kernel/scs.c

   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         kasan_unpoison_vmalloc(s, SCS_SIZE);
  82         vfree_atomic(s);
  83 }
  84
  85 static int scs_cleanup(unsigned int cpu)
  86 {
  87         int i;
  88         void **cache = per_cpu_ptr(scs_cache, cpu);
  89
  90         for (i = 0; i < NR_CACHED_SCS; i++) {
  91                 vfree(cache[i]);
  92                 cache[i] = NULL;
  93         }
  94
  95         return 0;
  96 }
  97
  98 void __init scs_init(void)
  99 {
 100         cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL,
 101                           scs_cleanup);
 102 }
 103
 104 int scs_prepare(struct task_struct *tsk, int node)
 105 {
 106         void *s = scs_alloc(node);
 107
 108         if (!s)
 109                 return -ENOMEM;
 110
 111         task_scs(tsk) = task_scs_sp(tsk) = s;
 112         return 0;
 113 }
 114
 115 static void scs_check_usage(struct task_struct *tsk)
 116 {
 117         static unsigned long highest;
 118
 119         unsigned long *p, prev, curr = highest, used = 0;
 120
 121         if (!IS_ENABLED(CONFIG_DEBUG_STACK_USAGE))
 122                 return;
 123
 124         for (p = task_scs(tsk); p < __scs_magic(tsk); ++p) {
 125                 if (!READ_ONCE_NOCHECK(*p))
 126                         break;
 127                 used += sizeof(*p);
 128         }
 129
 130         while (used > curr) {
 131                 prev = cmpxchg_relaxed(&highest, curr, used);
 132
 133                 if (prev == curr) {
 134                         pr_info("%s (%d): highest shadow stack usage: %lu bytes\n",
 135                                 tsk->comm, task_pid_nr(tsk), used);
 136                         break;
 137                 }
 138
 139                 curr = prev;
 140         }
 141 }
 142
 143 void scs_release(struct task_struct *tsk)
 144 {
 145         void *s = task_scs(tsk);
 146
 147         if (!s)
 148                 return;
 149
 150         WARN(task_scs_end_corrupted(tsk),
 151              "corrupted shadow stack detected when freeing task\n");
 152         scs_check_usage(tsk);
 153         scs_free(s);
 154 }