LXR linux/arch/arm64/kernel/kaslr.c

   1/*
   2 * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
   3 *
   4 * This program is free software; you can redistribute it and/or modify
   5 * it under the terms of the GNU General Public License version 2 as
   6 * published by the Free Software Foundation.
   7 */
   8
   9#include <linux/crc32.h>
  10#include <linux/init.h>
  11#include <linux/libfdt.h>
  12#include <linux/mm_types.h>
  13#include <linux/sched.h>
  14#include <linux/types.h>
  15
  16#include <asm/fixmap.h>
  17#include <asm/kernel-pgtable.h>
  18#include <asm/memory.h>
  19#include <asm/mmu.h>
  20#include <asm/pgtable.h>
  21#include <asm/sections.h>
  22
  23u64 __read_mostly module_alloc_base;
  24u16 __initdata memstart_offset_seed;
  25
  26static __init u64 get_kaslr_seed(void *fdt)
  27{
  28        int node, len;
  29        u64 *prop;
  30        u64 ret;
  31
  32        node = fdt_path_offset(fdt, "/chosen");
  33        if (node < 0)
  34                return 0;
  35
  36        prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
  37        if (!prop || len != sizeof(u64))
  38                return 0;
  39
  40        ret = fdt64_to_cpu(*prop);
  41        *prop = 0;
  42        return ret;
  43}
  44
  45static __init const u8 *get_cmdline(void *fdt)
  46{
  47        static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
  48
  49        if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
  50                int node;
  51                const u8 *prop;
  52
  53                node = fdt_path_offset(fdt, "/chosen");
  54                if (node < 0)
  55                        goto out;
  56
  57                prop = fdt_getprop(fdt, node, "bootargs", NULL);
  58                if (!prop)
  59                        goto out;
  60                return prop;
  61        }
  62out:
  63        return default_cmdline;
  64}
  65
  66extern void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size,
  67                                       pgprot_t prot);
  68
  69/*
  70 * This routine will be executed with the kernel mapped at its default virtual
  71 * address, and if it returns successfully, the kernel will be remapped, and
  72 * start_kernel() will be executed from a randomized virtual offset. The
  73 * relocation will result in all absolute references (e.g., static variables
  74 * containing function pointers) to be reinitialized, and zero-initialized
  75 * .bss variables will be reset to 0.
  76 */
  77u64 __init kaslr_early_init(u64 dt_phys)
  78{
  79        void *fdt;
  80        u64 seed, offset, mask, module_range;
  81        const u8 *cmdline, *str;
  82        int size;
  83
  84        /*
  85         * Set a reasonable default for module_alloc_base in case
  86         * we end up running with module randomization disabled.
  87         */
  88        module_alloc_base = (u64)_etext - MODULES_VSIZE;
  89
  90        /*
  91         * Try to map the FDT early. If this fails, we simply bail,
  92         * and proceed with KASLR disabled. We will make another
  93         * attempt at mapping the FDT in setup_machine()
  94         */
  95        early_fixmap_init();
  96        fdt = __fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
  97        if (!fdt)
  98                return 0;
  99
 100        /*
 101         * Retrieve (and wipe) the seed from the FDT
 102         */
 103        seed = get_kaslr_seed(fdt);
 104        if (!seed)
 105                return 0;
 106
 107        /*
 108         * Check if 'nokaslr' appears on the command line, and
 109         * return 0 if that is the case.
 110         */
 111        cmdline = get_cmdline(fdt);
 112        str = strstr(cmdline, "nokaslr");
 113        if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
 114                return 0;
 115
 116        /*
 117         * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 118         * kernel image offset from the seed. Let's place the kernel in the
 119         * lower half of the VMALLOC area (VA_BITS - 2).
 120         * Even if we could randomize at page granularity for 16k and 64k pages,
 121         * let's always round to 2 MB so we don't interfere with the ability to
 122         * map using contiguous PTEs
 123         */
 124        mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
 125        offset = seed & mask;
 126
 127        /* use the top 16 bits to randomize the linear region */
 128        memstart_offset_seed = seed >> 48;
 129
 130        /*
 131         * The kernel Image should not extend across a 1GB/32MB/512MB alignment
 132         * boundary (for 4KB/16KB/64KB granule kernels, respectively). If this
 133         * happens, increase the KASLR offset by the size of the kernel image.
 134         */
 135        if ((((u64)_text + offset) >> SWAPPER_TABLE_SHIFT) !=
 136            (((u64)_end + offset) >> SWAPPER_TABLE_SHIFT))
 137                offset = (offset + (u64)(_end - _text)) & mask;
 138
 139        if (IS_ENABLED(CONFIG_KASAN))
 140                /*
 141                 * KASAN does not expect the module region to intersect the
 142                 * vmalloc region, since shadow memory is allocated for each
 143                 * module at load time, whereas the vmalloc region is shadowed
 144                 * by KASAN zero pages. So keep modules out of the vmalloc
 145                 * region if KASAN is enabled.
 146                 */
 147                return offset;
 148
 149        if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
 150                /*
 151                 * Randomize the module region independently from the core
 152                 * kernel. This prevents modules from leaking any information
 153                 * about the address of the kernel itself, but results in
 154                 * branches between modules and the core kernel that are
 155                 * resolved via PLTs. (Branches between modules will be
 156                 * resolved normally.)
 157                 */
 158                module_range = VMALLOC_END - VMALLOC_START - MODULES_VSIZE;
 159                module_alloc_base = VMALLOC_START;
 160        } else {
 161                /*
 162                 * Randomize the module region by setting module_alloc_base to
 163                 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
 164                 * _stext) . This guarantees that the resulting region still
 165                 * covers [_stext, _etext], and that all relative branches can
 166                 * be resolved without veneers.
 167                 */
 168                module_range = MODULES_VSIZE - (u64)(_etext - _stext);
 169                module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
 170        }
 171
 172        /* use the lower 21 bits to randomize the base of the module region */
 173        module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
 174        module_alloc_base &= PAGE_MASK;
 175
 176        return offset;
 177}
 178