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/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>
17 #include <asm/cacheflush.h>
18 #include <asm/fixmap.h>
19 #include <asm/kernel-pgtable.h>
20 #include <asm/memory.h>
22 #include <asm/pgtable.h>
23 #include <asm/sections.h>
25 u64 __ro_after_init module_alloc_base;
26 u16 __initdata memstart_offset_seed;
28 static __init u64 get_kaslr_seed(void *fdt)
34 node = fdt_path_offset(fdt, "/chosen");
38 prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
39 if (!prop || len != sizeof(u64))
42 ret = fdt64_to_cpu(*prop);
47 static __init const u8 *kaslr_get_cmdline(void *fdt)
49 static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
51 if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
55 node = fdt_path_offset(fdt, "/chosen");
59 prop = fdt_getprop(fdt, node, "bootargs", NULL);
65 return default_cmdline;
68 extern void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size,
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.
79 u64 __init kaslr_early_init(u64 dt_phys)
82 u64 seed, offset, mask, module_range;
83 const u8 *cmdline, *str;
87 * Set a reasonable default for module_alloc_base in case
88 * we end up running with module randomization disabled.
90 module_alloc_base = (u64)_etext - MODULES_VSIZE;
93 * Try to map the FDT early. If this fails, we simply bail,
94 * and proceed with KASLR disabled. We will make another
95 * attempt at mapping the FDT in setup_machine()
98 fdt = __fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
103 * Retrieve (and wipe) the seed from the FDT
105 seed = get_kaslr_seed(fdt);
110 * Check if 'nokaslr' appears on the command line, and
111 * return 0 if that is the case.
113 cmdline = kaslr_get_cmdline(fdt);
114 str = strstr(cmdline, "nokaslr");
115 if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
119 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
120 * kernel image offset from the seed. Let's place the kernel in the
121 * middle half of the VMALLOC area (VA_BITS - 2), and stay clear of
122 * the lower and upper quarters to avoid colliding with other
124 * Even if we could randomize at page granularity for 16k and 64k pages,
125 * let's always round to 2 MB so we don't interfere with the ability to
126 * map using contiguous PTEs
128 mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
129 offset = BIT(VA_BITS - 3) + (seed & mask);
131 /* use the top 16 bits to randomize the linear region */
132 memstart_offset_seed = seed >> 48;
134 if (IS_ENABLED(CONFIG_KASAN))
136 * KASAN does not expect the module region to intersect the
137 * vmalloc region, since shadow memory is allocated for each
138 * module at load time, whereas the vmalloc region is shadowed
139 * by KASAN zero pages. So keep modules out of the vmalloc
140 * region if KASAN is enabled, and put the kernel well within
141 * 4 GB of the module region.
143 return offset % SZ_2G;
145 if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
147 * Randomize the module region over a 4 GB window covering the
148 * kernel. This reduces the risk of modules leaking 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 = SZ_4G - (u64)(_end - _stext);
155 module_alloc_base = max((u64)_end + offset - SZ_4G,
159 * Randomize the module region by setting module_alloc_base to
160 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
161 * _stext) . This guarantees that the resulting region still
162 * covers [_stext, _etext], and that all relative branches can
163 * be resolved without veneers.
165 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
166 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
169 /* use the lower 21 bits to randomize the base of the module region */
170 module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
171 module_alloc_base &= PAGE_MASK;
173 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
174 __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));