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 <linux/stdarg.h>
   9
  10 #include <linux/kernel.h>
  11 #include <linux/lockdep.h>
  12 #include <linux/math.h>
  13 #include <linux/printk.h>
  14 #include <linux/sched/debug.h>
  15 #include <linux/seq_file.h>
  16 #include <linux/sprintf.h>
  17 #include <linux/stacktrace.h>
  18 #include <linux/string.h>
  19 #include <trace/events/error_report.h>
  20
  21 #include <asm/kfence.h>
  22
  23 #include "kfence.h"
  24
  25 /* May be overridden by <asm/kfence.h>. */
  26 #ifndef ARCH_FUNC_PREFIX
  27 #define ARCH_FUNC_PREFIX ""
  28 #endif
  29
  30 /* Helper function to either print to a seq_file or to console. */
  31 __printf(2, 3)
  32 static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)
  33 {
  34         va_list args;
  35
  36         va_start(args, fmt);
  37         if (seq)
  38                 seq_vprintf(seq, fmt, args);
  39         else
  40                 vprintk(fmt, args);
  41         va_end(args);
  42 }
  43
  44 /*
  45  * Get the number of stack entries to skip to get out of MM internals. @type is
  46  * optional, and if set to NULL, assumes an allocation or free stack.
  47  */
  48 static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries,
  49                             const enum kfence_error_type *type)
  50 {
  51         char buf[64];
  52         int skipnr, fallback = 0;
  53
  54         if (type) {
  55                 /* Depending on error type, find different stack entries. */
  56                 switch (*type) {
  57                 case KFENCE_ERROR_UAF:
  58                 case KFENCE_ERROR_OOB:
  59                 case KFENCE_ERROR_INVALID:
  60                         /*
  61                          * kfence_handle_page_fault() may be called with pt_regs
  62                          * set to NULL; in that case we'll simply show the full
  63                          * stack trace.
  64                          */
  65                         return 0;
  66                 case KFENCE_ERROR_CORRUPTION:
  67                 case KFENCE_ERROR_INVALID_FREE:
  68                         break;
  69                 }
  70         }
  71
  72         for (skipnr = 0; skipnr < num_entries; skipnr++) {
  73                 int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);
  74
  75                 if (str_has_prefix(buf, ARCH_FUNC_PREFIX "kfence_") ||
  76                     str_has_prefix(buf, ARCH_FUNC_PREFIX "__kfence_") ||
  77                     str_has_prefix(buf, ARCH_FUNC_PREFIX "__kmem_cache_free") ||
  78                     !strncmp(buf, ARCH_FUNC_PREFIX "__slab_free", len)) {
  79                         /*
  80                          * In case of tail calls from any of the below to any of
  81                          * the above, optimized by the compiler such that the
  82                          * stack trace would omit the initial entry point below.
  83                          */
  84                         fallback = skipnr + 1;
  85                 }
  86
  87                 /*
  88                  * The below list should only include the initial entry points
  89                  * into the slab allocators. Includes the *_bulk() variants by
  90                  * checking prefixes.
