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 #include <linux/pgtable.h>
14 #include <linux/random.h>
15
16 #include <asm/cacheflush.h>
17 #include <asm/fixmap.h>
18 #include <asm/kernel-pgtable.h>
19 #include <asm/memory.h>
20 #include <asm/mmu.h>
21 #include <asm/sections.h>
22
23 enum kaslr_status {
24 KASLR_ENABLED,
25 KASLR_DISABLED_CMDLINE,
26 KASLR_DISABLED_NO_SEED,
27 KASLR_DISABLED_FDT_REMAP,
28 };
29
30 static enum kaslr_status __initdata kaslr_status;
31 u64 __ro_after_init module_alloc_base;
32 u16 __initdata memstart_offset_seed;
33
get_kaslr_seed(void * fdt)34 static __init u64 get_kaslr_seed(void *fdt)
35 {
36 int node, len;
37 fdt64_t *prop;
38 u64 ret;
39
40 node = fdt_path_offset(fdt, "/chosen");
41 if (node < 0)
42 return 0;
43
44 prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
45 if (!prop || len != sizeof(u64))
46 return 0;
47
48 ret = fdt64_to_cpu(*prop);
49 *prop = 0;
50 return ret;
51 }
52
kaslr_get_cmdline(void * fdt)53 static __init const u8 *kaslr_get_cmdline(void *fdt)
54 {
55 static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
56
57 if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
58 int node;
59 const u8 *prop;
60
61 node = fdt_path_offset(fdt, "/chosen");
62 if (node < 0)
63 goto out;
64
65 prop = fdt_getprop(fdt, node, "bootargs", NULL);
66 if (!prop)
67 goto out;
68 return prop;
69 }
70 out:
71 return default_cmdline;
72 }
73
74 /*
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.
81 */
kaslr_early_init(u64 dt_phys)82 u64 __init kaslr_early_init(u64 dt_phys)
83 {
84 void *fdt;
85 u64 seed, offset, mask, module_range;
86 const u8 *cmdline, *str;
87 unsigned long raw;
88 int size;
89
90 /*
91 * Set a reasonable default for module_alloc_base in case
92 * we end up running with module randomization disabled.
93 */
94 module_alloc_base = (u64)_etext - MODULES_VSIZE;
95 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
96
97 /*
98 * Try to map the FDT early. If this fails, we simply bail,
99 * and proceed with KASLR disabled. We will make another
100 * attempt at mapping the FDT in setup_machine()
101 */
102 early_fixmap_init();
103 fdt = fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
104 if (!fdt) {
105 kaslr_status = KASLR_DISABLED_FDT_REMAP;
106 return 0;
107 }
108
109 /*
110 * Retrieve (and wipe) the seed from the FDT
111 */
112 seed = get_kaslr_seed(fdt);
113
114 /*
115 * Check if 'nokaslr' appears on the command line, and
116 * return 0 if that is the case.
117 */
118 cmdline = kaslr_get_cmdline(fdt);
119 str = strstr(cmdline, "nokaslr");
120 if (str == cmdline || (str > cmdline && *(str - 1) == ' ')) {
121 kaslr_status = KASLR_DISABLED_CMDLINE;
122 return 0;
123 }
124
125 /*
126 * Mix in any entropy obtainable architecturally if enabled
127 * and supported.
128 */
129
130 if (arch_get_random_seed_long_early(&raw))
131 seed ^= raw;
132
133 if (!seed) {
134 kaslr_status = KASLR_DISABLED_NO_SEED;
135 return 0;
136 }
137
138 /*
139 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
140 * kernel image offset from the seed. Let's place the kernel in the
141 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
142 * the lower and upper quarters to avoid colliding with other
143 * allocations.
144 * Even if we could randomize at page granularity for 16k and 64k pages,
145 * let's always round to 2 MB so we don't interfere with the ability to
146 * map using contiguous PTEs
147 */
148 mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
149 offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
150
151 /* use the top 16 bits to randomize the linear region */
152 memstart_offset_seed = seed >> 48;
153
154 if (IS_ENABLED(CONFIG_KASAN))
155 /*
156 * KASAN does not expect the module region to intersect the
157 * vmalloc region, since shadow memory is allocated for each
158 * module at load time, whereas the vmalloc region is shadowed
159 * by KASAN zero pages. So keep modules out of the vmalloc
160 * region if KASAN is enabled, and put the kernel well within
161 * 4 GB of the module region.
162 */
163 return offset % SZ_2G;
164
165 if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
166 /*
167 * Randomize the module region over a 2 GB window covering the
168 * kernel. This reduces the risk of modules leaking information
169 * about the address of the kernel itself, but results in
170 * branches between modules and the core kernel that are
171 * resolved via PLTs. (Branches between modules will be
172 * resolved normally.)
173 */
174 module_range = SZ_2G - (u64)(_end - _stext);
175 module_alloc_base = max((u64)_end + offset - SZ_2G,
176 (u64)MODULES_VADDR);
177 } else {
178 /*
179 * Randomize the module region by setting module_alloc_base to
180 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
181 * _stext) . This guarantees that the resulting region still
182 * covers [_stext, _etext], and that all relative branches can
183 * be resolved without veneers.
184 */
185 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
186 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
187 }
188
189 /* use the lower 21 bits to randomize the base of the module region */
190 module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
191 module_alloc_base &= PAGE_MASK;
192
193 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
194 __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
195
196 return offset;
197 }
198
kaslr_init(void)199 static int __init kaslr_init(void)
200 {
201 switch (kaslr_status) {
202 case KASLR_ENABLED:
203 pr_info("KASLR enabled\n");
204 break;
205 case KASLR_DISABLED_CMDLINE:
206 pr_info("KASLR disabled on command line\n");
207 break;
208 case KASLR_DISABLED_NO_SEED:
209 pr_warn("KASLR disabled due to lack of seed\n");
210 break;
211 case KASLR_DISABLED_FDT_REMAP:
212 pr_warn("KASLR disabled due to FDT remapping failure\n");
213 break;
214 }
215
216 return 0;
217 }
218 core_initcall(kaslr_init)
219