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Kernel Loader

Page Discussion

The Kernel Loader ("KernelLdr"/"Kernelldr") was added in 8.0.0. It is responsible for applying relocations to the Kernel, and mapping the Kernel's .text/.rodata/.data/.bss at a random slide.

Functions

KernelLdr is called immediately by the Kernel's crt0 (after it deprivileges from EL2 to EL1, if required), with the following signature: 

   void KernelLdr_Main(uintptr_t kernel_base_address, KernelMap *kernel_map, uintptr_t ini1_base_address);

KernelLdr_Main

First, it clears BSS, and then sets SP = <BSS end>. 

    for (uint64_t *i = __bss_start; i != __bss_end; i++) {
        *i = 0;
    }
    SP = __bss_end;

Next, it applies relocations to itself and calls its init array. 

    KernelLdr_ApplyRelocations(&KernelLdr_Main, __dynamic_start);
    KernelLdr_libc_init_array();

[9.0.0+] Then it clears TPIDR_EL1 to 0, and sets VBAR_EL1. 

    // 9.0.0+
    TPIDR_EL1 = 0
    VBAR_EL1 = KernelLdr_ExceptionTable

Then, it calls the function which relocates the kernel, and jumps back to the kernel entrypoint. 

    // KernelLdr_LoadKernel returns (relocated_kernel_base - original_kernel_base).
    uintptr_t kernel_relocation_offset = KernelLdr_LoadKernel(kernel_base, kernel_map, ini_base);
    
    // finalize called for static page allocator.
    g_InitialPageAllocator.Finalize();
    
    // Jumps back to the kernel code that called KernelLdr_Main.
    ((void (*)(void))(kernel_relocation_offset + LR))();

KernelLdr_ApplyRelocations

This does standard ELF relocation using .dynamic.

First, it iterates over all entries in .dynamic, extracting .rel.dyn, .rela.dyn, relent, relatent, relcount, relacount from the relevant entries.

Then it does the following two loops to apply R_AARCH64_RELATIVE relocations: 

    for (size_t i = 0; i < rel_count; i++) {
        const Elf64_Rel *rel = dyn_rel_start + rel_ent * i;
        while (uint32_t(rel->r_info) != R_AARCH64_RELATIVE) { /* Invalid entry, infloops */ }
        *((Elf64_Addr *)(base_address + rel->r_offset)) += base_address;
    }

    for (size_t i = 0; i < rela_count; i++) {
        const Elf64_Rela *rela = dyn_rela_start + rela_ent * i;
        while (uint32_t(rela->r_info) != R_AARCH64_RELATIVE) { /* Invalid entry, infloops */ }
        *((Elf64_Addr *)(base_address + rela->r_offset)) = base_address + rela->r_addend;
    }

KernelLdr_libc_init_array()

This is just standard libc init array code. .init_array is empty in all available binaries.

KernelLdr_LoadKernel

First, it backs up the original kernel base, and then relocates the kernel physically to the upper half of DRAM if enough memory is available. 

    // Backup kernel_base argument for use later
    original_kernel_base = kernel_base;
    
    // Move kernel elsewhere in DRAM if needed (unused in practice?)
    // This is maybe to support reserving unused memory for a second OS/hypervisor?
    KernelLdr_RelocateKernelPhysically(&kernel_base, &kernel_map);

Then it checks all of the kernel map's offsets (and the kernel base) for page alignment. 

    // Read offsets from the kernel map, save on stack.
    text_offset           = kernel_map->text_offset;
    text_end_offset       = kernel_map->text_end_offset;
    ro_offset             = kernel_map->ro_offset;
    ro_end_offset         = kernel_map->ro_end_offset;
    rw_offset             = kernel_map->rw_offset;
    rw_end_offset         = kernel_map->rw_end_offset;
    bss_offset            = kernel_map->bss_offset;
    ini1_end_offset       = kernel_map->ini1_end_offset;
    dynamic_offset        = kernel_map->dynamic_offset;
    init_array_offset     = kernel_map->init_array_offset;
    init_array_end_offset = kernel_map->init_array_end_offset;

    // Check all offsets are appropriately aligned.
    while (kernel_base & 0xFFF) { }
    while (text_offset & 0xFFF) { }
    while (text_end_offset & 0xFFF) { }
    while (ro_offset & 0xFFF) { }
    while (ro_end_offset & 0xFFF) { }
    while (rw_offset & 0xFFF) { }
    while (rw_end_offset & 0xFFF) { }

Next, it relocates the INI1 to its appropriate load address. 