  91                  */
  92                 if (str_has_prefix(buf, ARCH_FUNC_PREFIX "kfree") ||
  93                     str_has_prefix(buf, ARCH_FUNC_PREFIX "kmem_cache_free") ||
  94                     str_has_prefix(buf, ARCH_FUNC_PREFIX "__kmalloc") ||
  95                     str_has_prefix(buf, ARCH_FUNC_PREFIX "kmem_cache_alloc"))
  96                         goto found;
  97         }
  98         if (fallback < num_entries)
  99                 return fallback;
 100 found:
 101         skipnr++;
 102         return skipnr < num_entries ? skipnr : 0;
 103 }
 104
 105 static void kfence_print_stack(struct seq_file *seq, const struct kfence_metadata *meta,
 106                                bool show_alloc)
 107 {
 108         const struct kfence_track *track = show_alloc ? &meta->alloc_track : &meta->free_track;
 109         u64 ts_sec = track->ts_nsec;
 110         unsigned long rem_nsec = do_div(ts_sec, NSEC_PER_SEC);
 111
 112         /* Timestamp matches printk timestamp format. */
 113         seq_con_printf(seq, "%s by task %d on cpu %d at %lu.%06lus:\n",
 114                        show_alloc ? "allocated" : "freed", track->pid,
 115                        track->cpu, (unsigned long)ts_sec, rem_nsec / 1000);
 116
 117         if (track->num_stack_entries) {
 118                 /* Skip allocation/free internals stack. */
 119                 int i = get_stack_skipnr(track->stack_entries, track->num_stack_entries, NULL);
 120
 121                 /* stack_trace_seq_print() does not exist; open code our own. */
 122                 for (; i < track->num_stack_entries; i++)
 123                         seq_con_printf(seq, " %pS\n", (void *)track->stack_entries[i]);
 124         } else {
 125                 seq_con_printf(seq, " no %s stack\n", show_alloc ? "allocation" : "deallocation");
 126         }
 127 }
 128
 129 void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta)
 130 {
 131         const int size = abs(meta->size);
 132         const unsigned long start = meta->addr;
 133         const struct kmem_cache *const cache = meta->cache;
 134
 135         lockdep_assert_held(&meta->lock);
 136
 137         if (meta->state == KFENCE_OBJECT_UNUSED) {
 138                 seq_con_printf(seq, "kfence-#%td unused\n", meta - kfence_metadata);
 139                 return;
 140         }
 141
 142         seq_con_printf(seq, "kfence-#%td: 0x%p-0x%p, size=%d, cache=%s\n\n",
 143                        meta - kfence_metadata, (void *)start, (void *)(start + size - 1),
 144                        size, (cache && cache->name) ? cache->name : "<destroyed>");
 145
 146         kfence_print_stack(seq, meta, true);
 147
 148         if (meta->state == KFENCE_OBJECT_FREED) {
 149                 seq_con_printf(seq, "\n");
 150                 kfence_print_stack(seq, meta, false);
 151         }
 152 }
 153
 154 /*
 155  * Show bytes at @addr that are different from the expected canary values, up to
 156  * @max_bytes.
 157  */
 158 static void print_diff_canary(unsigned long address, size_t bytes_to_show,
 159                               const struct kfence_metadata *meta)
 160 {
 161         const unsigned long show_until_addr = address + bytes_to_show;
 162         const u8 *cur, *end;
 163
 164         /* Do not show contents of object nor read into following guard page. */
 165         end = (const u8 *)(address < meta->addr ? min(show_until_addr, meta->addr)
 166                                                 : min(show_until_addr, PAGE_ALIGN(address)));
 167
 168         pr_cont("[");
 169         for (cur = (const u8 *)address; cur < end; cur++) {
 170                 if (*cur == KFENCE_CANARY_PATTERN_U8(cur))
 171                         pr_cont(" .");
 172                 else if (no_hash_pointers)
 173                         pr_cont(" 0x%02x", *cur);
 174                 else /* Do not leak kernel memory in non-debug builds. */
 175                         pr_cont(" !");
 176         }
 177         pr_cont(" ]");
 178 }
 179
 180 static const char *get_access_type(bool is_write)
 181 {
 182         return is_write ? "write" : "read";
 183 }
 184
 185 void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *regs,
 186                          const struct kfence_metadata *meta, enum kfence_error_type type)
 187 {
 188         unsigned long stack_entries[KFENCE_STACK_DEPTH] = { 0 };
 189         const ptrdiff_t object_index = meta ? meta - kfence_metadata : -1;
 190         int num_stack_entries;
 191         int skipnr = 0;
 192
 193         if (regs) {
 194                 num_stack_entries = stack_trace_save_regs(regs, stack_entries, KFENCE_STACK_DEPTH, 0);
 195         } else {
 196                 num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 1);
 197                 skipnr = get_stack_skipnr(stack_entries, num_stack_entries, &type);
 198         }
 199
 200         /* Require non-NULL meta, except if KFENCE_ERROR_INVALID. */
 201         if (WARN_ON(type != KFENCE_ERROR_INVALID && !meta))
 202                 return;
 203
 204         if (meta)
 205                 lockdep_assert_held(&meta->lock);
 206         /*
 207          * Because we may generate reports in printk-unfriendly parts of the
 208          * kernel, such as scheduler code, the use of printk() could deadlock.
