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 #include <linux/pgtable.h>
  14 #include <linux/random.h>
  15
  16 #include <asm/cacheflush.h>
  17 #include <asm/fixmap.h>
  18 #include <asm/kernel-pgtable.h>
  19 #include <asm/memory.h>
  20 #include <asm/mmu.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 bool cmdline_contains_nokaslr(const u8 *cmdline)
  54 {
  55         const u8 *str;
  56
  57         str = strstr(cmdline, "nokaslr");
  58         return str == cmdline || (str > cmdline && *(str - 1) == ' ');
  59 }
  60
  61 static __init bool is_kaslr_disabled_cmdline(void *fdt)
  62 {
  63         if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
  64                 int node;
  65                 const u8 *prop;
  66
  67                 node = fdt_path_offset(fdt, "/chosen");
  68                 if (node < 0)
  69                         goto out;
  70
  71                 prop = fdt_getprop(fdt, node, "bootargs", NULL);
  72                 if (!prop)
  73                         goto out;
  74
  75                 if (cmdline_contains_nokaslr(prop))
  76                         return true;
  77
  78                 if (IS_ENABLED(CONFIG_CMDLINE_EXTEND))
  79                         goto out;
  80
  81                 return false;
  82         }
  83 out:
  84         return cmdline_contains_nokaslr(CONFIG_CMDLINE);
  85 }
  86
  87 /*
  88  * This routine will be executed with the kernel mapped at its default virtual
  89  * address, and if it returns successfully, the kernel will be remapped, and
  90  * start_kernel() will be executed from a randomized virtual offset. The
  91  * relocation will result in all absolute references (e.g., static variables
  92  * containing function pointers) to be reinitialized, and zero-initialized
  93  * .bss variables will be reset to 0.
  94  */
  95 u64 __init kaslr_early_init(u64 dt_phys)
  96 {
  97         void *fdt;
  98         u64 seed, offset, mask, module_range;
  99         unsigned long raw;
 100         int size;
 101
 102         /*
 103          * Set a reasonable default for module_alloc_base in case
 104          * we end up running with module randomization disabled.
 105          */
 106         module_alloc_base = (u64)_etext - MODULES_VSIZE;
 107         __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
 108
 109         /*
 110          * Try to map the FDT early. If this fails, we simply bail,
 111          * and proceed with KASLR disabled. We will make another
 112          * attempt at mapping the FDT in setup_machine()
 113          */
 114         early_fixmap_init();
 115         fdt = fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
 116         if (!fdt) {
 117                 kaslr_status = KASLR_DISABLED_FDT_REMAP;
 118                 return 0;
 119         }
 120
 121         /*
 122          * Retrieve (and wipe) the seed from the FDT
 123          */
 124         seed = get_kaslr_seed(fdt);
 125
 126         /*
 127          * Check if 'nokaslr' appears on the command line, and
 128          * return 0 if that is the case.
 129          */
 130         if (is_kaslr_disabled_cmdline(fdt)) {
 131                 kaslr_status = KASLR_DISABLED_CMDLINE;
 132                 return 0;
 133         }
 134
 135         /*
 136          * Mix in any entropy obtainable architecturally if enabled
 137          * and supported.
 138          */
 139
 140         if (arch_get_random_seed_long_early(&raw))
 141                 seed ^= raw;
 142
 143         if (!seed) {
 144                 kaslr_status = KASLR_DISABLED_NO_SEED;
 145                 return 0;
 146         }
 147
 148         /*
 149          * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 150          * kernel image offset from the seed. Let's place the kernel in the
 151          * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
 152          * the lower and upper quarters to avoid colliding with other
 153          * allocations.
 154          * Even if we could randomize at page granularity for 16k and 64k pages,
 155          * let's always round to 2 MB so we don't interfere with the ability to
 156          * map using contiguous PTEs
 157          */
 158         mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
 159         offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
 160
 161         /* use the top 16 bits to randomize the linear region */
 162         memstart_offset_seed = seed >> 48;
 163
 164         if (IS_ENABLED(CONFIG_KASAN_GENERIC) ||
 165             IS_ENABLED(CONFIG_KASAN_SW_TAGS))
 166                 /*
 167                  * KASAN does not expect the module region to intersect the
 168                  * vmalloc region, since shadow memory is allocated for each
 169                  * module at load time, whereas the vmalloc region is shadowed
 170                  * by KASAN zero pages. So keep modules out of the vmalloc
 171                  * region if KASAN is enabled, and put the kernel well within
 172                  * 4 GB of the module region.
 173                  */
 174                 return offset % SZ_2G;
 175
 176         if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
 177                 /*
 178                  * Randomize the module region over a 2 GB window covering the
 179                  * kernel. This reduces the risk of modules leaking information
 180                  * about the address of the kernel itself, but results in
 181                  * branches between modules and the core kernel that are
 182                  * resolved via PLTs. (Branches between modules will be
 183                  * resolved normally.)
 184                  */
 185                 module_range = SZ_2G - (u64)(_end - _stext);
 186                 module_alloc_base = max((u64)_end + offset - SZ_2G,
 187                                         (u64)MODULES_VADDR);
 188         } else {
 189                 /*
 190                  * Randomize the module region by setting module_alloc_base to
 191                  * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
 192                  * _stext) . This guarantees that the resulting region still
 193                  * covers [_stext, _etext], and that all relative branches can
 194                  * be resolved without veneers.
 195                  */
 196                 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
 197                 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
 198         }
 199
 200         /* use the lower 21 bits to randomize the base of the module region */
 201         module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
 202         module_alloc_base &= PAGE_MASK;
 203
 204         __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
 205         __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
 206
 207         return offset;
 208 }
 209
 210 static int __init kaslr_init(void)
 211 {
 212         switch (kaslr_status) {
 213         case KASLR_ENABLED:
 214                 pr_info("KASLR enabled\n");
 215                 break;
 216         case KASLR_DISABLED_CMDLINE:
 217                 pr_info("KASLR disabled on command line\n");
 218                 break;
 219         case KASLR_DISABLED_NO_SEED:
 220                 pr_warn("KASLR disabled due to lack of seed\n");
 221                 break;
 222         case KASLR_DISABLED_FDT_REMAP:
 223                 pr_warn("KASLR disabled due to FDT remapping failure\n");
 224                 break;
 225         }
 226
 227         return 0;
 228 }
 229 core_initcall(kaslr_init)