kaslr.c 5.6 KB

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  1. // SPDX-License-Identifier: GPL-2.0-only
  2. /*
  3. * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
  4. */
  5. #include <linux/cache.h>
  6. #include <linux/crc32.h>
  7. #include <linux/init.h>
  8. #include <linux/libfdt.h>
  9. #include <linux/mm_types.h>
  10. #include <linux/sched.h>
  11. #include <linux/types.h>
  12. #include <linux/pgtable.h>
  13. #include <linux/random.h>
  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/sections.h>
  20. #include <asm/setup.h>
  21. enum kaslr_status {
  22. KASLR_ENABLED,
  23. KASLR_DISABLED_CMDLINE,
  24. KASLR_DISABLED_NO_SEED,
  25. KASLR_DISABLED_FDT_REMAP,
  26. };
  27. static enum kaslr_status __initdata kaslr_status;
  28. u64 __ro_after_init module_alloc_base;
  29. u16 __initdata memstart_offset_seed;
  30. static __init u64 get_kaslr_seed(void *fdt)
  31. {
  32. int node, len;
  33. fdt64_t *prop;
  34. u64 ret;
  35. node = fdt_path_offset(fdt, "/chosen");
  36. if (node < 0)
  37. return 0;
  38. prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
  39. if (!prop || len != sizeof(u64))
  40. return 0;
  41. ret = fdt64_to_cpu(*prop);
  42. *prop = 0;
  43. return ret;
  44. }
  45. struct arm64_ftr_override kaslr_feature_override __initdata;
  46. /*
  47. * This routine will be executed with the kernel mapped at its default virtual
  48. * address, and if it returns successfully, the kernel will be remapped, and
  49. * start_kernel() will be executed from a randomized virtual offset. The
  50. * relocation will result in all absolute references (e.g., static variables
  51. * containing function pointers) to be reinitialized, and zero-initialized
  52. * .bss variables will be reset to 0.
  53. */
  54. u64 __init kaslr_early_init(void)
  55. {
  56. void *fdt;
  57. u64 seed, offset, mask, module_range;
  58. unsigned long raw;
  59. /*
  60. * Set a reasonable default for module_alloc_base in case
  61. * we end up running with module randomization disabled.
  62. */
  63. module_alloc_base = (u64)_etext - MODULES_VSIZE;
  64. __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
  65. /*
  66. * Try to map the FDT early. If this fails, we simply bail,
  67. * and proceed with KASLR disabled. We will make another
  68. * attempt at mapping the FDT in setup_machine()
  69. */
  70. fdt = get_early_fdt_ptr();
  71. if (!fdt) {
  72. kaslr_status = KASLR_DISABLED_FDT_REMAP;
  73. return 0;
  74. }
  75. /*
  76. * Retrieve (and wipe) the seed from the FDT
  77. */
  78. seed = get_kaslr_seed(fdt);
  79. /*
  80. * Check if 'nokaslr' appears on the command line, and
  81. * return 0 if that is the case.
  82. */
  83. if (kaslr_feature_override.val & kaslr_feature_override.mask & 0xf) {
  84. kaslr_status = KASLR_DISABLED_CMDLINE;
  85. return 0;
  86. }
  87. /*
  88. * Mix in any entropy obtainable architecturally if enabled
  89. * and supported.
  90. */
  91. if (arch_get_random_seed_long_early(&raw))
  92. seed ^= raw;
  93. if (!seed) {
  94. kaslr_status = KASLR_DISABLED_NO_SEED;
  95. return 0;
  96. }
  97. /*
  98. * OK, so we are proceeding with KASLR enabled. Calculate a suitable
  99. * kernel image offset from the seed. Let's place the kernel in the
  100. * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
  101. * the lower and upper quarters to avoid colliding with other
  102. * allocations.
  103. * Even if we could randomize at page granularity for 16k and 64k pages,
  104. * let's always round to 2 MB so we don't interfere with the ability to
  105. * map using contiguous PTEs
  106. */
  107. mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
  108. offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
  109. /* use the top 16 bits to randomize the linear region */
  110. memstart_offset_seed = seed >> 48;
  111. if (!IS_ENABLED(CONFIG_KASAN_VMALLOC) &&
  112. (IS_ENABLED(CONFIG_KASAN_GENERIC) ||
  113. IS_ENABLED(CONFIG_KASAN_SW_TAGS)))
  114. /*
  115. * KASAN without KASAN_VMALLOC does not expect the module region
  116. * to intersect the vmalloc region, since shadow memory is
  117. * allocated for each module at load time, whereas the vmalloc
  118. * region is shadowed by KASAN zero pages. So keep modules
  119. * out of the vmalloc region if KASAN is enabled without
  120. * KASAN_VMALLOC, and put the kernel well within 4 GB of the
  121. * module region.
  122. */
  123. return offset % SZ_2G;
  124. if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
  125. /*
  126. * Randomize the module region over a 2 GB window covering the
  127. * kernel. This reduces the risk of modules leaking information
  128. * about the address of the kernel itself, but results in
  129. * branches between modules and the core kernel that are
  130. * resolved via PLTs. (Branches between modules will be
  131. * resolved normally.)
  132. */
  133. module_range = SZ_2G - (u64)(_end - _stext);
  134. module_alloc_base = max((u64)_end + offset - SZ_2G,
  135. (u64)MODULES_VADDR);
  136. } else {
  137. /*
  138. * Randomize the module region by setting module_alloc_base to
  139. * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
  140. * _stext) . This guarantees that the resulting region still
  141. * covers [_stext, _etext], and that all relative branches can
  142. * be resolved without veneers.
  143. */
  144. module_range = MODULES_VSIZE - (u64)(_etext - _stext);
  145. module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
  146. }
  147. /* use the lower 21 bits to randomize the base of the module region */
  148. module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
  149. module_alloc_base &= PAGE_MASK;
  150. __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
  151. __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
  152. return offset;
  153. }
  154. static int __init kaslr_init(void)
  155. {
  156. switch (kaslr_status) {
  157. case KASLR_ENABLED:
  158. pr_info("KASLR enabled\n");
  159. break;
  160. case KASLR_DISABLED_CMDLINE:
  161. pr_info("KASLR disabled on command line\n");
  162. break;
  163. case KASLR_DISABLED_NO_SEED:
  164. pr_warn("KASLR disabled due to lack of seed\n");
  165. break;
  166. case KASLR_DISABLED_FDT_REMAP:
  167. pr_warn("KASLR disabled due to FDT remapping failure\n");
  168. break;
  169. }
  170. return 0;
  171. }
  172. core_initcall(kaslr_init)