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SubjectRe: [PATCH RFC 4/8] riscv/kaslr: randomize the kernel image offset
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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@sifive.com>
> ---
> 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

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