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 #include <asm/setup.h>
23
24 enum kaslr_status {
25 KASLR_ENABLED,
26 KASLR_DISABLED_CMDLINE,
27 KASLR_DISABLED_NO_SEED,
28 KASLR_DISABLED_FDT_REMAP,
29 };
30
31 static enum kaslr_status __initdata kaslr_status;
32 u64 __ro_after_init module_alloc_base;
33 u16 __initdata memstart_offset_seed;
34
get_kaslr_seed(void * fdt)35 static __init u64 get_kaslr_seed(void *fdt)
36 {
37 int node, len;
38 fdt64_t *prop;
39 u64 ret;
40
41 node = fdt_path_offset(fdt, "/chosen");
42 if (node < 0)
43 return 0;
44
45 prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
46 if (!prop || len != sizeof(u64))
47 return 0;
48
49 ret = fdt64_to_cpu(*prop);
50 *prop = 0;
51 return ret;
52 }
53
54 struct arm64_ftr_override kaslr_feature_override __initdata;
55
56 /*
57 * This routine will be executed with the kernel mapped at its default virtual
58 * address, and if it returns successfully, the kernel will be remapped, and
59 * start_kernel() will be executed from a randomized virtual offset. The
60 * relocation will result in all absolute references (e.g., static variables
61 * containing function pointers) to be reinitialized, and zero-initialized
62 * .bss variables will be reset to 0.
63 */
kaslr_early_init(void)64 u64 __init kaslr_early_init(void)
65 {
66 void *fdt;
67 u64 seed, offset, mask, module_range;
68 unsigned long raw;
69
70 /*
71 * Set a reasonable default for module_alloc_base in case
72 * we end up running with module randomization disabled.
73 */
74 module_alloc_base = (u64)_etext - MODULES_VSIZE;
75 dcache_clean_inval_poc((unsigned long)&module_alloc_base,
76 (unsigned long)&module_alloc_base +
77 sizeof(module_alloc_base));
78
79 /*
80 * Try to map the FDT early. If this fails, we simply bail,
81 * and proceed with KASLR disabled. We will make another
82 * attempt at mapping the FDT in setup_machine()
83 */
84 fdt = get_early_fdt_ptr();
85 if (!fdt) {
86 kaslr_status = KASLR_DISABLED_FDT_REMAP;
87 return 0;
88 }
89
90 /*
91 * Retrieve (and wipe) the seed from the FDT
92 */
93 seed = get_kaslr_seed(fdt);
94
95 /*
96 * Check if 'nokaslr' appears on the command line, and
97 * return 0 if that is the case.
98 */
99 if (kaslr_feature_override.val & kaslr_feature_override.mask & 0xf) {
100 kaslr_status = KASLR_DISABLED_CMDLINE;
101 return 0;
102 }
103
104 /*
105 * Mix in any entropy obtainable architecturally if enabled
106 * and supported.
107 */
108
109 if (arch_get_random_seed_long_early(&raw))
110 seed ^= raw;
111
112 if (!seed) {
113 kaslr_status = KASLR_DISABLED_NO_SEED;
114 return 0;
115 }
116
117 /*
118 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
119 * kernel image offset from the seed. Let's place the kernel in the
120 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
121 * the lower and upper quarters to avoid colliding with other
122 * allocations.
123 * Even if we could randomize at page granularity for 16k and 64k pages,
124 * let's always round to 2 MB so we don't interfere with the ability to
125 * map using contiguous PTEs
126 */
127 mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
128 offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
129
130 /* use the top 16 bits to randomize the linear region */
131 memstart_offset_seed = seed >> 48;
132
133 if (!IS_ENABLED(CONFIG_KASAN_VMALLOC) &&
134 (IS_ENABLED(CONFIG_KASAN_GENERIC) ||
135 IS_ENABLED(CONFIG_KASAN_SW_TAGS)))
136 /*
137 * KASAN without KASAN_VMALLOC does not expect the module region
138 * to intersect the vmalloc region, since shadow memory is
139 * allocated for each module at load time, whereas the vmalloc
140 * region is shadowed by KASAN zero pages. So keep modules
141 * out of the vmalloc region if KASAN is enabled without
142 * KASAN_VMALLOC, and put the kernel well within 4 GB of the
143 * module region.
144 */
145 return offset % SZ_2G;
146
147 if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
148 /*
149 * Randomize the module region over a 2 GB window covering the
150 * kernel. This reduces the risk of modules leaking information
151 * about the address of the kernel itself, but results in
152 * branches between modules and the core kernel that are
153 * resolved via PLTs. (Branches between modules will be
154 * resolved normally.)
155 */
156 module_range = SZ_2G - (u64)(_end - _stext);
157 module_alloc_base = max((u64)_end + offset - SZ_2G,
158 (u64)MODULES_VADDR);
159 } else {
160 /*
161 * Randomize the module region by setting module_alloc_base to
162 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
163 * _stext) . This guarantees that the resulting region still
164 * covers [_stext, _etext], and that all relative branches can
165 * be resolved without veneers unless this region is exhausted
166 * and we fall back to a larger 2GB window in module_alloc()
167 * when ARM64_MODULE_PLTS is enabled.
168 */
169 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
170 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
171 }
172
173 /* use the lower 21 bits to randomize the base of the module region */
174 module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
175 module_alloc_base &= PAGE_MASK;
176
177 dcache_clean_inval_poc((unsigned long)&module_alloc_base,
178 (unsigned long)&module_alloc_base +
179 sizeof(module_alloc_base));
180 dcache_clean_inval_poc((unsigned long)&memstart_offset_seed,
181 (unsigned long)&memstart_offset_seed +
182 sizeof(memstart_offset_seed));
183
184 return offset;
185 }
186
kaslr_init(void)187 static int __init kaslr_init(void)
188 {
189 switch (kaslr_status) {
190 case KASLR_ENABLED:
191 pr_info("KASLR enabled\n");
192 break;
193 case KASLR_DISABLED_CMDLINE:
194 pr_info("KASLR disabled on command line\n");
195 break;
196 case KASLR_DISABLED_NO_SEED:
197 pr_warn("KASLR disabled due to lack of seed\n");
198 break;
199 case KASLR_DISABLED_FDT_REMAP:
200 pr_warn("KASLR disabled due to FDT remapping failure\n");
201 break;
202 }
203
204 return 0;
205 }
206 core_initcall(kaslr_init)
207