1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * This code fills the used part of the kernel stack with a poison value 4 * before returning to userspace. It's part of the STACKLEAK feature 5 * ported from grsecurity/PaX. 6 * 7 * Author: Alexander Popov <alex.popov@linux.com> 8 * 9 * STACKLEAK reduces the information which kernel stack leak bugs can 10 * reveal and blocks some uninitialized stack variable attacks. 11 */ 12 13 #include <linux/stackleak.h> 14 #include <linux/kprobes.h> 15 16 #ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE 17 #include <linux/jump_label.h> 18 #include <linux/sysctl.h> 19 #include <linux/init.h> 20 21 static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass); 22 23 #ifdef CONFIG_SYSCTL 24 static int stack_erasing_sysctl(struct ctl_table *table, int write, 25 void __user *buffer, size_t *lenp, loff_t *ppos) 26 { 27 int ret = 0; 28 int state = !static_branch_unlikely(&stack_erasing_bypass); 29 int prev_state = state; 30 31 table->data = &state; 32 table->maxlen = sizeof(int); 33 ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); 34 state = !!state; 35 if (ret || !write || state == prev_state) 36 return ret; 37 38 if (state) 39 static_branch_disable(&stack_erasing_bypass); 40 else 41 static_branch_enable(&stack_erasing_bypass); 42 43 pr_warn("stackleak: kernel stack erasing is %s\n", 44 state ? "enabled" : "disabled"); 45 return ret; 46 } 47 static struct ctl_table stackleak_sysctls[] = { 48 { 49 .procname = "stack_erasing", 50 .data = NULL, 51 .maxlen = sizeof(int), 52 .mode = 0600, 53 .proc_handler = stack_erasing_sysctl, 54 .extra1 = SYSCTL_ZERO, 55 .extra2 = SYSCTL_ONE, 56 }, 57 {} 58 }; 59 60 static int __init stackleak_sysctls_init(void) 61 { 62 register_sysctl_init("kernel", stackleak_sysctls); 63 return 0; 64 } 65 late_initcall(stackleak_sysctls_init); 66 #endif /* CONFIG_SYSCTL */ 67 68 #define skip_erasing() static_branch_unlikely(&stack_erasing_bypass) 69 #else 70 #define skip_erasing() false 71 #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */ 72 73 #ifndef __stackleak_poison 74 static __always_inline void __stackleak_poison(unsigned long erase_low, 75 unsigned long erase_high, 76 unsigned long poison) 77 { 78 while (erase_low < erase_high) { 79 *(unsigned long *)erase_low = poison; 80 erase_low += sizeof(unsigned long); 81 } 82 } 83 #endif 84 85 static __always_inline void __stackleak_erase(bool on_task_stack) 86 { 87 const unsigned long task_stack_low = stackleak_task_low_bound(current); 88 const unsigned long task_stack_high = stackleak_task_high_bound(current); 89 unsigned long erase_low, erase_high; 90 91 erase_low = stackleak_find_top_of_poison(task_stack_low, 92 current->lowest_stack); 93 94 #ifdef CONFIG_STACKLEAK_METRICS 95 current->prev_lowest_stack = erase_low; 96 #endif 97 98 /* 99 * Write poison to the task's stack between 'erase_low' and 100 * 'erase_high'. 101 * 102 * If we're running on a different stack (e.g. an entry trampoline 103 * stack) we can erase everything below the pt_regs at the top of the 104 * task stack. 105 * 106 * If we're running on the task stack itself, we must not clobber any 107 * stack used by this function and its caller. We assume that this 108 * function has a fixed-size stack frame, and the current stack pointer 109 * doesn't change while we write poison. 110 */ 111 if (on_task_stack) 112 erase_high = current_stack_pointer; 113 else 114 erase_high = task_stack_high; 115 116 __stackleak_poison(erase_low, erase_high, STACKLEAK_POISON); 117 118 /* Reset the 'lowest_stack' value for the next syscall */ 119 current->lowest_stack = task_stack_high; 120 } 121 122 /* 123 * Erase and poison the portion of the task stack used since the last erase. 124 * Can be called from the task stack or an entry stack when the task stack is 125 * no longer in use. 126 */ 127 asmlinkage void noinstr stackleak_erase(void) 128 { 129 if (skip_erasing()) 130 return; 131 132 __stackleak_erase(on_thread_stack()); 133 } 134 135 /* 136 * Erase and poison the portion of the task stack used since the last erase. 137 * Can only be called from the task stack. 138 */ 139 asmlinkage void noinstr stackleak_erase_on_task_stack(void) 140 { 141 if (skip_erasing()) 142 return; 143 144 __stackleak_erase(true); 145 } 146 147 /* 148 * Erase and poison the portion of the task stack used since the last erase. 149 * Can only be called from a stack other than the task stack. 150 */ 151 asmlinkage void noinstr stackleak_erase_off_task_stack(void) 152 { 153 if (skip_erasing()) 154 return; 155 156 __stackleak_erase(false); 157 } 158 159 void __used __no_caller_saved_registers noinstr stackleak_track_stack(void) 160 { 161 unsigned long sp = current_stack_pointer; 162 163 /* 164 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than 165 * STACKLEAK_SEARCH_DEPTH makes the poison search in 166 * stackleak_erase() unreliable. Let's prevent that. 167 */ 168 BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH); 169 170 /* 'lowest_stack' should be aligned on the register width boundary */ 171 sp = ALIGN(sp, sizeof(unsigned long)); 172 if (sp < current->lowest_stack && 173 sp >= stackleak_task_low_bound(current)) { 174 current->lowest_stack = sp; 175 } 176 } 177 EXPORT_SYMBOL(stackleak_track_stack); 178