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
14 #include <asm/cacheflush.h>
15 #include <asm/fixmap.h>
16 #include <asm/kernel-pgtable.h>
17 #include <asm/memory.h>
18 #include <asm/mmu.h>
19 #include <asm/pgtable.h>
20 #include <asm/sections.h>
21
22 u64 __ro_after_init module_alloc_base;
23 u16 __initdata memstart_offset_seed;
24
get_kaslr_seed(void * fdt)25 static __init u64 get_kaslr_seed(void *fdt)
26 {
27 int node, len;
28 fdt64_t *prop;
29 u64 ret;
30
31 node = fdt_path_offset(fdt, "/chosen");
32 if (node < 0)
33 return 0;
34
35 prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
36 if (!prop || len != sizeof(u64))
37 return 0;
38
39 ret = fdt64_to_cpu(*prop);
40 *prop = 0;
41 return ret;
42 }
43
kaslr_get_cmdline(void * fdt)44 static __init const u8 *kaslr_get_cmdline(void *fdt)
45 {
46 static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
47
48 if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
49 int node;
50 const u8 *prop;
51
52 node = fdt_path_offset(fdt, "/chosen");
53 if (node < 0)
54 goto out;
55
56 prop = fdt_getprop(fdt, node, "bootargs", NULL);
57 if (!prop)
58 goto out;
59 return prop;
60 }
61 out:
62 return default_cmdline;
63 }
64
65 /*
66 * This routine will be executed with the kernel mapped at its default virtual
67 * address, and if it returns successfully, the kernel will be remapped, and
68 * start_kernel() will be executed from a randomized virtual offset. The
69 * relocation will result in all absolute references (e.g., static variables
70 * containing function pointers) to be reinitialized, and zero-initialized
71 * .bss variables will be reset to 0.
72 */
kaslr_early_init(u64 dt_phys)73 u64 __init kaslr_early_init(u64 dt_phys)
74 {
75 void *fdt;
76 u64 seed, offset, mask, module_range;
77 const u8 *cmdline, *str;
78 int size;
79
80 /*
81 * Set a reasonable default for module_alloc_base in case
82 * we end up running with module randomization disabled.
83 */
84 module_alloc_base = (u64)_etext - MODULES_VSIZE;
85 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
86
87 /*
88 * Try to map the FDT early. If this fails, we simply bail,
89 * and proceed with KASLR disabled. We will make another
90 * attempt at mapping the FDT in setup_machine()
91 */
92 early_fixmap_init();
93 fdt = fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
94 if (!fdt)
95 return 0;
96
97 /*
98 * Retrieve (and wipe) the seed from the FDT
99 */
100 seed = get_kaslr_seed(fdt);
101 if (!seed)
102 return 0;
103
104 /*
105 * Check if 'nokaslr' appears on the command line, and
106 * return 0 if that is the case.
107 */
108 cmdline = kaslr_get_cmdline(fdt);
109 str = strstr(cmdline, "nokaslr");
110 if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
111 return 0;
112
113 /*
114 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
115 * kernel image offset from the seed. Let's place the kernel in the
116 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
117 * the lower and upper quarters to avoid colliding with other
118 * allocations.
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
122 */
123 mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
124 offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
125
126 /* use the top 16 bits to randomize the linear region */
127 memstart_offset_seed = seed >> 48;
128
129 if (IS_ENABLED(CONFIG_KASAN))
130 /*
131 * KASAN does not expect the module region to intersect the
132 * vmalloc region, since shadow memory is allocated for each
133 * module at load time, whereas the vmalloc region is shadowed
134 * by KASAN zero pages. So keep modules out of the vmalloc
135 * region if KASAN is enabled, and put the kernel well within
136 * 4 GB of the module region.
137 */
138 return offset % SZ_2G;
139
140 if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
141 /*
142 * Randomize the module region over a 2 GB window covering the
143 * kernel. This reduces the risk of modules leaking information
144 * about the address of the kernel itself, but results in
145 * branches between modules and the core kernel that are
146 * resolved via PLTs. (Branches between modules will be
147 * resolved normally.)
148 */
149 module_range = SZ_2G - (u64)(_end - _stext);
150 module_alloc_base = max((u64)_end + offset - SZ_2G,
151 (u64)MODULES_VADDR);
152 } else {
153 /*
154 * Randomize the module region by setting module_alloc_base to
155 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
156 * _stext) . This guarantees that the resulting region still
157 * covers [_stext, _etext], and that all relative branches can
158 * be resolved without veneers.
159 */
160 module_range = MODULES_VSIZE - (u64)(_etext - _stext);
161 module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
162 }
163
164 /* use the lower 21 bits to randomize the base of the module region */
165 module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
166 module_alloc_base &= PAGE_MASK;
167
168 __flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
169 __flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
170
171 return offset;
172 }
173
