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 u64 __ro_after_init module_alloc_base;
  23 u16 __initdata memstart_offset_seed;
  24
  25 static __init u64 get_kaslr_seed(void *fdt)
  26 {
  27         int node, len;
  28         fdt64_t *prop;
  29         u64 ret;
  30
  31         node = fdt_path_offset(fdt, "/chosen");
  32         if (node < 0)
  33                 return 0;
  34
  35         prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
  36         if (!prop || len != sizeof(u64))
  37                 return 0;
  38
  39         ret = fdt64_to_cpu(*prop);
  40         *prop = 0;
  41         return ret;
  42 }
  43
  44 static __init const u8 *kaslr_get_cmdline(void *fdt)
  45 {
  46         static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
  47
  48         if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
  49                 int node;
  50                 const u8 *prop;
  51
  52                 node = fdt_path_offset(fdt, "/chosen");
  53                 if (node < 0)
  54                         goto out;
  55
  56                 prop = fdt_getprop(fdt, node, "bootargs", NULL);
  57                 if (!prop)
  58                         goto out;
  59                 return prop;
  60         }
  61 out:
  62         return default_cmdline;
  63 }
  64
  65 extern void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size,
  66                                        pgprot_t prot);
  67
  68 /*
  69  * This routine will be executed with the kernel mapped at its default virtual
  70  * address, and if it returns successfully, the kernel will be remapped, and
  71  * start_kernel() will be executed from a randomized virtual offset. The
  72  * relocation will result in all absolute references (e.g., static variables
  73  * containing function pointers) to be reinitialized, and zero-initialized
  74  * .bss variables will be reset to 0.
  75  */
  76 u64 __init kaslr_early_init(u64 dt_phys)
  77 {
  78         void *fdt;
  79         u64 seed, offset, mask, module_range;
  80         const u8 *cmdline, *str;
  81         int size;
  82
  83         /*
  84          * Set a reasonable default for module_alloc_base in case
  85          * we end up running with module randomization disabled.
  86          */
  87         module_alloc_base = (u64)_etext - MODULES_VSIZE;
  88         __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
  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 = kaslr_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          * middle half of the VMALLOC area (VA_BITS - 2), and stay clear of
 120          * the lower and upper quarters to avoid colliding with other
 121          * allocations.
 122          * Even if we could randomize at page granularity for 16k and 64k pages,
 123          * let's always round to 2 MB so we don't interfere with the ability to
 124          * map using contiguous PTEs
 125          */
 126         mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
 127         offset = BIT(VA_BITS - 3) + (seed & mask);
 128
 129         /* use the top 16 bits to randomize the linear region */
 130         memstart_offset_seed = seed >> 48;
 131
 132         if (IS_ENABLED(CONFIG_KASAN))
 133                 /*
 134                  * KASAN does not expect the module region to intersect the
 135                  * vmalloc region, since shadow memory is allocated for each
 136                  * module at load time, whereas the vmalloc region is shadowed
 137                  * by KASAN zero pages. So keep modules out of the vmalloc
 138                  * region if KASAN is enabled, and put the kernel well within
 139                  * 4 GB of the module region.
 140                  */
 141                 return offset % SZ_2G;
 142
 143         if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
 144                 /*
 145                  * Randomize the module region over a 2 GB window covering the
 146                  * kernel. This reduces the risk of modules leaking information
 147                  * about the address of the kernel itself, but results in
 148                  * branches between modules and the core kernel that are
 149                  * resolved via PLTs. (Branches between modules will be
 150                  * resolved normally.)
 151                  */
 152                 module_range = SZ_2G - (u64)(_end - _stext);
 153                 module_alloc_base = max((u64)_end + offset - SZ_2G,
 154                                         (u64)MODULES_VADDR);
 155         } else {
 156                 /*
 157                  * Randomize the module region by setting module_alloc_base to
 158                  * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
 159                  * _stext) . This guarantees that the resulting region still
 160                  * covers [_stext, _etext], and that all relative branches can
 161                  * be resolved without veneers.
 162                  */
 163                 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
 164                 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
 165         }
 166
 167         /* use the lower 21 bits to randomize the base of the module region */
 168         module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
 169         module_alloc_base &= PAGE_MASK;
 170
 171         __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
 172         __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
 173
 174         return offset;
 175 }