2 * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
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.
9 #include <linux/crc32.h>
10 #include <linux/init.h>
11 #include <linux/libfdt.h>
12 #include <linux/mm_types.h>
13 #include <linux/sched.h>
14 #include <linux/types.h>
16 #include <asm/fixmap.h>
17 #include <asm/kernel-pgtable.h>
18 #include <asm/memory.h>
20 #include <asm/pgtable.h>
21 #include <asm/sections.h>
23 u64 __read_mostly module_alloc_base;
25 static __init u64 get_kaslr_seed(void *fdt)
31 node = fdt_path_offset(fdt, "/chosen");
35 prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
36 if (!prop || len != sizeof(u64))
39 ret = fdt64_to_cpu(*prop);
44 static __init const u8 *get_cmdline(void *fdt)
46 static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
48 if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
52 node = fdt_path_offset(fdt, "/chosen");
56 prop = fdt_getprop(fdt, node, "bootargs", NULL);
62 return default_cmdline;
65 extern void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size,
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.
76 u64 __init kaslr_early_init(u64 dt_phys)
79 u64 seed, offset, mask, module_range;
80 const u8 *cmdline, *str;
84 * Set a reasonable default for module_alloc_base in case
85 * we end up running with module randomization disabled.
87 module_alloc_base = (u64)_etext - MODULES_VSIZE;
90 * Try to map the FDT early. If this fails, we simply bail,
91 * and proceed with KASLR disabled. We will make another
92 * attempt at mapping the FDT in setup_machine()
95 fdt = __fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
100 * Retrieve (and wipe) the seed from the FDT
102 seed = get_kaslr_seed(fdt);
107 * Check if 'nokaslr' appears on the command line, and
108 * return 0 if that is the case.
110 cmdline = get_cmdline(fdt);
111 str = strstr(cmdline, "nokaslr");
112 if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
116 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
117 * kernel image offset from the seed. Let's place the kernel in the
118 * lower half of the VMALLOC area (VA_BITS - 2).
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
123 mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
124 offset = seed & mask;
127 * The kernel Image should not extend across a 1GB/32MB/512MB alignment
128 * boundary (for 4KB/16KB/64KB granule kernels, respectively). If this
129 * happens, increase the KASLR offset by the size of the kernel image.
131 if ((((u64)_text + offset) >> SWAPPER_TABLE_SHIFT) !=
132 (((u64)_end + offset) >> SWAPPER_TABLE_SHIFT))
133 offset = (offset + (u64)(_end - _text)) & mask;
135 if (IS_ENABLED(CONFIG_KASAN))
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.
145 if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
147 * Randomize the module region independently from the core
148 * kernel. This prevents modules from leaking any information
149 * about the address of the kernel itself, but results in
150 * branches between modules and the core kernel that are
151 * resolved via PLTs. (Branches between modules will be
152 * resolved normally.)
154 module_range = VMALLOC_END - VMALLOC_START - MODULES_VSIZE;
155 module_alloc_base = VMALLOC_START;
158 * Randomize the module region by setting module_alloc_base to
159 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
160 * _stext) . This guarantees that the resulting region still
161 * covers [_stext, _etext], and that all relative branches can
162 * be resolved without veneers.
164 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
165 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
168 /* use the lower 21 bits to randomize the base of the module region */
169 module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
170 module_alloc_base &= PAGE_MASK;