mm/kfence/report.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * KFENCE reporting.
   4  *
   5  * Copyright (C) 2020, Google LLC.
   6  */
   7
   8 #include <stdarg.h>
   9
  10 #include <linux/kernel.h>
  11 #include <linux/lockdep.h>
  12 #include <linux/printk.h>
  13 #include <linux/sched/debug.h>
  14 #include <linux/seq_file.h>
  15 #include <linux/stacktrace.h>
  16 #include <linux/string.h>
  17
  18 #include <asm/kfence.h>
  19
  20 #include "kfence.h"
  21
  22 extern bool no_hash_pointers;
  23
  24 /* Helper function to either print to a seq_file or to console. */
  25 __printf(2, 3)
  26 static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)
  27 {
  28         va_list args;
  29
  30         va_start(args, fmt);
  31         if (seq)
  32                 seq_vprintf(seq, fmt, args);
  33         else
  34                 vprintk(fmt, args);
  35         va_end(args);
  36 }
  37
  38 /*
  39  * Get the number of stack entries to skip to get out of MM internals. @type is
  40  * optional, and if set to NULL, assumes an allocation or free stack.
  41  */
  42 static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries,
  43                             const enum kfence_error_type *type)
  44 {
  45         char buf[64];
  46         int skipnr, fallback = 0;
  47
  48         if (type) {
  49                 /* Depending on error type, find different stack entries. */
  50                 switch (*type) {
  51                 case KFENCE_ERROR_UAF:
  52                 case KFENCE_ERROR_OOB:
  53                 case KFENCE_ERROR_INVALID:
  54                         /*
  55                          * kfence_handle_page_fault() may be called with pt_regs
  56                          * set to NULL; in that case we'll simply show the full
  57                          * stack trace.
  58                          */
  59                         return 0;
  60                 case KFENCE_ERROR_CORRUPTION:
  61                 case KFENCE_ERROR_INVALID_FREE:
  62                         break;
  63                 }
  64         }
  65
  66         for (skipnr = 0; skipnr < num_entries; skipnr++) {
  67                 int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);
  68
  69                 if (str_has_prefix(buf, "kfence_") || str_has_prefix(buf, "__kfence_") ||
  70                     !strncmp(buf, "__slab_free", len)) {
  71                         /*
  72                          * In case of tail calls from any of the below
  73                          * to any of the above.
  74                          */
  75                         fallback = skipnr + 1;
  76                 }
  77
  78                 /* Also the *_bulk() variants by only checking prefixes. */
  79                 if (str_has_prefix(buf, "kfree") ||
  80                     str_has_prefix(buf, "kmem_cache_free") ||
  81                     str_has_prefix(buf, "__kmalloc") ||
  82                     str_has_prefix(buf, "kmem_cache_alloc"))
  83                         goto found;
  84         }
  85         if (fallback < num_entries)
  86                 return fallback;
  87 found:
  88         skipnr++;
  89         return skipnr < num_entries ? skipnr : 0;
  90 }
  91
  92 static void kfence_print_stack(struct seq_file *seq, const struct kfence_metadata *meta,
  93                                bool show_alloc)
  94 {
  95         const struct kfence_track *track = show_alloc ? &meta->alloc_track : &meta->free_track;
  96
  97         if (track->num_stack_entries) {
  98                 /* Skip allocation/free internals stack. */
  99                 int i = get_stack_skipnr(track->stack_entries, track->num_stack_entries, NULL);
 100
 101                 /* stack_trace_seq_print() does not exist; open code our own. */
 102                 for (; i < track->num_stack_entries; i++)
 103                         seq_con_printf(seq, " %pS\n", (void *)track->stack_entries[i]);
 104         } else {
 105                 seq_con_printf(seq, " no %s stack\n", show_alloc ? "allocation" : "deallocation");
 106         }
 107 }
 108
 109 void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta)
 110 {
 111         const int size = abs(meta->size);
 112         const unsigned long start = meta->addr;
 113         const struct kmem_cache *const cache = meta->cache;
 114
 115         lockdep_assert_held(&meta->lock);
 116
 117         if (meta->state == KFENCE_OBJECT_UNUSED) {
 118                 seq_con_printf(seq, "kfence-#%zd unused\n", meta - kfence_metadata);
 119                 return;
 120         }
 121
 122         seq_con_printf(seq,
 123                        "kfence-#%zd [0x%p-0x%p"
 124                        ", size=%d, cache=%s] allocated by task %d:\n",
 125                        meta - kfence_metadata, (void *)start, (void *)(start + size - 1), size,
 126                        (cache && cache->name) ? cache->name : "<destroyed>", meta->alloc_track.pid);
 127         kfence_print_stack(seq, meta, true);
 128
 129         if (meta->state == KFENCE_OBJECT_FREED) {
 130                 seq_con_printf(seq, "\nfreed by task %d:\n", meta->free_track.pid);
 131                 kfence_print_stack(seq, meta, false);
 132         }
 133 }
 134
 135 /*
 136  * Show bytes at @addr that are different from the expected canary values, up to
 137  * @max_bytes.
