Re: [PATCH RFC 4/8] riscv/kaslr: randomize the kernel image offset

[Alex Ghiti]

On 3/24/20 3:30 AM, Zong Li wrote:
> Entropy is derived from the banner and timer, it is better than nothing
> but not enough secure, so previous stage may pass entropy via the device
> tree /chosen/kaslr-seed node.
>
> We limit randomization range within 1GB, so we can exploit early page
> table to map new destination of kernel image. Additionally, the kernel
> offset need 2M alignment to ensure it's good in PMD page table.
>
> We also checks the kernel offset whether it's safe by avoiding to
> overlaps with dtb, initrd and reserved memory regions.

That maybe changes the way my sv48 patchset will be implemented: I can't 
get user preference (3-level or 4-level) by any means, device-tree or 
kernel parameter.

But I don't see how you could get a random offset without info from the 
device tree anyway (reserved memory regions especially), so maybe I 
could parse dtb for allowing the user to choose. I'll move this 
discussion to the sv48 introduction.

> Signed-off-by: Zong Li <zong.li@xxxxxxxxxx>
> ---
>   arch/riscv/kernel/kaslr.c | 274 +++++++++++++++++++++++++++++++++++++-
>   arch/riscv/mm/init.c      |   2 +-
>   2 files changed, 273 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/kernel/kaslr.c b/arch/riscv/kernel/kaslr.c
> index 281b5fcca5c8..9ec2b608eb7f 100644
> --- a/arch/riscv/kernel/kaslr.c
> +++ b/arch/riscv/kernel/kaslr.c
> @@ -11,23 +11,293 @@
>   #include <asm/cacheflush.h>
>   
>   extern char _start[], _end[];
> +extern void *dtb_early_va;
> +extern phys_addr_t dtb_early_pa;
>   extern void secondary_random_target(void);
>   extern void kaslr_create_page_table(uintptr_t start, uintptr_t end);
>   
>   uintptr_t secondary_next_target __initdata;
>   static uintptr_t kaslr_offset __initdata;
>   
> +static const __init u32 *get_reg_address(int root_cells,
> +					 const u32 *value, u64 *result)
> +{
> +	int cell;
> +	*result = 0;
> +
> +	for (cell = root_cells; cell > 0; --cell)
> +		*result = (*result << 32) + fdt32_to_cpu(*value++);
> +
> +	return value;
> +}
> +
> +static __init int get_node_addr_size_cells(const char *path, int *addr_cell,
> +					   int *size_cell)
> +{
> +	int node = fdt_path_offset(dtb_early_va, path);
> +	fdt64_t *prop;
> +
> +	if (node < 0)
> +		return -EINVAL;
> +
> +	prop = fdt_getprop_w(dtb_early_va, node, "#address-cells", NULL);
> +	if (!prop)
> +		return -EINVAL;
> +	*addr_cell = fdt32_to_cpu(*prop);
> +
> +	prop = fdt_getprop_w(dtb_early_va, node, "#size-cells", NULL);
> +	if (!prop)
> +		return -EINVAL;
> +	*size_cell = fdt32_to_cpu(*prop);
> +
> +	return node;
> +}
> +
> +static __init void kaslr_get_mem_info(uintptr_t *mem_start,
> +				      uintptr_t *mem_size)
> +{
> +	int node, root, addr_cells, size_cells;
> +	u64 base, size;
> +
> +	/* Get root node's address cells and size cells. */
> +	root = get_node_addr_size_cells("/", &addr_cells, &size_cells);
> +	if (root < 0)
> +		return;
> +
> +	/* Get memory base address and size. */
> +	fdt_for_each_subnode(node, dtb_early_va, root) {
> +		const char *dev_type;
> +		const u32 *reg;
> +
> +		dev_type = fdt_getprop(dtb_early_va, node, "device_type", NULL);
> +		if (!dev_type)
> +			continue;
> +
> +		if (!strcmp(dev_type, "memory")) {
> +			reg = fdt_getprop(dtb_early_va, node, "reg", NULL);
> +			if (!reg)
> +				return;
> +
> +			reg = get_reg_address(addr_cells, reg, &base);
> +			reg = get_reg_address(size_cells, reg, &size);
> +
> +			*mem_start = base;
> +			*mem_size = size;
> +
> +			break;
> +		}
> +	}
> +}
> +
> +/* Return a default seed if there is no HW generator. */
> +static u64 kaslr_default_seed = ULL(-1);
> +static __init u64 kaslr_get_seed(void)
> +{
> +	int node, len;
> +	fdt64_t *prop;
> +	u64 ret;
> +
> +	node = fdt_path_offset(dtb_early_va, "/chosen");
> +	if (node < 0)
> +		return kaslr_default_seed++;
> +
> +	prop = fdt_getprop_w(dtb_early_va, node, "kaslr-seed", &len);
> +	if (!prop || len != sizeof(u64))
> +		return kaslr_default_seed++;
> +
> +	ret = fdt64_to_cpu(*prop);
> +
> +	/* Re-write to zero for checking whether get seed at second time */
> +	*prop = 0;
> +
> +	return ret;
> +}
> +
> +static __init bool is_overlap(uintptr_t s1, uintptr_t e1, uintptr_t s2,
> +			      uintptr_t e2)
> +{
> +	return e1 >= s2 && e2 >= s1;
> +}

