Date:   Fri, 10 Apr 2020 11:58:57 -0400
From:   Alex Ghiti <alex@...ti.fr>
To:     Zong Li <zong.li@...ive.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 4/8] riscv/kaslr: randomize the kernel image offset

Hi Zong,

On 4/9/20 6:31 AM, Zong Li wrote:
> On Thu, Apr 9, 2020 at 1:51 PM Alex Ghiti <alex@...ti.fr> wrote:
>>
>>
>>
>> On 4/7/20 6:53 AM, Zong Li wrote:
>>> On Tue, Apr 7, 2020 at 1:11 PM Alex Ghiti <alex@...ti.fr> wrote:
>>>>
>>>>
>>>> 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.
>>>
>>> Maybe I'm a little bit misunderstanding here, but I think I got the
>>> random offset through some information by parsing dtb.
>>>
>>
>> I was just saying that I may use the dtb too in sv48 patchset to make it
>> possible for users to choose sv39 even if sv48 is supported by hardware
>> (which is not the case in my current patchset).
>>
>>>>
>>>>> Signed-off-by: Zong Li <zong.li@...ive.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.
>>>
>>> Yes, sure. Thanks.
>>>
>>>>
>>>>> +
>>>>> +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.
>>>
>>> The max_index is the maximum safe index for destination of new kernel
>>> image. Could you give more explain here?
>>>
>>
>> But max_index is not random at all. I really don't know if that's a
>> problem, I just found intriguing the fact the kernel could be loaded at
>> some specific location. Would it be more secure, instead of picking
>> max_index as fallback when reaching 0, to pick another random number
>> between random_index and max_index ?
> 
> ok, I can get your point. The original idea here is that we get a
> random index first, then we decrease the index to retry to find a good
> place if there are overlapping with other regions. A bit like the ring
> buffer, the end of index traversing is not zero, but the random_index
> - 1, we might consider it as continuity, so we don't know where is the
> end point because the start point is random, whether we stop at zero
> or random_index - 1.
> 
> Pick another random number is more secure when occurring overlapping,
> but I a little bit worry that it would take very long time to retry
> many times in the worst case. for example, there is just only one
> index could fit kernel image in (except for original location). In the
> meantime, we don't need to wait the index being decreased to zero,
> because it seems to me that they are the same to stop at zero or
> random_index - 1, so if we decide to re-calculate a new random number,
> maybe we could remove the index decreasing here.

But you're right that it could take some time before converging to a 
"good" index. Maybe we could restrict the index range to indexes that we 
know for sure will be good ?

Alex

> 
>>
>> Alex
>>
>>>>
>>>>> +
>>>>> +     } 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.
>>>
>>> OK, change it in the next version. Thanks.
>>>
>>>>
>>>>> +
>>>>> +     /* 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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