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 s = kasan_unpoison_vmalloc(s, SCS_SIZE,
36 KASAN_VMALLOC_PROT_NORMAL);
37 memset(s, 0, SCS_SIZE);
38 goto out;
39 }
40 }
41
42 s = __vmalloc_node_range(SCS_SIZE, 1, VMALLOC_START, VMALLOC_END,
43 GFP_SCS, PAGE_KERNEL, 0, node,
44 __builtin_return_address(0));
45
46 out:
47 return kasan_reset_tag(s);
48 }
49
scs_alloc(int node)50 void *scs_alloc(int node)
51 {
52 void *s;
53
54 s = __scs_alloc(node);
55 if (!s)
56 return NULL;
57
58 *__scs_magic(s) = SCS_END_MAGIC;
59
60 /*
61 * Poison the allocation to catch unintentional accesses to
62 * the shadow stack when KASAN is enabled.
63 */
64 kasan_poison_vmalloc(s, SCS_SIZE);
65 __scs_account(s, 1);
66 return s;
67 }
68
scs_free(void * s)69 void scs_free(void *s)
70 {
71 int i;
72
73 __scs_account(s, -1);
74
75 /*
76 * We cannot sleep as this can be called in interrupt context,
77 * so use this_cpu_cmpxchg to update the cache, and vfree_atomic
78 * to free the stack.
79 */
80
81 for (i = 0; i < NR_CACHED_SCS; i++)
82 if (this_cpu_cmpxchg(scs_cache[i], 0, s) == NULL)
83 return;
84
85 kasan_unpoison_vmalloc(s, SCS_SIZE, KASAN_VMALLOC_PROT_NORMAL);
86 vfree_atomic(s);
87 }
88
scs_cleanup(unsigned int cpu)89 static int scs_cleanup(unsigned int cpu)
90 {
91 int i;
92 void **cache = per_cpu_ptr(scs_cache, cpu);
93
94 for (i = 0; i < NR_CACHED_SCS; i++) {
95 vfree(cache[i]);
96 cache[i] = NULL;
97 }
98
99 return 0;
100 }
101
scs_init(void)102 void __init scs_init(void)
103 {
104 cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL,
105 scs_cleanup);
106 }
107
scs_prepare(struct task_struct * tsk,int node)108 int scs_prepare(struct task_struct *tsk, int node)
109 {
110 void *s = scs_alloc(node);
111
112 if (!s)
113 return -ENOMEM;
114
115 task_scs(tsk) = task_scs_sp(tsk) = s;
116 return 0;
117 }
118
scs_check_usage(struct task_struct * tsk)119 static void scs_check_usage(struct task_struct *tsk)
120 {
121 static unsigned long highest;
122
123 unsigned long *p, prev, curr = highest, used = 0;
124
125 if (!IS_ENABLED(CONFIG_DEBUG_STACK_USAGE))
126 return;
127
128 for (p = task_scs(tsk); p < __scs_magic(tsk); ++p) {
129 if (!READ_ONCE_NOCHECK(*p))
130 break;
131 used += sizeof(*p);
132 }
133
134 while (used > curr) {
135 prev = cmpxchg_relaxed(&highest, curr, used);
136
137 if (prev == curr) {
138 pr_info("%s (%d): highest shadow stack usage: %lu bytes\n",
139 tsk->comm, task_pid_nr(tsk), used);
140 break;
141 }
142
143 curr = prev;
144 }
145 }
146
scs_release(struct task_struct * tsk)147 void scs_release(struct task_struct *tsk)
148 {
149 void *s = task_scs(tsk);
150
151 if (!s)
152 return;
153
154 WARN(task_scs_end_corrupted(tsk),
155 "corrupted shadow stack detected when freeing task\n");
156 scs_check_usage(tsk);
157 scs_free(s);
158 }
159
