xref: /linux/mm/kasan/sw_tags.c (revision 9c5968db9e625019a0ee5226c7eebef5519d366a)
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * This file contains core software tag-based KASAN code.
4  *
5  * Copyright (c) 2018 Google, Inc.
6  * Author: Andrey Konovalov <andreyknvl@google.com>
7  */
8 
9 #define pr_fmt(fmt) "kasan: " fmt
10 
11 #include <linux/export.h>
12 #include <linux/interrupt.h>
13 #include <linux/init.h>
14 #include <linux/kasan.h>
15 #include <linux/kernel.h>
16 #include <linux/kmemleak.h>
17 #include <linux/linkage.h>
18 #include <linux/memblock.h>
19 #include <linux/memory.h>
20 #include <linux/mm.h>
21 #include <linux/module.h>
22 #include <linux/printk.h>
23 #include <linux/random.h>
24 #include <linux/sched.h>
25 #include <linux/sched/task_stack.h>
26 #include <linux/slab.h>
27 #include <linux/stacktrace.h>
28 #include <linux/string.h>
29 #include <linux/string_choices.h>
30 #include <linux/types.h>
31 #include <linux/vmalloc.h>
32 #include <linux/bug.h>
33 
34 #include "kasan.h"
35 #include "../slab.h"
36 
37 static DEFINE_PER_CPU(u32, prng_state);
38 
39 void __init kasan_init_sw_tags(void)
40 {
41 	int cpu;
42 
43 	for_each_possible_cpu(cpu)
44 		per_cpu(prng_state, cpu) = (u32)get_cycles();
45 
46 	kasan_init_tags();
47 
48 	pr_info("KernelAddressSanitizer initialized (sw-tags, stacktrace=%s)\n",
49 		str_on_off(kasan_stack_collection_enabled()));
50 }
51 
52 /*
53  * If a preemption happens between this_cpu_read and this_cpu_write, the only
54  * side effect is that we'll give a few allocated in different contexts objects
55  * the same tag. Since tag-based KASAN is meant to be used a probabilistic
56  * bug-detection debug feature, this doesn't have significant negative impact.
57  *
58  * Ideally the tags use strong randomness to prevent any attempts to predict
59  * them during explicit exploit attempts. But strong randomness is expensive,
60  * and we did an intentional trade-off to use a PRNG. This non-atomic RMW
61  * sequence has in fact positive effect, since interrupts that randomly skew
62  * PRNG at unpredictable points do only good.
63  */
64 u8 kasan_random_tag(void)
65 {
66 	u32 state = this_cpu_read(prng_state);
67 
68 	state = 1664525 * state + 1013904223;
69 	this_cpu_write(prng_state, state);
70 
71 	return (u8)(state % (KASAN_TAG_MAX + 1));
72 }
73 
74 bool kasan_check_range(const void *addr, size_t size, bool write,
75 			unsigned long ret_ip)
76 {
77 	u8 tag;
78 	u8 *shadow_first, *shadow_last, *shadow;
79 	void *untagged_addr;
80 
81 	if (unlikely(size == 0))
82 		return true;
83 
84 	if (unlikely(addr + size < addr))
85 		return !kasan_report(addr, size, write, ret_ip);
86 
87 	tag = get_tag((const void *)addr);
88 
89 	/*
90 	 * Ignore accesses for pointers tagged with 0xff (native kernel
91 	 * pointer tag) to suppress false positives caused by kmap.
92 	 *
93 	 * Some kernel code was written to account for archs that don't keep
94 	 * high memory mapped all the time, but rather map and unmap particular
95 	 * pages when needed. Instead of storing a pointer to the kernel memory,
96 	 * this code saves the address of the page structure and offset within
97 	 * that page for later use. Those pages are then mapped and unmapped
98 	 * with kmap/kunmap when necessary and virt_to_page is used to get the
99 	 * virtual address of the page. For arm64 (that keeps the high memory
100 	 * mapped all the time), kmap is turned into a page_address call.
101 
102 	 * The issue is that with use of the page_address + virt_to_page
103 	 * sequence the top byte value of the original pointer gets lost (gets
104 	 * set to KASAN_TAG_KERNEL (0xFF)).
105 	 */
106 	if (tag == KASAN_TAG_KERNEL)
107 		return true;
108 
109 	untagged_addr = kasan_reset_tag((const void *)addr);
110 	if (unlikely(!addr_has_metadata(untagged_addr)))
111 		return !kasan_report(addr, size, write, ret_ip);
112 	shadow_first = kasan_mem_to_shadow(untagged_addr);
113 	shadow_last = kasan_mem_to_shadow(untagged_addr + size - 1);
114 	for (shadow = shadow_first; shadow <= shadow_last; shadow++) {
115 		if (*shadow != tag) {
116 			return !kasan_report(addr, size, write, ret_ip);
117 		}
118 	}
119 
120 	return true;
121 }
122 
123 bool kasan_byte_accessible(const void *addr)
124 {
125 	u8 tag = get_tag(addr);
126 	void *untagged_addr = kasan_reset_tag(addr);
127 	u8 shadow_byte;
128 
129 	if (!addr_has_metadata(untagged_addr))
130 		return false;
131 
132 	shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
133 	return tag == KASAN_TAG_KERNEL || tag == shadow_byte;
134 }
135 
136 #define DEFINE_HWASAN_LOAD_STORE(size)					\
137 	void __hwasan_load##size##_noabort(void *addr)			\
138 	{								\
139 		kasan_check_range(addr, size, false, _RET_IP_);		\
140 	}								\
141 	EXPORT_SYMBOL(__hwasan_load##size##_noabort);			\
142 	void __hwasan_store##size##_noabort(void *addr)			\
143 	{								\
144 		kasan_check_range(addr, size, true, _RET_IP_);		\
145 	}								\
146 	EXPORT_SYMBOL(__hwasan_store##size##_noabort)
147 
148 DEFINE_HWASAN_LOAD_STORE(1);
149 DEFINE_HWASAN_LOAD_STORE(2);
150 DEFINE_HWASAN_LOAD_STORE(4);
151 DEFINE_HWASAN_LOAD_STORE(8);
152 DEFINE_HWASAN_LOAD_STORE(16);
153 
154 void __hwasan_loadN_noabort(void *addr, ssize_t size)
155 {
156 	kasan_check_range(addr, size, false, _RET_IP_);
157 }
158 EXPORT_SYMBOL(__hwasan_loadN_noabort);
159 
160 void __hwasan_storeN_noabort(void *addr, ssize_t size)
161 {
162 	kasan_check_range(addr, size, true, _RET_IP_);
163 }
164 EXPORT_SYMBOL(__hwasan_storeN_noabort);
165 
166 void __hwasan_tag_memory(void *addr, u8 tag, ssize_t size)
167 {
168 	kasan_poison(addr, size, tag, false);
169 }
170 EXPORT_SYMBOL(__hwasan_tag_memory);
171 
172 void kasan_tag_mismatch(void *addr, unsigned long access_info,
173 			unsigned long ret_ip)
174 {
175 	kasan_report(addr, 1 << (access_info & 0xf), access_info & 0x10,
176 		     ret_ip);
177 }
178