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 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 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 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 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 kasan_unpoison_vmalloc(s, SCS_SIZE); 82 vfree_atomic(s); 83 } 84 85 static int scs_cleanup(unsigned int cpu) 86 { 87 int i; 88 void **cache = per_cpu_ptr(scs_cache, cpu); 89 90 for (i = 0; i < NR_CACHED_SCS; i++) { 91 vfree(cache[i]); 92 cache[i] = NULL; 93 } 94 95 return 0; 96 } 97 98 void __init scs_init(void) 99 { 100 cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL, 101 scs_cleanup); 102 } 103 104 int scs_prepare(struct task_struct *tsk, int node) 105 { 106 void *s = scs_alloc(node); 107 108 if (!s) 109 return -ENOMEM; 110 111 task_scs(tsk) = task_scs_sp(tsk) = s; 112 return 0; 113 } 114 115 static void scs_check_usage(struct task_struct *tsk) 116 { 117 static unsigned long highest; 118 119 unsigned long *p, prev, curr = highest, used = 0; 120 121 if (!IS_ENABLED(CONFIG_DEBUG_STACK_USAGE)) 122 return; 123 124 for (p = task_scs(tsk); p < __scs_magic(tsk); ++p) { 125 if (!READ_ONCE_NOCHECK(*p)) 126 break; 127 used += sizeof(*p); 128 } 129 130 while (used > curr) { 131 prev = cmpxchg_relaxed(&highest, curr, used); 132 133 if (prev == curr) { 134 pr_info("%s (%d): highest shadow stack usage: %lu bytes\n", 135 tsk->comm, task_pid_nr(tsk), used); 136 break; 137 } 138 139 curr = prev; 140 } 141 } 142 143 void scs_release(struct task_struct *tsk) 144 { 145 void *s = task_scs(tsk); 146 147 if (!s) 148 return; 149 150 WARN(task_scs_end_corrupted(tsk), 151 "corrupted shadow stack detected when freeing task\n"); 152 scs_check_usage(tsk); 153 scs_free(s); 154 } 155