    // If configured to do so, an extra 0x68000 bytes will be reserved for kernel usage.
    reserved_kernel_data_size = KernelLdr_ShouldReserveAdditionalKernelData() ? 0x1790000 : 0x1728000;

    // Calculate address at which to place INI1.
    ini1_end_address   = kernel_base + ini1_end_offset + reserved_kernel_data_size;
    ini1_load_address = ini1_end_address - 0xC00000;

    // Relocate INI1 if destination address isn't the input argument address
    if (ini1_load_address != ini1_address) {
        // Validate INI1 binary has correct magic and valid size.
        INI1Header *ini = (INI1Header *)ini1_address;
        if (ini->magic == MAGIC_INI1 && ini->size <= 0xC00000) {
            memmove(ini1_load_address, ini1_address, ini->size); // NOTE: No ToCToU, ini1->size is cached on stack.
        } else {
            // Invalid INI, place invalid header at load address. This will cause Kernel Panic later.
            memset(ini1_load_address, 0, sizeof(INI1Header));
        }
    }

Next, it initializes the MMU with a basic identity mapping for Kernel + KernelLdr. 

    // Set page table region
    page_table_region = ini1_end_address;
    page_table_region_size = 0x200000;
    g_InitialPageAllocator.Initialize(page_table_region);

    // Initialize new page table, eventually ends up in TTBR1_EL1.
    KInitialPageTable ttbr1_page_table(&g_InitialPageAllocator);

    // Setup MMU with initial identity mapping.
    KernelLdr_MapInitialIdentityMapping(&ttbr1_page_table, kernel_base, rw_end_offset, page_table_region, page_table_region_size, &g_InitialPageAllocator);

Next, it generates a random KASLR slide for the Kernel. 

    // Repeatedly try to generate a random slide
    while (true) {
        // Get random value from secure monitor in range
        // This is "probably" KSystemControl::GenerateRandomRange, as in normal kernel
        // However, it's unclear whether KSystemControl is actually included, or whether this is just copy/pasted?
        random_kaslr_slide = KernelLdr_GenerateRandomRange(0xFFFFFF8000000000, 0xFFFFFFFFFFDFFFFF);
        aligned_random_kaslr_slide = random_kaslr_slide & 0xFFFFFFFFFFE00000;
        
        // Calculate end address for kernel with this slide, rounding up.
        random_kernel_end = aligned_random_kaslr_slide + (kernel_base & 0x1FFFFF) + rw_end_offset + 0x1FFFFF) & 0x1FFE00000;
       
        // Validate no overflow, and that the kernel will fit with the slide.
        if (aligned_random_kaslr_slide >= random_kaslr_end || ((random_kaslr_end - 1) > 0xFFFFFFFFFFDFFFFF)) {
            continue;
        }

        // Validate we can map this range without conflicts.
        // NOTE: This is inlined, but code looks same as in older kernel binaries.
        if (!ttbr1_page_table.IsFree(aligned_random_kaslr_slide, random_kernel_end - aligned_random_kaslr_slide)) {
            continue;
        }

        // Valid kaslr slide, so we're done.
        break;
    }
    final_virtual_kernel_base = aligned_random_kaslr_slide | (kernel_base & 0x1FFFFF);

Then, it maps the kernel at the final virtual address. 

    // Maps .text as R-X
    attribute = 0x40000000000788;
    ttbr1_page_table.Map(final_virtual_kernel_base + text_offset, text_end_offset - text_offset, kernel_base + text_offset, &attribute, &g_InitialPageAllocator);
    
    // Maps .rodata as R--
    attribute = 0x60000000000788;

    // 9.0.0+
    {
        // On 9.0.0+, .rodata is initially RW- to facilitate .rel.ro.
        attribute = 0x60000000000708;
    }

    ttbr1_page_table.Map(final_virtual_kernel_base + ro_offset, ro_end_offset - ro_offset, kernel_base + ro_offset, &attribute, &g_InitialPageAllocator);

    // Maps .rwdata and .bss as RW-
    attribute = 0x60000000000708;
    ttbr1_page_table.Map(final_virtual_kernel_base + rw_offset, rw_end_offset - rw_offset, kernel_base + rw_offset, &attribute, &g_InitialPageAllocator);

    // Clears BSS.
    memset(final_kernel_virtual_base + bss_offset, 0, rw_end_offset - bss_offset);

Then, it applies the kernel's .dynamic relocations and calls the kernel's libc .init_array functions. 

