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
  23 u64 __read_mostly module_alloc_base;
  24 u16 __initdata memstart_offset_seed;
  25
  26 static __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
  45 static __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         }
  62 out:
  63         return default_cmdline;
  64 }
  65
  66 extern 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  */
  77 u64 __init kaslr_early_init(u64 dt_phys, u64 modulo_offset)
  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 + modulo_offset) >> SWAPPER_TABLE_SHIFT) !=
 136             (((u64)_end + offset + modulo_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 }