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