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
37static DEFINE_PER_CPU(u32, prng_state);
38
39void __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 kasan_enable();
48
49 pr_info("KernelAddressSanitizer initialized (sw-tags, stacktrace=%s)\n",
50 str_on_off(kasan_stack_collection_enabled()));
51}
52
53/*
54 * If a preemption happens between this_cpu_read and this_cpu_write, the only
55 * side effect is that we'll give a few allocated in different contexts objects
56 * the same tag. Since tag-based KASAN is meant to be used a probabilistic
57 * bug-detection debug feature, this doesn't have significant negative impact.
58 *
59 * Ideally the tags use strong randomness to prevent any attempts to predict
60 * them during explicit exploit attempts. But strong randomness is expensive,
61 * and we did an intentional trade-off to use a PRNG. This non-atomic RMW
62 * sequence has in fact positive effect, since interrupts that randomly skew
63 * PRNG at unpredictable points do only good.
64 */
65u8 kasan_random_tag(void)
66{
67 u32 state = this_cpu_read(prng_state);
68
69 state = 1664525 * state + 1013904223;
70 this_cpu_write(prng_state, state);
71
72 return (u8)(state % (KASAN_TAG_MAX + 1));
73}
74
75bool kasan_check_range(const void *addr, size_t size, bool write,
76 unsigned long ret_ip)
77{
78 u8 tag;
79 u8 *shadow_first, *shadow_last, *shadow;
80 void *untagged_addr;
81
82 if (unlikely(size == 0))
83 return true;
84
85 if (unlikely(addr + size < addr))
86 return !kasan_report(addr, size, is_write: write, ip: ret_ip);
87
88 tag = get_tag((const void *)addr);
89
90 /*
91 * Ignore accesses for pointers tagged with 0xff (native kernel
92 * pointer tag) to suppress false positives caused by kmap.
93 *
94 * Some kernel code was written to account for archs that don't keep
95 * high memory mapped all the time, but rather map and unmap particular
96 * pages when needed. Instead of storing a pointer to the kernel memory,
97 * this code saves the address of the page structure and offset within
98 * that page for later use. Those pages are then mapped and unmapped
99 * with kmap/kunmap when necessary and virt_to_page is used to get the
100 * virtual address of the page. For arm64 (that keeps the high memory
101 * mapped all the time), kmap is turned into a page_address call.
102
103 * The issue is that with use of the page_address + virt_to_page
104 * sequence the top byte value of the original pointer gets lost (gets
105 * set to KASAN_TAG_KERNEL (0xFF)).
106 */
107 if (tag == KASAN_TAG_KERNEL)
108 return true;
109
110 untagged_addr = kasan_reset_tag(addr: (const void *)addr);
111 if (unlikely(!addr_has_metadata(untagged_addr)))
112 return !kasan_report(addr, size, is_write: write, ip: ret_ip);
113 shadow_first = kasan_mem_to_shadow(addr: untagged_addr);
114 shadow_last = kasan_mem_to_shadow(addr: untagged_addr + size - 1);
115 for (shadow = shadow_first; shadow <= shadow_last; shadow++) {
116 if (*shadow != tag) {
117 return !kasan_report(addr, size, is_write: write, ip: ret_ip);
118 }
119 }
120
121 return true;
122}
123
124bool kasan_byte_accessible(const void *addr)
125{
126 u8 tag = get_tag(addr);
127 void *untagged_addr = kasan_reset_tag(addr);
128 u8 shadow_byte;
129
130 if (!addr_has_metadata(addr: untagged_addr))
131 return false;
132
133 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
134 return tag == KASAN_TAG_KERNEL || tag == shadow_byte;
135}
136
137#define DEFINE_HWASAN_LOAD_STORE(size) \
138 void __hwasan_load##size##_noabort(void *addr) \
139 { \
140 kasan_check_range(addr, size, false, _RET_IP_); \
141 } \
142 EXPORT_SYMBOL(__hwasan_load##size##_noabort); \
143 void __hwasan_store##size##_noabort(void *addr) \
144 { \
145 kasan_check_range(addr, size, true, _RET_IP_); \
146 } \
147 EXPORT_SYMBOL(__hwasan_store##size##_noabort)
148
149DEFINE_HWASAN_LOAD_STORE(1);
150DEFINE_HWASAN_LOAD_STORE(2);
151DEFINE_HWASAN_LOAD_STORE(4);
152DEFINE_HWASAN_LOAD_STORE(8);
153DEFINE_HWASAN_LOAD_STORE(16);
154
155void __hwasan_loadN_noabort(void *addr, ssize_t size)
156{
157 kasan_check_range(addr, size, write: false, _RET_IP_);
158}
159EXPORT_SYMBOL(__hwasan_loadN_noabort);
160
161void __hwasan_storeN_noabort(void *addr, ssize_t size)
162{
163 kasan_check_range(addr, size, write: true, _RET_IP_);
164}
165EXPORT_SYMBOL(__hwasan_storeN_noabort);
166
167void __hwasan_tag_memory(void *addr, u8 tag, ssize_t size)
168{
169 kasan_poison(addr, size, value: tag, init: false);
170}
171EXPORT_SYMBOL(__hwasan_tag_memory);
172
173void kasan_tag_mismatch(void *addr, unsigned long access_info,
174 unsigned long ret_ip)
175{
176 kasan_report(addr, size: 1 << (access_info & 0xf), is_write: access_info & 0x10,
177 ip: ret_ip);
178}
179

source code of linux/mm/kasan/sw_tags.c