    // Applies all R_AARCH64_RELATIVE relocations.
    KernelLdr_ApplyRelocations(final_kernel_virtual_base, final_kernel_virtual_base + dynamic_offset);

    // 9.0.0+: Reprotects .rodata as R--.
    ttbr1_page_table.ReprotectToReadOnly(final_virtual_kernel_base + ro_offset, ro_end_offset - ro_offset);
    
    // This is standard libc init_array code, but called for the kernel's binary instead of kernelldr's.
    for (uintptr_t cur_func = final_virtual_kernel_base + init_array_offset; cur_func < final_virtual_kernel_base + init_array_end_offset; cur_func += 8) {
        ((void (*)(void))(*(uint64_t *)cur_func)();
    }

Finally, it returns the difference between the kernel's original physical base address and the relocated kaslr'd virtual base address. 

    return final_virtual_kernel_base - original_kernel_base;

KernelLdr_MapInitialIdentityMapping

Signature is like 

   void KernelLdr_MapInitialIdentityMapping(KInitialPageTable *ttbr1_page_table, uintptr_t kernel_base, uintptr_t kernel_size, 
                                            uintptr_t page_tables_base, uintptr_t page_tables_size, InitialPageAllocator *allocator);


First, this creates a new page table (eventually ends up in TTBR0_EL1), and adds identity mappings for Kernel, KernelLdr, and the Page Table region to it. 

    // Create new KInitialPageTable
    KInitialPageTable ttbr0_page_table(allocator);

    // Maps kernel with RWX identity mapping.
    attribute = 0x40000000000708;
    ttbr0_page_table.Map(kernel_base, kernel_size, kernel_base, &attribute, allocator);

    // Maps kernel loader with RWX identity mapping.
    attribute = 0x40000000000708;
    ttbr0_page_table.Map(__start, __end - __start, __start, &attribute, allocator);

    // Maps page table region with RW- identity mapping.
    attribute = 0x60000000000708;
    ttbr0_page_table.Map(page_tables_base, page_tables_size, page_tables_base, &attribute, allocator);

Next, this sets some system registers. 

    // Set TTBR0/TTBR1 with initial page tables.
    TTBR0_EL1 = ttbr0_page_table.GetL1Table();
    TTBR1_EL1 = ttbr1_page_table->GetL1Table();
    
    // Configure MAIR, TCR. TODO: Document here what bits these are.
    MAIR_EL1 = 0x44FF0400;
    TCR_EL1  = 0x11B5193519;

    // Check what CPU we're running on to configure CPUECTLR, CPUACTLR appropriately.
    manufacture_id = MIDR_EL1;
    implementer = manufacturer_id >> 24) & 0xFF;
    
    // 9.0.0+: Save X19-X30 + SP, save context struct in TPIDR_EL1.
    KernelLdr_SaveRegistersToTpidrEl1();

    if (implementer == 0x41) {
        // Implementer ID is 0x41 (ARM Limited).
        architecture = (manufacture_id >> 4)  & 0x0FFF;
        hw_variant   = (manufacture_id >> 20) & 0xF;
        hw_revision  = (manufacture_id >> 0)  & 0xF;
        if (architecture == 0xD07) {
            // Architecture is 0xD07 (Cortex-A57).
            cpuactlr_value = 0x1000000;    // Non-cacheable load forwarding enabled
            cpuectlr_value = 0x1B00000040; // Enable the processor to receive instruction cache and TLB maintenance operations broadcast from other processors in the cluster; set the L2 load/store data prefetch distance to 8 requests; set the L2 instruction fetch prefetch distance to 3 requests.
            if (hw_variant == 0 || (hw_variant == 1 && hw_revision <= 1)) {
                // If supported, disable load-pass DMB.
                cpuactlr_value |= 0x800000000000000;
            }
            CPUACTLR_EL1 = cpuactlr_value;
            if (CPUECTLR_EL1 != cpuectlr_value) {
                CPUECTLR_EL1 = cpuectlr_value;
            }
        } else if (architecture == 0xD03) { // 9.0.0+
            // Architecture is 0xD03 (Cortex-A53).
            cpuactlr_value = 0x90CA000; // Set L1 data prefetch control to allow 5 outstanding prefetches; enable device split throttle; set the number of independent data prefetch streams to 2; disable transient and no-read-allocate hints for loads; set write streaming no-allocate threshold so the 128th consecutive streaming cache line does not allocate in the L1 or L2 cache.
            cpuectlr_value = 0x40;      // Enable hardware management of data coherency with other cores in the cluster.
            if (hw_variant != 0 || (hw_variant == 0 && hw_revision > 2)) {
                // If supported, enable data cache clean as data cache clean/invalidate.
                cpuactlr_value |= 0x100000000000;
            }
            CPUACTLR_EL1 = cpuactlr_value;
            if (CPUECTLR_EL1 != cpuectlr_value) {
                CPUECTLR_EL1 = cpuectlr_value;
            }
        }
    }


