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
  20 static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
  21
  22 int stack_erasing_sysctl(struct ctl_table *table, int write,
  23                         void __user *buffer, size_t *lenp, loff_t *ppos)
  24 {
  25         int ret = 0;
  26         int state = !static_branch_unlikely(&stack_erasing_bypass);
  27         int prev_state = state;
  28
  29         table->data = &state;
  30         table->maxlen = sizeof(int);
  31         ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
  32         state = !!state;
  33         if (ret || !write || state == prev_state)
  34                 return ret;
  35
  36         if (state)
  37                 static_branch_disable(&stack_erasing_bypass);
  38         else
  39                 static_branch_enable(&stack_erasing_bypass);
  40
  41         pr_warn("stackleak: kernel stack erasing is %s\n",
  42                                         state ? "enabled" : "disabled");
  43         return ret;
  44 }
  45
  46 #define skip_erasing()  static_branch_unlikely(&stack_erasing_bypass)
  47 #else
  48 #define skip_erasing()  false
  49 #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
  50
  51 asmlinkage void notrace stackleak_erase(void)
  52 {
  53         /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
  54         unsigned long kstack_ptr = current->lowest_stack;
  55         unsigned long boundary = (unsigned long)end_of_stack(current);
  56         unsigned int poison_count = 0;
  57         const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
  58
  59         if (skip_erasing())
  60                 return;
  61
  62         /* Check that 'lowest_stack' value is sane */
  63         if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
  64                 kstack_ptr = boundary;
  65
  66         /* Search for the poison value in the kernel stack */
  67         while (kstack_ptr > boundary && poison_count <= depth) {
  68                 if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
  69                         poison_count++;
  70                 else
  71                         poison_count = 0;
  72
  73                 kstack_ptr -= sizeof(unsigned long);
  74         }
  75
  76         /*
  77          * One 'long int' at the bottom of the thread stack is reserved and
  78          * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK=y).
  79          */
  80         if (kstack_ptr == boundary)
  81                 kstack_ptr += sizeof(unsigned long);
  82
  83 #ifdef CONFIG_STACKLEAK_METRICS
  84         current->prev_lowest_stack = kstack_ptr;
  85 #endif
  86
  87         /*
  88          * Now write the poison value to the kernel stack. Start from
  89          * 'kstack_ptr' and move up till the new 'boundary'. We assume that
  90          * the stack pointer doesn't change when we write poison.
  91          */
  92         if (on_thread_stack())
  93                 boundary = current_stack_pointer;
  94         else
  95                 boundary = current_top_of_stack();
  96
  97         while (kstack_ptr < boundary) {
  98                 *(unsigned long *)kstack_ptr = STACKLEAK_POISON;
  99                 kstack_ptr += sizeof(unsigned long);
 100         }
 101
 102         /* Reset the 'lowest_stack' value for the next syscall */
 103         current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
 104 }
 105 NOKPROBE_SYMBOL(stackleak_erase);
 106
 107 void __used notrace stackleak_track_stack(void)
 108 {
 109         /*
 110          * N.B. stackleak_erase() fills the kernel stack with the poison value,
 111          * which has the register width. That code assumes that the value
 112          * of 'lowest_stack' is aligned on the register width boundary.
 113          *
 114          * That is true for x86 and x86_64 because of the kernel stack
 115          * alignment on these platforms (for details, see 'cc_stack_align' in
 116          * arch/x86/Makefile). Take care of that when you port STACKLEAK to
 117          * new platforms.
 118          */
 119         unsigned long sp = (unsigned long)&sp;
 120
 121         /*
 122          * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
 123          * STACKLEAK_SEARCH_DEPTH makes the poison search in
 124          * stackleak_erase() unreliable. Let's prevent that.
 125          */
 126         BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH);
 127
 128         if (sp < current->lowest_stack &&
 129             sp >= (unsigned long)task_stack_page(current) +
 130                                                 sizeof(unsigned long)) {
 131                 current->lowest_stack = sp;
 132         }
 133 }
 134 EXPORT_SYMBOL(stackleak_track_stack);