 138  */
 139 static void print_diff_canary(unsigned long address, size_t bytes_to_show,
 140                               const struct kfence_metadata *meta)
 141 {
 142         const unsigned long show_until_addr = address + bytes_to_show;
 143         const u8 *cur, *end;
 144
 145         /* Do not show contents of object nor read into following guard page. */
 146         end = (const u8 *)(address < meta->addr ? min(show_until_addr, meta->addr)
 147                                                 : min(show_until_addr, PAGE_ALIGN(address)));
 148
 149         pr_cont("[");
 150         for (cur = (const u8 *)address; cur < end; cur++) {
 151                 if (*cur == KFENCE_CANARY_PATTERN(cur))
 152                         pr_cont(" .");
 153                 else if (no_hash_pointers)
 154                         pr_cont(" 0x%02x", *cur);
 155                 else /* Do not leak kernel memory in non-debug builds. */
 156                         pr_cont(" !");
 157         }
 158         pr_cont(" ]");
 159 }
 160
 161 static const char *get_access_type(bool is_write)
 162 {
 163         return is_write ? "write" : "read";
 164 }
 165
 166 void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *regs,
 167                          const struct kfence_metadata *meta, enum kfence_error_type type)
 168 {
 169         unsigned long stack_entries[KFENCE_STACK_DEPTH] = { 0 };
 170         const ptrdiff_t object_index = meta ? meta - kfence_metadata : -1;
 171         int num_stack_entries;
 172         int skipnr = 0;
 173
 174         if (regs) {
 175                 num_stack_entries = stack_trace_save_regs(regs, stack_entries, KFENCE_STACK_DEPTH, 0);
 176         } else {
 177                 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 1);
 178                 skipnr = get_stack_skipnr(stack_entries, num_stack_entries, &type);
 179         }
 180
 181         /* Require non-NULL meta, except if KFENCE_ERROR_INVALID. */
 182         if (WARN_ON(type != KFENCE_ERROR_INVALID && !meta))
 183                 return;
 184
 185         if (meta)
 186                 lockdep_assert_held(&meta->lock);
 187         /*
 188          * Because we may generate reports in printk-unfriendly parts of the
 189          * kernel, such as scheduler code, the use of printk() could deadlock.
 190          * Until such time that all printing code here is safe in all parts of
 191          * the kernel, accept the risk, and just get our message out (given the
 192          * system might already behave unpredictably due to the memory error).
 193          * As such, also disable lockdep to hide warnings, and avoid disabling
 194          * lockdep for the rest of the kernel.
 195          */
 196         lockdep_off();
 197
 198         pr_err("==================================================================\n");
 199         /* Print report header. */
 200         switch (type) {
 201         case KFENCE_ERROR_OOB: {
 202                 const bool left_of_object = address < meta->addr;
 203
 204                 pr_err("BUG: KFENCE: out-of-bounds %s in %pS\n\n", get_access_type(is_write),
 205                        (void *)stack_entries[skipnr]);
 206                 pr_err("Out-of-bounds %s at 0x%p (%luB %s of kfence-#%zd):\n",
 207                        get_access_type(is_write), (void *)address,
 208                        left_of_object ? meta->addr - address : address - meta->addr,
 209                        left_of_object ? "left" : "right", object_index);
 210                 break;
 211         }
 212         case KFENCE_ERROR_UAF:
 213                 pr_err("BUG: KFENCE: use-after-free %s in %pS\n\n", get_access_type(is_write),
 214                        (void *)stack_entries[skipnr]);
 215                 pr_err("Use-after-free %s at 0x%p (in kfence-#%zd):\n",
 216                        get_access_type(is_write), (void *)address, object_index);
 217                 break;
 218         case KFENCE_ERROR_CORRUPTION:
 219                 pr_err("BUG: KFENCE: memory corruption in %pS\n\n", (void *)stack_entries[skipnr]);
 220                 pr_err("Corrupted memory at 0x%p ", (void *)address);
 221                 print_diff_canary(address, 16, meta);
 222                 pr_cont(" (in kfence-#%zd):\n", object_index);
 223                 break;
 224         case KFENCE_ERROR_INVALID:
 225                 pr_err("BUG: KFENCE: invalid %s in %pS\n\n", get_access_type(is_write),
 226                        (void *)stack_entries[skipnr]);
 227                 pr_err("Invalid %s at 0x%p:\n", get_access_type(is_write),
 228                        (void *)address);
 229                 break;
 230         case KFENCE_ERROR_INVALID_FREE:
 231                 pr_err("BUG: KFENCE: invalid free in %pS\n\n", (void *)stack_entries[skipnr]);
 232                 pr_err("Invalid free of 0x%p (in kfence-#%zd):\n", (void *)address,
 233                        object_index);
 234                 break;
 235         }
 236
 237         /* Print stack trace and object info. */
 238         stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr, 0);
 239
 240         if (meta) {
 241                 pr_err("\n");
 242                 kfence_print_object(NULL, meta);
 243         }
 244
 245         /* Print report footer. */
 246         pr_err("\n");
 247         if (no_hash_pointers && regs)
 248                 show_regs(regs);
 249         else
 250                 dump_stack_print_info(KERN_ERR);
 251         pr_err("==================================================================\n");
 252
 253         lockdep_on();
 254
 255         if (panic_on_warn)
 256                 panic("panic_on_warn set ...\n");
 257
 258         /* We encountered a memory unsafety error, taint the kernel! */
 259         add_taint(TAINT_BAD_PAGE, LOCKDEP_STILL_OK);
 260 }