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