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 vfree_atomic(s); 82 } 83 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 97 void __init scs_init(void) 98 { 99 cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL, 100 scs_cleanup); 101 } 102 103 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 } 113 114 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 } 141 142 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