arch/arm64/kernel/kaslr.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
   4  */
   5
   6 #include <linux/cache.h>
   7 #include <linux/crc32.h>
   8 #include <linux/init.h>
   9 #include <linux/libfdt.h>
  10 #include <linux/mm_types.h>
  11 #include <linux/sched.h>
  12 #include <linux/types.h>
  13
  14 #include <asm/cacheflush.h>
  15 #include <asm/fixmap.h>
  16 #include <asm/kernel-pgtable.h>
  17 #include <asm/memory.h>
  18 #include <asm/mmu.h>
  19 #include <asm/pgtable.h>
  20 #include <asm/sections.h>
  21
  22 enum kaslr_status {
  23         KASLR_ENABLED,
  24         KASLR_DISABLED_CMDLINE,
  25         KASLR_DISABLED_NO_SEED,
  26         KASLR_DISABLED_FDT_REMAP,
  27 };
  28
  29 static enum kaslr_status __initdata kaslr_status;
  30 u64 __ro_after_init module_alloc_base;
  31 u16 __initdata memstart_offset_seed;
  32
  33 static __init u64 get_kaslr_seed(void *fdt)
  34 {
  35         int node, len;
  36         fdt64_t *prop;
  37         u64 ret;
  38
  39         node = fdt_path_offset(fdt, "/chosen");
  40         if (node < 0)
  41                 return 0;
  42
  43         prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
  44         if (!prop || len != sizeof(u64))
  45                 return 0;
  46
  47         ret = fdt64_to_cpu(*prop);
  48         *prop = 0;
  49         return ret;
  50 }
  51
  52 static __init const u8 *kaslr_get_cmdline(void *fdt)
  53 {
  54         static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
  55
  56         if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
  57                 int node;
  58                 const u8 *prop;
  59
  60                 node = fdt_path_offset(fdt, "/chosen");
  61                 if (node < 0)
  62                         goto out;
  63
  64                 prop = fdt_getprop(fdt, node, "bootargs", NULL);
  65                 if (!prop)
  66                         goto out;
  67                 return prop;
  68         }
  69 out:
  70         return default_cmdline;
  71 }
  72
  73 /*
  74  * This routine will be executed with the kernel mapped at its default virtual
  75  * address, and if it returns successfully, the kernel will be remapped, and
  76  * start_kernel() will be executed from a randomized virtual offset. The
  77  * relocation will result in all absolute references (e.g., static variables
  78  * containing function pointers) to be reinitialized, and zero-initialized
  79  * .bss variables will be reset to 0.
  80  */
  81 u64 __init kaslr_early_init(u64 dt_phys)
  82 {
  83         void *fdt;
  84         u64 seed, offset, mask, module_range;
  85         const u8 *cmdline, *str;
  86         int size;
  87
  88         /*
  89          * Set a reasonable default for module_alloc_base in case
  90          * we end up running with module randomization disabled.
  91          */
  92         module_alloc_base = (u64)_etext - MODULES_VSIZE;
  93         __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
  94
  95         /*
  96          * Try to map the FDT early. If this fails, we simply bail,
  97          * and proceed with KASLR disabled. We will make another
  98          * attempt at mapping the FDT in setup_machine()
  99          */
 100         early_fixmap_init();
 101         fdt = fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
 102         if (!fdt) {
 103                 kaslr_status = KASLR_DISABLED_FDT_REMAP;
 104                 return 0;
 105         }
 106
 107         /*
 108          * Retrieve (and wipe) the seed from the FDT
 109          */
 110         seed = get_kaslr_seed(fdt);
 111
 112         /*
 113          * Check if 'nokaslr' appears on the command line, and
 114          * return 0 if that is the case.
 115          */
 116         cmdline = kaslr_get_cmdline(fdt);
 117         str = strstr(cmdline, "nokaslr");
 118         if (str == cmdline || (str > cmdline && *(str - 1) == ' ')) {
 119                 kaslr_status = KASLR_DISABLED_CMDLINE;
 120                 return 0;
 121         }
 122
 123         /*
 124          * Mix in any entropy obtainable architecturally, open coded
 125          * since this runs extremely early.
 126          */
 127         if (__early_cpu_has_rndr()) {
 128                 unsigned long raw;
 129
 130                 if (__arm64_rndr(&raw))
 131                         seed ^= raw;
 132         }
 133
 134         if (!seed) {
 135                 kaslr_status = KASLR_DISABLED_NO_SEED;
 136                 return 0;
 137         }
 138
 139         /*
 140          * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 141          * kernel image offset from the seed. Let's place the kernel in the
 142          * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
 143          * the lower and upper quarters to avoid colliding with other
 144          * allocations.
 145          * Even if we could randomize at page granularity for 16k and 64k pages,
 146          * let's always round to 2 MB so we don't interfere with the ability to
 147          * map using contiguous PTEs
 148          */
 149         mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
 150         offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
 151
 152         /* use the top 16 bits to randomize the linear region */
 153         memstart_offset_seed = seed >> 48;
 154
 155         if (IS_ENABLED(CONFIG_KASAN))
 156                 /*
 157                  * KASAN does not expect the module region to intersect the
 158                  * vmalloc region, since shadow memory is allocated for each
 159                  * module at load time, whereas the vmalloc region is shadowed
 160                  * by KASAN zero pages. So keep modules out of the vmalloc
 161                  * region if KASAN is enabled, and put the kernel well within
 162                  * 4 GB of the module region.
 163                  */
 164                 return offset % SZ_2G;
 165
 166         if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
 167                 /*
 168                  * Randomize the module region over a 2 GB window covering the
 169                  * kernel. This reduces the risk of modules leaking information
 170                  * about the address of the kernel itself, but results in
 171                  * branches between modules and the core kernel that are
 172                  * resolved via PLTs. (Branches between modules will be
 173                  * resolved normally.)
 174                  */
 175                 module_range = SZ_2G - (u64)(_end - _stext);
 176                 module_alloc_base = max((u64)_end + offset - SZ_2G,
 177                                         (u64)MODULES_VADDR);
 178         } else {
 179                 /*
 180                  * Randomize the module region by setting module_alloc_base to
 181                  * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
 182                  * _stext) . This guarantees that the resulting region still
 183                  * covers [_stext, _etext], and that all relative branches can
 184                  * be resolved without veneers.
 185                  */
 186                 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
 187                 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
 188         }
 189
 190         /* use the lower 21 bits to randomize the base of the module region */
 191         module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
 192         module_alloc_base &= PAGE_MASK;
 193
 194         __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
 195         __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
 196
 197         return offset;
 198 }
 199
 200 static int __init kaslr_init(void)
 201 {
 202         switch (kaslr_status) {
 203         case KASLR_ENABLED:
 204                 pr_info("KASLR enabled\n");
 205                 break;
 206         case KASLR_DISABLED_CMDLINE:
 207                 pr_info("KASLR disabled on command line\n");
 208                 break;
 209         case KASLR_DISABLED_NO_SEED:
 210                 pr_warn("KASLR disabled due to lack of seed\n");
 211                 break;
 212         case KASLR_DISABLED_FDT_REMAP:
 213                 pr_warn("KASLR disabled due to FDT remapping failure\n");
 214                 break;
 215         }
 216
 217         return 0;
 218 }
 219 core_initcall(kaslr_init)