;
> +	unsigned long kernel_length;
> +	long offset = 0;
> +	/* Default to original kernel entry point */
> +	void *kernel_entry = start_kernel;
> +	char *cmdline = early_ioremap(fw_arg1, COMMAND_LINE_SIZE);
> +
> +	/* Boot command line was passed in fw_arg1 */
> +	strscpy(boot_command_line, cmdline, COMMAND_LINE_SIZE);
> +
> +	kernel_length = (long)(_end) - (long)(_text);
> +
> +	loc_new = determine_relocation_address();
> +
> +	/* Sanity check relocation address */
> +	if (relocation_addr_valid(loc_new))
> +		offset = (unsigned long)loc_new - (unsigned long)(_text);
> +
> +	if (offset) {
> +		/* Copy the kernel to it's new location */
> +		memcpy(loc_new, _text, kernel_length);
> +
> +		/* Sync the caches ready for execution of new kernel */
> +		__asm__ __volatile__ (
> +			"ibar 0 \t\n"
> +			"dbar 0 \t\n");
> +
> +		reloc_offset = (unsigned long)_text - VMLINUX_LOAD_ADDRESS;
> +		reloc_offset += offset;
> +
> +		relocate_relative();
> +
> +		/* The current thread is now within the relocated image */
> +		__current_thread_info = RELOCATED_KASLR(__current_thread_info);
> +
> +		/* Return the new kernel's entry point */
> +		kernel_entry = RELOCATED_KASLR(start_kernel);
> +
> +		update_reloc_offset(&reloc_offset, offset);
> +	}

Self review:

There is a problem with do_kaslr implementation, which will be fixed in
the next version.

> +
> +	return kernel_entry;
> +}
> +#endif
> +
> +void __init relocate_kernel(void)
> +{
> +	reloc_offset = (unsigned long)_text - VMLINUX_LOAD_ADDRESS;
> +
> +	relocate_relative();
> +}
> +
>  /*
>   * Show relocation information on panic.
>   */
>

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