1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Shadow Call Stack support.
4 *
5 * Copyright (C) 2019 Google LLC
6 */
7
8 #include <linux/cpuhotplug.h>
9 #include <linux/kasan.h>
10 #include <linux/mm.h>
11 #include <linux/scs.h>
12 #include <linux/vmalloc.h>
13 #include <linux/vmstat.h>
14
__scs_account(void * s,int account)15 static void __scs_account(void *s, int account)
16 {
17 struct page *scs_page = vmalloc_to_page(s);
18
19 mod_node_page_state(page_pgdat(scs_page), NR_KERNEL_SCS_KB,
20 account * (SCS_SIZE / SZ_1K));
21 }
22
23 /* Matches NR_CACHED_STACKS for VMAP_STACK */
24 #define NR_CACHED_SCS 2
25 static DEFINE_PER_CPU(void *, scs_cache[NR_CACHED_SCS]);
26
__scs_alloc(int node)27 static void *__scs_alloc(int node)
28 {
29 int i;
30 void *s;
31
32 for (i = 0; i < NR_CACHED_SCS; i++) {
33 s = this_cpu_xchg(scs_cache[i], NULL);
34 if (s) {
35 kasan_unpoison_vmalloc(s, SCS_SIZE);
36 memset(s, 0, SCS_SIZE);
37 return s;
38 }
39 }
40
41 return __vmalloc_node_range(SCS_SIZE, 1, VMALLOC_START, VMALLOC_END,
42 GFP_SCS, PAGE_KERNEL, 0, node,
43 __builtin_return_address(0));
44 }
45
scs_alloc(int node)46 void *scs_alloc(int node)
47 {
48 void *s;
49
50 s = __scs_alloc(node);
51 if (!s)
52 return NULL;
53
54 *__scs_magic(s) = SCS_END_MAGIC;
55
56 /*
57 * Poison the allocation to catch unintentional accesses to
58 * the shadow stack when KASAN is enabled.
59 */
60 kasan_poison_vmalloc(s, SCS_SIZE);
61 __scs_account(s, 1);
62 return s;
63 }
64
scs_free(void * s)65 void scs_free(void *s)
66 {
67 int i;
68
69 __scs_account(s, -1);
70
71 /*
72 * We cannot sleep as this can be called in interrupt context,
73 * so use this_cpu_cmpxchg to update the cache, and vfree_atomic
74 * to free the stack.
75 */
76
77 for (i = 0; i < NR_CACHED_SCS; i++)
78 if (this_cpu_cmpxchg(scs_cache[i], 0, s) == NULL)
79 return;
80
81 vfree_atomic(s);
82 }
83
scs_cleanup(unsigned int cpu)84 static int scs_cleanup(unsigned int cpu)
85 {
86 int i;
87 void **cache = per_cpu_ptr(scs_cache, cpu);
88
89 for (i = 0; i < NR_CACHED_SCS; i++) {
90 vfree(cache[i]);
91 cache[i] = NULL;
92 }
93
94 return 0;
95 }
96
scs_init(void)97 void __init scs_init(void)
98 {
99 cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL,
100 scs_cleanup);
101 }
102
scs_prepare(struct task_struct * tsk,int node)103 int scs_prepare(struct task_struct *tsk, int node)
104 {
105 void *s = scs_alloc(node);
106
107 if (!s)
108 return -ENOMEM;
109
110 task_scs(tsk) = task_scs_sp(tsk) = s;
111 return 0;
112 }
113
scs_check_usage(struct task_struct * tsk)114 static void scs_check_usage(struct task_struct *tsk)
115 {
116 static unsigned long highest;
117
118 unsigned long *p, prev, curr = highest, used = 0;
119
120 if (!IS_ENABLED(CONFIG_DEBUG_STACK_USAGE))
121 return;
122
123 for (p = task_scs(tsk); p < __scs_magic(tsk); ++p) {
124 if (!READ_ONCE_NOCHECK(*p))
125 break;
126 used += sizeof(*p);
127 }
128
129 while (used > curr) {
130 prev = cmpxchg_relaxed(&highest, curr, used);
131
132 if (prev == curr) {
133 pr_info("%s (%d): highest shadow stack usage: %lu bytes\n",
134 tsk->comm, task_pid_nr(tsk), used);
135 break;
136 }
137
138 curr = prev;
139 }
140 }
141
scs_release(struct task_struct * tsk)142 void scs_release(struct task_struct *tsk)
143 {
144 void *s = task_scs(tsk);
145
146 if (!s)
147 return;
148
149 WARN(task_scs_end_corrupted(tsk),
150 "corrupted shadow stack detected when freeing task\n");
151 scs_check_usage(tsk);
152 scs_free(s);
153 }
154