    // 9.0.0+: Verify that TPIDR_EL1 is still set.
    KernelLdr_VerifyTpidrEl1();

Next, the cache is flushed, to ensure that page tables will be successfully read once the MMU is enabled. 

    KernelLdr_EnsureCacheFlushed();

Finally, SCTLR is written to, enabling the MMU. 

    SCTLR_EL1 = 0x34D5D925;
    __dsb_sy();
    __isb();

KernelLdr_RelocateKernelPhysically

This retrieves memory layout information from the secure monitor, and adjusts the kernel's physical location if necessary. 

    adjusted_kernel_base = KernelLdr_GetAdjustedKernelPhysicalBase(*p_kernel_base);

    if (adjusted_kernel_base != *p_kernel_base) {
        // Copy data to adjusted destination
        memmove(adjusted_kernel_base, *p_kernel_base, (*p_kernel_map)->data_end_offset);

        // Adjust pointers.
        kernel_base_diff = adjusted_kernel_base - *p_kernel_base;
        *p_kernel_base = (uintptr_t)*p_kernel_base + kernel_base_diff;
        *p_kernel_map  = (uintptr_t)*p_kernel_map  + kernel_base_diff;
    }


KernelLdr_GetAdjustedKernelPhysicalBase

This sees how much more memory is available than expected, and relocates the kernel accordingly.

Note: Panic (infloop) happens on any smc call error, this isn't depicted in pseudocode for brevity reasons. 

    // Gets DRAM size information from Memory Controller
    dram_size_from_mc = (smc_read_write_register(MC_EMEM_CFG, 0, 0) & 0x3FFF) << 20;
    
    // Gets DRAM size information from Secure Monitor KernelConfiguration
    memory_type = (smc_get_config(ConfigItem_KernelConfiguration) >> 16) & 3;
    switch (memory_type) {
        case MemoryType_4GB: // 0
        default:
            dram_size_from_kernel_cfg = 0x100000000;
            break;
        case MemoryType_6GB: // 1
            dram_size_from_kernel_cfg = 0x180000000;
            break;
        case MemoryType_8GB: // 2
            dram_size_from_kernel_cfg = 0x200000000;
            break;
    }
    
    // On normal systems, these should be equal (and kernel will not be relocated).
    if (dram_size_from_mc < 2 * dram_size_from_kernel_cfg) {
        return kernel_base + (dram_size_from_mc - dram_size_from_kernel_cfg) / 2;
    } else {
        return kernel_base;
    }

KernelLdr_ShouldReserveAdditionalKernelData

This just gets a flag from the KernelConfiguration.

Note: Panic (infloop) happens on any smc call error, this isn't depicted in pseudocode for brevity reasons. 

    return (smc_get_config(ConfigItem_KernelConfiguration) >> 3) & 1;

KernelLdr_GenerateRandomRange

This uses entropy from the secure monitor to generate a random value in a range (inclusive). 

    range_size   = (range_end + 1 - range_start);
    random_value = smc_generate_random_bytes(8);
    random_value -= random_value / range_size * range_size;
    return range_start + random_value;

KernelLdr_EnsureCacheFlushed

Note: this is inlined, however it uses instructions that no compiler has intrinsics for (and looks like hand-written asm), so it's presumably its own thing. 

    // Invalidate Local Cache
    KernelLdr_InvalidateCacheLocal();
    __dsb_sy();

    // Invalidate Share
    KernelLdr_InvalidateCacheShared();
    __dsb_sy();

    // Invalidate Local Cache again
    KernelLdr_InvalidateCacheLocal();
    __dsb_sy();
    
    // asm { tlbi vmalle1is; }
    __dsb_sy();
    __isb();

KernelLdr_InvalidateCacheLocal

Standard ARM cache clean code, uses LoUIS + LoC from CLIDR_EL1.

