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

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