Inline this function or use a macro maybe.

> +
> +static __init bool is_overlap_reserved_mem(uintptr_t start_addr,
> +					   uintptr_t end_addr)
> +{
> +	int node, rsv_mem, addr_cells, size_cells;
> +
> +	/* Get the reserved-memory node. */
> +	rsv_mem = get_node_addr_size_cells("/reserved-memory",
> +					   &addr_cells,
> +					   &size_cells);
> +	if (rsv_mem < 0)
> +		return false;
> +
> +	/* Get memory base address and size. */
> +	fdt_for_each_subnode(node, dtb_early_va, rsv_mem) {
> +		uint64_t base, size;
> +		const uint32_t *reg;
> +
> +		reg = fdt_getprop(dtb_early_va, node, "reg", NULL);
> +		if (!reg)
> +			return 0;
> +
> +		reg = get_reg_address(addr_cells, reg, &base);
> +		reg = get_reg_address(size_cells, reg, &size);
> +
> +		if (is_overlap(start_addr, end_addr, base, base + size))
> +			return true;
> +	}
> +
> +	return false;
> +}
> +
> +static __init bool is_overlap_initrd(uintptr_t start_addr, uintptr_t end_addr)
> +{
> +	int node;
> +	uintptr_t initrd_start, initrd_end;
> +	fdt64_t *prop;
> +
> +	node = fdt_path_offset(dtb_early_va, "/chosen");
> +	if (node < 0)
> +		return false;
> +
> +	prop = fdt_getprop_w(dtb_early_va, node, "linux,initrd-start", NULL);
> +	if (!prop)
> +		return false;
> +
> +	initrd_start = fdt64_to_cpu(*prop);
> +
> +	prop = fdt_getprop_w(dtb_early_va, node, "linux,initrd-end", NULL);
> +	if (!prop)
> +		return false;
> +
> +	initrd_end = fdt64_to_cpu(*prop);
> +
> +	return is_overlap(start_addr, end_addr, initrd_start, initrd_end);
> +}
> +
> +static __init bool is_overlap_dtb(uintptr_t start_addr, uintptr_t end_addr)
> +{
> +	uintptr_t dtb_start = dtb_early_pa;
> +	uintptr_t dtb_end = dtb_start + fdt_totalsize(dtb_early_va);
> +
> +	return is_overlap(start_addr, end_addr, dtb_start, dtb_end);
> +}
> +
> +static __init bool has_regions_overlapping(uintptr_t start_addr,
> +					   uintptr_t end_addr)
> +{
> +	if (is_overlap_dtb(start_addr, end_addr))
> +		return true;
> +
> +	if (is_overlap_initrd(start_addr, end_addr))
> +		return true;
> +
> +	if (is_overlap_reserved_mem(start_addr, end_addr))
> +		return true;
> +
> +	return false;
> +}
> +
> +static inline __init unsigned long get_legal_offset(int random_index,
> +						    int max_index,
> +						    uintptr_t mem_start,
> +						    uintptr_t kernel_size)
> +{
> +	uintptr_t start_addr, end_addr;
> +	int idx, stop_idx;
> +
> +	idx = stop_idx = random_index;
> +
> +	do {
> +		start_addr = mem_start + idx * SZ_2M + kernel_size;
> +		end_addr = start_addr + kernel_size;
> +
> +		/* Check overlap to other regions. */
> +		if (!has_regions_overlapping(start_addr, end_addr))
> +			return idx * SZ_2M + kernel_size;
> +
> +		if (idx-- < 0)
> +			idx = max_index;

