1 // SPDX-License-Identifier: GPL-2.0-only
3 * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
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 #include <linux/pgtable.h>
15 #include <asm/archrandom.h>
16 #include <asm/cacheflush.h>
17 #include <asm/fixmap.h>
18 #include <asm/kernel-pgtable.h>
19 #include <asm/memory.h>
21 #include <asm/sections.h>
25 KASLR_DISABLED_CMDLINE,
26 KASLR_DISABLED_NO_SEED,
27 KASLR_DISABLED_FDT_REMAP,
30 static enum kaslr_status __initdata kaslr_status;
31 u64 __ro_after_init module_alloc_base;
32 u16 __initdata memstart_offset_seed;
34 static __init u64 get_kaslr_seed(void *fdt)
40 node = fdt_path_offset(fdt, "/chosen");
44 prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
45 if (!prop || len != sizeof(u64))
48 ret = fdt64_to_cpu(*prop);
53 static __init const u8 *kaslr_get_cmdline(void *fdt)
55 static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
57 if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
61 node = fdt_path_offset(fdt, "/chosen");
65 prop = fdt_getprop(fdt, node, "bootargs", NULL);
71 return default_cmdline;
75 * This routine will be executed with the kernel mapped at its default virtual
76 * address, and if it returns successfully, the kernel will be remapped, and
77 * start_kernel() will be executed from a randomized virtual offset. The
78 * relocation will result in all absolute references (e.g., static variables
79 * containing function pointers) to be reinitialized, and zero-initialized
80 * .bss variables will be reset to 0.
82 u64 __init kaslr_early_init(u64 dt_phys)
85 u64 seed, offset, mask, module_range;
86 const u8 *cmdline, *str;
90 * Set a reasonable default for module_alloc_base in case
91 * we end up running with module randomization disabled.
93 module_alloc_base = (u64)_etext - MODULES_VSIZE;
94 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
97 * Try to map the FDT early. If this fails, we simply bail,
98 * and proceed with KASLR disabled. We will make another
99 * attempt at mapping the FDT in setup_machine()
102 fdt = fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
104 kaslr_status = KASLR_DISABLED_FDT_REMAP;
109 * Retrieve (and wipe) the seed from the FDT
111 seed = get_kaslr_seed(fdt);
114 * Check if 'nokaslr' appears on the command line, and
115 * return 0 if that is the case.
117 cmdline = kaslr_get_cmdline(fdt);
118 str = strstr(cmdline, "nokaslr");
119 if (str == cmdline || (str > cmdline && *(str - 1) == ' ')) {
120 kaslr_status = KASLR_DISABLED_CMDLINE;
125 * Mix in any entropy obtainable architecturally, open coded
126 * since this runs extremely early.
128 if (__early_cpu_has_rndr()) {
131 if (__arm64_rndr(&raw))
136 kaslr_status = KASLR_DISABLED_NO_SEED;
141 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
142 * kernel image offset from the seed. Let's place the kernel in the
143 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
144 * the lower and upper quarters to avoid colliding with other
146 * Even if we could randomize at page granularity for 16k and 64k pages,
147 * let's always round to 2 MB so we don't interfere with the ability to
148 * map using contiguous PTEs
150 mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
151 offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
153 /* use the top 16 bits to randomize the linear region */
154 memstart_offset_seed = seed >> 48;
156 if (IS_ENABLED(CONFIG_KASAN))
158 * KASAN does not expect the module region to intersect the
159 * vmalloc region, since shadow memory is allocated for each
160 * module at load time, whereas the vmalloc region is shadowed
161 * by KASAN zero pages. So keep modules out of the vmalloc
162 * region if KASAN is enabled, and put the kernel well within
163 * 4 GB of the module region.
165 return offset % SZ_2G;
167 if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
169 * Randomize the module region over a 2 GB window covering the
170 * kernel. This reduces the risk of modules leaking information
171 * about the address of the kernel itself, but results in
172 * branches between modules and the core kernel that are
173 * resolved via PLTs. (Branches between modules will be
174 * resolved normally.)
176 module_range = SZ_2G - (u64)(_end - _stext);
177 module_alloc_base = max((u64)_end + offset - SZ_2G,
181 * Randomize the module region by setting module_alloc_base to
182 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
183 * _stext) . This guarantees that the resulting region still
184 * covers [_stext, _etext], and that all relative branches can
185 * be resolved without veneers.
187 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
188 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
191 /* use the lower 21 bits to randomize the base of the module region */
192 module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
193 module_alloc_base &= PAGE_MASK;
195 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
196 __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
201 static int __init kaslr_init(void)
203 switch (kaslr_status) {
205 pr_info("KASLR enabled\n");
207 case KASLR_DISABLED_CMDLINE:
208 pr_info("KASLR disabled on command line\n");
210 case KASLR_DISABLED_NO_SEED:
211 pr_warn("KASLR disabled due to lack of seed\n");
213 case KASLR_DISABLED_FDT_REMAP:
214 pr_warn("KASLR disabled due to FDT remapping failure\n");
220 core_initcall(kaslr_init)