xref: /openbmc/linux/arch/arm64/kernel/pi/kaslr_early.c (revision ff590009)
1 // SPDX-License-Identifier: GPL-2.0-only
2 // Copyright 2022 Google LLC
3 // Author: Ard Biesheuvel <ardb@google.com>
4 
5 // NOTE: code in this file runs *very* early, and is not permitted to use
6 // global variables or anything that relies on absolute addressing.
7 
8 #include <linux/libfdt.h>
9 #include <linux/init.h>
10 #include <linux/linkage.h>
11 #include <linux/types.h>
12 #include <linux/sizes.h>
13 #include <linux/string.h>
14 
15 #include <asm/archrandom.h>
16 #include <asm/memory.h>
17 
18 /* taken from lib/string.c */
__strstr(const char * s1,const char * s2)19 static char *__strstr(const char *s1, const char *s2)
20 {
21 	size_t l1, l2;
22 
23 	l2 = strlen(s2);
24 	if (!l2)
25 		return (char *)s1;
26 	l1 = strlen(s1);
27 	while (l1 >= l2) {
28 		l1--;
29 		if (!memcmp(s1, s2, l2))
30 			return (char *)s1;
31 		s1++;
32 	}
33 	return NULL;
34 }
cmdline_contains_nokaslr(const u8 * cmdline)35 static bool cmdline_contains_nokaslr(const u8 *cmdline)
36 {
37 	const u8 *str;
38 
39 	str = __strstr(cmdline, "nokaslr");
40 	return str == cmdline || (str > cmdline && *(str - 1) == ' ');
41 }
42 
is_kaslr_disabled_cmdline(void * fdt)43 static bool is_kaslr_disabled_cmdline(void *fdt)
44 {
45 	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
46 		int node;
47 		const u8 *prop;
48 
49 		node = fdt_path_offset(fdt, "/chosen");
50 		if (node < 0)
51 			goto out;
52 
53 		prop = fdt_getprop(fdt, node, "bootargs", NULL);
54 		if (!prop)
55 			goto out;
56 
57 		if (cmdline_contains_nokaslr(prop))
58 			return true;
59 
60 		if (IS_ENABLED(CONFIG_CMDLINE_EXTEND))
61 			goto out;
62 
63 		return false;
64 	}
65 out:
66 	return cmdline_contains_nokaslr(CONFIG_CMDLINE);
67 }
68 
get_kaslr_seed(void * fdt)69 static u64 get_kaslr_seed(void *fdt)
70 {
71 	int node, len;
72 	fdt64_t *prop;
73 	u64 ret;
74 
75 	node = fdt_path_offset(fdt, "/chosen");
76 	if (node < 0)
77 		return 0;
78 
79 	prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
80 	if (!prop || len != sizeof(u64))
81 		return 0;
82 
83 	ret = fdt64_to_cpu(*prop);
84 	*prop = 0;
85 	return ret;
86 }
87 
kaslr_early_init(void * fdt)88 asmlinkage u64 kaslr_early_init(void *fdt)
89 {
90 	u64 seed;
91 
92 	if (is_kaslr_disabled_cmdline(fdt))
93 		return 0;
94 
95 	seed = get_kaslr_seed(fdt);
96 	if (!seed) {
97 		if (!__early_cpu_has_rndr() ||
98 		    !__arm64_rndr((unsigned long *)&seed))
99 			return 0;
100 	}
101 
102 	/*
103 	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
104 	 * kernel image offset from the seed. Let's place the kernel in the
105 	 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
106 	 * the lower and upper quarters to avoid colliding with other
107 	 * allocations.
108 	 */
109 	return BIT(VA_BITS_MIN - 3) + (seed & GENMASK(VA_BITS_MIN - 3, 0));
110 }
111