Isn't the fallback to max_index a security breach ? Because at some 
point, the kernel will be loaded at this specific address.

> +
> +	} while (idx != stop_idx);
> +
> +	return 0;
> +}
> +
> +static inline __init u64 rotate_xor(u64 hash, const void *area, size_t size)
> +{
> +	size_t i;
> +	uintptr_t *ptr = (uintptr_t *) 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;
> +}
> +
> +#define MEM_RESERVE_START	__pa(PAGE_OFFSET)
> +static __init uintptr_t get_random_offset(u64 seed, uintptr_t kernel_size)
> +{
> +	uintptr_t mem_start = 0, mem_size= 0, random_size;
> +	uintptr_t kernel_size_align = round_up(kernel_size, SZ_2M);
> +	int index;
> +	u64 random = 0;
> +	cycles_t time_base;
> +
> +	/* Attempt to create a simple but unpredictable starting entropy */
> +	random = rotate_xor(random, linux_banner, strlen(linux_banner));
> +
> +	/*
> +	 * If there is no HW random number generator, use timer to get a random
> +	 * number. This is better than nothing but not enough secure.
> +	 */
> +	time_base = get_cycles() << 32;
> +	time_base ^= get_cycles();
> +	random = rotate_xor(random, &time_base, sizeof(time_base));
> +
> +	if (seed)
> +		random = rotate_xor(random, &seed, sizeof(seed));
> +
> +	kaslr_get_mem_info(&mem_start, &mem_size);
> +	if (!mem_size)
> +		return 0;
> +
> +	if (mem_start < MEM_RESERVE_START) {
> +		mem_size -= MEM_RESERVE_START - mem_start;
> +		mem_start = MEM_RESERVE_START;
> +	}
> +
> +	/*
> +	 * Limit randomization range within 1G, so we can exploit
> +	 * early_pmd/early_pte during early page table phase.
> +	 */
> +	random_size = min_t(u64,
> +			    mem_size - (kernel_size_align * 2),
> +			    SZ_1G - (kernel_size_align * 2));

pgdir size is 30 bits in sv39, but it's 39 bits in sv48, you should use 
PGDIR_SIZE macro here.

> +
> +	/* The index of 2M block in whole avaliable region */
> +	index = random % (random_size / SZ_2M);
> +
> +	return get_legal_offset(index, random_size / SZ_2M,
> +				mem_start, kernel_size_align);
> +}
> +
>   uintptr_t __init kaslr_early_init(void)
>   {
> +	u64 seed;
>   	uintptr_t dest_start, dest_end;
>   	uintptr_t kernel_size = (uintptr_t) _end - (uintptr_t) _start;
>   
>   	/* Get zero value at second time to avoid doing randomization again. */
> -	if (kaslr_offset)
> +	seed = kaslr_get_seed();
> +	if (!seed)
>   		return 0;
>   
>   	/* Get the random number for kaslr offset. */
> -	kaslr_offset = 0x10000000;
> +	kaslr_offset = get_random_offset(seed, kernel_size);
>   
>   	/* Update kernel_virt_addr for get_kaslr_offset. */
>   	kernel_virt_addr += kaslr_offset;
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 2f5b25f02b6c..34c6ecf2c599 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -125,7 +125,7 @@ static void __init setup_initrd(void)
>   }
>   #endif /* CONFIG_BLK_DEV_INITRD */
>   
> -static phys_addr_t dtb_early_pa __initdata;
> +phys_addr_t dtb_early_pa __initdata;
>   
>   void __init setup_bootmem(void)
>   {

Alex