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(const 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 struct ctl_table table_copy = *table; 31 32 table_copy.data = &state; 33 ret = proc_dointvec_minmax(&table_copy, 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 static int __init stackleak_sysctls_init(void) 60 { 61 register_sysctl_init("kernel", stackleak_sysctls); 62 return 0; 63 } 64 late_initcall(stackleak_sysctls_init); 65 #endif /* CONFIG_SYSCTL */ 66 67 #define skip_erasing() static_branch_unlikely(&stack_erasing_bypass) 68 #else 69 #define skip_erasing() false 70 #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */ 71 72 #ifndef __stackleak_poison 73 static __always_inline void __stackleak_poison(unsigned long erase_low, 74 unsigned long erase_high, 75 unsigned long poison) 76 { 77 while (erase_low < erase_high) { 78 *(unsigned long *)erase_low = poison; 79 erase_low += sizeof(unsigned long); 80 } 81 } 82 #endif 83 84 static __always_inline void __stackleak_erase(bool on_task_stack) 85 { 86 const unsigned long task_stack_low = stackleak_task_low_bound(current); 87 const unsigned long task_stack_high = stackleak_task_high_bound(current); 88 unsigned long erase_low, erase_high; 89 90 erase_low = stackleak_find_top_of_poison(task_stack_low, 91 current->lowest_stack); 92 93 #ifdef CONFIG_STACKLEAK_METRICS 94 current->prev_lowest_stack = erase_low; 95 #endif 96 97 /* 98 * Write poison to the task's stack between 'erase_low' and 99 * 'erase_high'. 100 * 101 * If we're running on a different stack (e.g. an entry trampoline 102 * stack) we can erase everything below the pt_regs at the top of the 103 * task stack. 104 * 105 * If we're running on the task stack itself, we must not clobber any 106 * stack used by this function and its caller. We assume that this 107 * function has a fixed-size stack frame, and the current stack pointer 108 * doesn't change while we write poison. 109 */ 110 if (on_task_stack) 111 erase_high = current_stack_pointer; 112 else 113 erase_high = task_stack_high; 114 115 __stackleak_poison(erase_low, erase_high, STACKLEAK_POISON); 116 117 /* Reset the 'lowest_stack' value for the next syscall */ 118 current->lowest_stack = task_stack_high; 119 } 120 121 /* 122 * Erase and poison the portion of the task stack used since the last erase. 123 * Can be called from the task stack or an entry stack when the task stack is 124 * no longer in use. 125 */ 126 asmlinkage void noinstr stackleak_erase(void) 127 { 128 if (skip_erasing()) 129 return; 130 131 __stackleak_erase(on_thread_stack()); 132 } 133 134 /* 135 * Erase and poison the portion of the task stack used since the last erase. 136 * Can only be called from the task stack. 137 */ 138 asmlinkage void noinstr stackleak_erase_on_task_stack(void) 139 { 140 if (skip_erasing()) 141 return; 142 143 __stackleak_erase(true); 144 } 145 146 /* 147 * Erase and poison the portion of the task stack used since the last erase. 148 * Can only be called from a stack other than the task stack. 149 */ 150 asmlinkage void noinstr stackleak_erase_off_task_stack(void) 151 { 152 if (skip_erasing()) 153 return; 154 155 __stackleak_erase(false); 156 } 157 158 void __used __no_caller_saved_registers noinstr stackleak_track_stack(void) 159 { 160 unsigned long sp = current_stack_pointer; 161 162 /* 163 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than 164 * STACKLEAK_SEARCH_DEPTH makes the poison search in 165 * stackleak_erase() unreliable. Let's prevent that. 166 */ 167 BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH); 168 169 /* 'lowest_stack' should be aligned on the register width boundary */ 170 sp = ALIGN(sp, sizeof(unsigned long)); 171 if (sp < current->lowest_stack && 172 sp >= stackleak_task_low_bound(current)) { 173 current->lowest_stack = sp; 174 } 175 } 176 EXPORT_SYMBOL(stackleak_track_stack); 177