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
  24 u64 __ro_after_init module_alloc_base;
  25 u16 __initdata memstart_offset_seed;
  26
  27 static __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
  46 static __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         }
  63 out:
  64         return default_cmdline;
  65 }
  66
  67 extern 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  */
  78 u64 __init kaslr_early_init(u64 dt_phys, u64 modulo_offset)
  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          * lower half of the VMALLOC area (VA_BITS - 2).
 121          * Even if we could randomize at page granularity for 16k and 64k pages,
 122          * let's always round to 2 MB so we don't interfere with the ability to
 123          * map using contiguous PTEs
 124          */
 125         mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
 126         offset = seed & mask;
 127
 128         /* use the top 16 bits to randomize the linear region */
 129         memstart_offset_seed = seed >> 48;
 130
 131         /*
 132          * The kernel Image should not extend across a 1GB/32MB/512MB alignment
 133          * boundary (for 4KB/16KB/64KB granule kernels, respectively). If this
 134          * happens, increase the KASLR offset by the size of the kernel image
 135          * rounded up by SWAPPER_BLOCK_SIZE.
 136          */
 137         if ((((u64)_text + offset + modulo_offset) >> SWAPPER_TABLE_SHIFT) !=
 138             (((u64)_end + offset + modulo_offset) >> SWAPPER_TABLE_SHIFT)) {
 139                 u64 kimg_sz = _end - _text;
 140                 offset = (offset + round_up(kimg_sz, SWAPPER_BLOCK_SIZE))
 141                                 & mask;
 142         }
 143
 144         if (IS_ENABLED(CONFIG_KASAN))
 145                 /*
 146                  * KASAN does not expect the module region to intersect the
 147                  * vmalloc region, since shadow memory is allocated for each
 148                  * module at load time, whereas the vmalloc region is shadowed
 149                  * by KASAN zero pages. So keep modules out of the vmalloc
 150                  * region if KASAN is enabled.
 151                  */
 152                 return offset;
 153
 154         if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
 155                 /*
 156                  * Randomize the module region independently from the core
 157                  * kernel. This prevents modules from leaking any information
 158                  * about the address of the kernel itself, but results in
 159                  * branches between modules and the core kernel that are
 160                  * resolved via PLTs. (Branches between modules will be
 161                  * resolved normally.)
 162                  */
 163                 module_range = VMALLOC_END - VMALLOC_START - MODULES_VSIZE;
 164                 module_alloc_base = VMALLOC_START;
 165         } else {
 166                 /*
 167                  * Randomize the module region by setting module_alloc_base to
 168                  * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
 169                  * _stext) . This guarantees that the resulting region still
 170                  * covers [_stext, _etext], and that all relative branches can
 171                  * be resolved without veneers.
 172                  */
 173                 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
 174                 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
 175         }
 176
 177         /* use the lower 21 bits to randomize the base of the module region */
 178         module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
 179         module_alloc_base &= PAGE_MASK;
 180
 181         return offset;
 182 }