kernel/stackleak.c

   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);