KernelLdr_InvalidateCacheShared

Standard ARM cache clean code, uses LoUIS from CLIDR_EL1.

KernelLdr_ExceptionTable

Standard aarch64 exception table, only function that doesn't infinite loop is synchronous exception from same EL (synch_spx_exception)

synch_spx_exception does the following:

  • Moves TPIDR_EL1 into X0
  • Infinite loops if it is 0/NULL.
  • Restores X19-X30 + SP from the memory pointed to by TPIDR_EL1.
  • Returns to the saved LR stored in the context save struct.

KernelLdr_SaveRegistersToTpidrEl1

This saves X19-X30 + SP to an input pointer, and moves the pointer into TPIDR_EL1.

KernelLdr_VerifyTpidrEl1

This just verifies that TPIDR_EL1 is equal to an input argument, and clears it. 

    // 9.0.0+
    if (TPIDR_EL1 != input_arg) {
        while (1) { /* Infinite loop panic */ }
    }
    TPIDR_EL1 = 0

KInitialPageAllocator::KInitialPageAllocator

This sets the allocator's next address to 0 (guessed, since this is done statically in KernelLoader). 

    constexpr KInitialPageAllocator::KInitialPageAllocator : next_address(0) {}

KInitialPageAllocator::Initialize

This sets the allocator's next address (function inferred as it is (presumably) inlined and next_address is (presumably) private). 

    this->next_address = address;

KInitialPageAllocator::Finalize

This just clears the allocator's next address. 

    this->next_address = 0;

KInitialPageAllocator::Allocate

This linearly allocates a page. 

    virtual void *KInitialPageAllocator::Allocate() {
        void *address = reinterpret_cast<void *>(this->next_address);
        if (address == nullptr) {
            // If called on uninitialized allocator, panic by infinite looping
            while (true) {}
        }
        this->next_address += 0x1000;
        memset(address, 0, 0x1000);
        return address;
    }

KInitialPageAllocator::Free

This frees a page (implemented as noop in KernelLoader) 

    virtual void KInitialPageAllocator::Free(void *address) {
        // Does Nothing
    }

KInitialPageTable::KInitialPageTable

NOTE: This constructor is inferred. 

KInitialPageTable::KInitialPageTable(KInitialPageAllocator *allocator) {
    this->l1_table_ptr = allocator->Allocate();
    memset(this->l1_table_ptr, 0, 0x1000);
    this->num_l1_table_entries = 0x200;
}

KInitialPageTable::Map

Signature is like 

   KInitialPageTable::Map(uintptr_t virtual_address, size_t size, uintptr_t physical_address, const uint64_t *attribute, InitialPageAllocator *allocator);

This is just standard aarch64 page table mapping code. New L2/L3 pages are allocated via allocator->Allocate() when needed.

KInitialPageTable::IsFree

This is just standard aarch64 page table code. Walks the page table, verifying that all entries it would map for size + range are free.

KInitialPageTable::ReprotectToReadOnly

This is just standard aarch64 page table code. Walks the page table, reprotects the read-write pages in the specified region as read-only.

This is probably a compiler-optimized version of a function that does an arbitrary reprotection.

KInitialPageTable::GetL1Table

This is an inferred getter for a (presumably) private member. 


    void *KInitialPageTable::GetL1Table() const {
        return this->l1_table_ptr;
    }

Structures

KernelMap

Offset Size Description
0x0 4 .text offset
0x4 4 .text end offset
0x8 4 .rodata end offset
0xC 4 .rodata end offset
0x10 4 .rwdata offset
0x14 4 .rwdata end offset
0x18 4 .bss offset
0x1C 4 .bss end offset
0x20 4 INI1 end offset
0x24 4 .dynamic end offset
0x28 4 .init_array end offset
0x2C 4 .init_array end offset

KInitialPageAllocator

KInitialPageAllocator is just a simple linear allocator.

Offset Size Description
0x0 8 vtable;
0x8 8 Next Address;

KInitialPageAllocator::vtable

Offset Size Description
0x0 8 void *(*Allocate)(KInitialPageAllocator *this);
0x8 8 void (*Free)(KInitialPageAllocator *this, void *address);

KInitialPageTable

KInitialPageTable is a very, very stripped-down KPageTable.

Compared to pre-KernelLoader KInitialPageTable, it has slightly reduced memory footprint.

Offset Size Description
0x0 8 Pointer to L1 Table;
0x8 8 Number of L1 Table Entries (Normally 0x200);
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