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 /* 66 * This routine will be executed with the kernel mapped at its default virtual 67 * address, and if it returns successfully, the kernel will be remapped, and 68 * start_kernel() will be executed from a randomized virtual offset. The 69 * relocation will result in all absolute references (e.g., static variables 70 * containing function pointers) to be reinitialized, and zero-initialized 71 * .bss variables will be reset to 0. 72 */ 73 u64 __init kaslr_early_init(u64 dt_phys) 74 { 75 void *fdt; 76 u64 seed, offset, mask, module_range; 77 const u8 *cmdline, *str; 78 int size; 79 80 /* 81 * Set a reasonable default for module_alloc_base in case 82 * we end up running with module randomization disabled. 83 */ 84 module_alloc_base = (u64)_etext - MODULES_VSIZE; 85 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base)); 86 87 /* 88 * Try to map the FDT early. If this fails, we simply bail, 89 * and proceed with KASLR disabled. We will make another 90 * attempt at mapping the FDT in setup_machine() 91 */ 92 early_fixmap_init(); 93 fdt = fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL); 94 if (!fdt) 95 return 0; 96 97 /* 98 * Retrieve (and wipe) the seed from the FDT 99 */ 100 seed = get_kaslr_seed(fdt); 101 if (!seed) 102 return 0; 103 104 /* 105 * Check if 'nokaslr' appears on the command line, and 106 * return 0 if that is the case. 107 */ 108 cmdline = kaslr_get_cmdline(fdt); 109 str = strstr(cmdline, "nokaslr"); 110 if (str == cmdline || (str > cmdline && *(str - 1) == ' ')) 111 return 0; 112 113 /* 114 * OK, so we are proceeding with KASLR enabled. Calculate a suitable 115 * kernel image offset from the seed. Let's place the kernel in the 116 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of 117 * the lower and upper quarters to avoid colliding with other 118 * allocations. 119 * Even if we could randomize at page granularity for 16k and 64k pages, 120 * let's always round to 2 MB so we don't interfere with the ability to 121 * map using contiguous PTEs 122 */ 123 mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1); 124 offset = BIT(VA_BITS_MIN - 3) + (seed & mask); 125 126 /* use the top 16 bits to randomize the linear region */ 127 memstart_offset_seed = seed >> 48; 128 129 if (IS_ENABLED(CONFIG_KASAN)) 130 /* 131 * KASAN does not expect the module region to intersect the 132 * vmalloc region, since shadow memory is allocated for each 133 * module at load time, whereas the vmalloc region is shadowed 134 * by KASAN zero pages. So keep modules out of the vmalloc 135 * region if KASAN is enabled, and put the kernel well within 136 * 4 GB of the module region. 137 */ 138 return offset % SZ_2G; 139 140 if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) { 141 /* 142 * Randomize the module region over a 2 GB window covering the 143 * kernel. This reduces the risk of modules leaking information 144 * about the address of the kernel itself, but results in 145 * branches between modules and the core kernel that are 146 * resolved via PLTs. (Branches between modules will be 147 * resolved normally.) 148 */ 149 module_range = SZ_2G - (u64)(_end - _stext); 150 module_alloc_base = max((u64)_end + offset - SZ_2G, 151 (u64)MODULES_VADDR); 152 } else { 153 /* 154 * Randomize the module region by setting module_alloc_base to 155 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE, 156 * _stext) . This guarantees that the resulting region still 157 * covers [_stext, _etext], and that all relative branches can 158 * be resolved without veneers. 159 */ 160 module_range = MODULES_VSIZE - (u64)(_etext - _stext); 161 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE; 162 } 163 164 /* use the lower 21 bits to randomize the base of the module region */ 165 module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21; 166 module_alloc_base &= PAGE_MASK; 167 168 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base)); 169 __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed)); 170 171 return offset; 172 } 173