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/cache.h>
  10#include <linux/crc32.h>
  11#include <linux/init.h>
  12#include <linux/libfdt.h>
  13#include <linux/mm_types.h>
  14#include <linux/sched.h>
  15#include <linux/types.h>
  16
  17#include <asm/fixmap.h>
  18#include <asm/kernel-pgtable.h>
  19#include <asm/memory.h>
  20#include <asm/mmu.h>
  21#include <asm/pgtable.h>
  22#include <asm/sections.h>
  23
  24u64 __ro_after_init module_alloc_base;
  25u16 __initdata memstart_offset_seed;
  26
  27static __init u64 get_kaslr_seed(void *fdt)
  28{
  29        int node, len;
  30        fdt64_t *prop;
  31        u64 ret;
  32
  33        node = fdt_path_offset(fdt, "/chosen");
  34        if (node < 0)
  35                return 0;
  36
  37        prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
  38        if (!prop || len != sizeof(u64))
  39                return 0;
  40
  41        ret = fdt64_to_cpu(*prop);
  42        *prop = 0;
  43        return ret;
  44}
  45
  46static __init const u8 *get_cmdline(void *fdt)
  47{
  48        static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
  49
  50        if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
  51                int node;
  52                const u8 *prop;
  53
  54                node = fdt_path_offset(fdt, "/chosen");
  55                if (node < 0)
  56                        goto out;
  57
  58                prop = fdt_getprop(fdt, node, "bootargs", NULL);
  59                if (!prop)
  60                        goto out;
  61                return prop;
  62        }
  63out:
  64        return default_cmdline;
  65}
  66
  67extern void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size,
  68                                       pgprot_t prot);
  69
  70/*
  71 * This routine will be executed with the kernel mapped at its default virtual
  72 * address, and if it returns successfully, the kernel will be remapped, and
  73 * start_kernel() will be executed from a randomized virtual offset. The
  74 * relocation will result in all absolute references (e.g., static variables
  75 * containing function pointers) to be reinitialized, and zero-initialized
  76 * .bss variables will be reset to 0.
  77 */
  78u64 __init kaslr_early_init(u64 dt_phys)
  79{
  80        void *fdt;
  81        u64 seed, offset, mask, module_range;
  82        const u8 *cmdline, *str;
  83        int size;
  84
  85        /*
  86         * Set a reasonable default for module_alloc_base in case
  87         * we end up running with module randomization disabled.
  88         */
  89        module_alloc_base = (u64)_etext - MODULES_VSIZE;
  90
  91        /*
  92         * Try to map the FDT early. If this fails, we simply bail,
  93         * and proceed with KASLR disabled. We will make another
  94         * attempt at mapping the FDT in setup_machine()
  95         */
  96        early_fixmap_init();
  97        fdt = __fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
  98        if (!fdt)
  99                return 0;
 100
 101        /*
 102         * Retrieve (and wipe) the seed from the FDT
 103         */
 104        seed = get_kaslr_seed(fdt);
 105        if (!seed)
 106                return 0;
 107
 108        /*
 109         * Check if 'nokaslr' appears on the command line, and
 110         * return 0 if that is the case.
 111         */
 112        cmdline = get_cmdline(fdt);
 113        str = strstr(cmdline, "nokaslr");
 114        if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
 115                return 0;
 116
 117        /*
 118         * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 119         * kernel image offset from the seed. Let's place the kernel in the
 120         * middle half of the VMALLOC area (VA_BITS - 2), and stay clear of
 121         * the lower and upper quarters to avoid colliding with other
 122         * allocations.
 123         * Even if we could randomize at page granularity for 16k and 64k pages,
 124         * let's always round to 2 MB so we don't interfere with the ability to
 125         * map using contiguous PTEs
 126         */
 127        mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
 128        offset = BIT(VA_BITS - 3) + (seed & mask);
 129
 130        /* use the top 16 bits to randomize the linear region */
 131        memstart_offset_seed = seed >> 48;
 132
 133        if (IS_ENABLED(CONFIG_KASAN))
 134                /*
 135                 * KASAN does not expect the module region to intersect the
 136                 * vmalloc region, since shadow memory is allocated for each
 137                 * module at load time, whereas the vmalloc region is shadowed
 138                 * by KASAN zero pages. So keep modules out of the vmalloc
 139                 * region if KASAN is enabled, and put the kernel well within
 140                 * 4 GB of the module region.
 141                 */
 142                return offset % SZ_2G;
 143
 144        if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
 145                /*
 146                 * Randomize the module region over a 4 GB window covering the
 147                 * kernel. This reduces the risk of modules leaking information
 148                 * about the address of the kernel itself, but results in
 149                 * branches between modules and the core kernel that are
 150                 * resolved via PLTs. (Branches between modules will be
 151                 * resolved normally.)
 152                 */
 153                module_range = SZ_4G - (u64)(_end - _stext);
 154                module_alloc_base = max((u64)_end + offset - SZ_4G,
 155                                        (u64)MODULES_VADDR);
 156        } else {
 157                /*
 158                 * Randomize the module region by setting module_alloc_base to
 159                 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
 160                 * _stext) . This guarantees that the resulting region still
 161                 * covers [_stext, _etext], and that all relative branches can
 162                 * be resolved without veneers.
 163                 */
 164                module_range = MODULES_VSIZE - (u64)(_etext - _stext);
 165                module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
 166        }
 167
 168        /* use the lower 21 bits to randomize the base of the module region */
 169        module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
 170        module_alloc_base &= PAGE_MASK;
 171
 172        return offset;
 173}
 174