 209          * Until such time that all printing code here is safe in all parts of
 210          * the kernel, accept the risk, and just get our message out (given the
 211          * system might already behave unpredictably due to the memory error).
 212          * As such, also disable lockdep to hide warnings, and avoid disabling
 213          * lockdep for the rest of the kernel.
 214          */
 215         lockdep_off();
 216
 217         pr_err("==================================================================\n");
 218         /* Print report header. */
 219         switch (type) {
 220         case KFENCE_ERROR_OOB: {
 221                 const bool left_of_object = address < meta->addr;
 222
 223                 pr_err("BUG: KFENCE: out-of-bounds %s in %pS\n\n", get_access_type(is_write),
 224                        (void *)stack_entries[skipnr]);
 225                 pr_err("Out-of-bounds %s at 0x%p (%luB %s of kfence-#%td):\n",
 226                        get_access_type(is_write), (void *)address,
 227                        left_of_object ? meta->addr - address : address - meta->addr,
 228                        left_of_object ? "left" : "right", object_index);
 229                 break;
 230         }
 231         case KFENCE_ERROR_UAF:
 232                 pr_err("BUG: KFENCE: use-after-free %s in %pS\n\n", get_access_type(is_write),
 233                        (void *)stack_entries[skipnr]);
 234                 pr_err("Use-after-free %s at 0x%p (in kfence-#%td):\n",
 235                        get_access_type(is_write), (void *)address, object_index);
 236                 break;
 237         case KFENCE_ERROR_CORRUPTION:
 238                 pr_err("BUG: KFENCE: memory corruption in %pS\n\n", (void *)stack_entries[skipnr]);
 239                 pr_err("Corrupted memory at 0x%p ", (void *)address);
 240                 print_diff_canary(address, 16, meta);
 241                 pr_cont(" (in kfence-#%td):\n", object_index);
 242                 break;
 243         case KFENCE_ERROR_INVALID:
 244                 pr_err("BUG: KFENCE: invalid %s in %pS\n\n", get_access_type(is_write),
 245                        (void *)stack_entries[skipnr]);
 246                 pr_err("Invalid %s at 0x%p:\n", get_access_type(is_write),
 247                        (void *)address);
 248                 break;
 249         case KFENCE_ERROR_INVALID_FREE:
 250                 pr_err("BUG: KFENCE: invalid free in %pS\n\n", (void *)stack_entries[skipnr]);
 251                 pr_err("Invalid free of 0x%p (in kfence-#%td):\n", (void *)address,
 252                        object_index);
 253                 break;
 254         }
 255
 256         /* Print stack trace and object info. */
 257         stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr, 0);
 258
 259         if (meta) {
 260                 pr_err("\n");
 261                 kfence_print_object(NULL, meta);
 262         }
 263
 264         /* Print report footer. */
 265         pr_err("\n");
 266         if (no_hash_pointers && regs)
 267                 show_regs(regs);
 268         else
 269                 dump_stack_print_info(KERN_ERR);
 270         trace_error_report_end(ERROR_DETECTOR_KFENCE, address);
 271         pr_err("==================================================================\n");
 272
 273         lockdep_on();
 274
 275         check_panic_on_warn("KFENCE");
 276
 277         /* We encountered a memory safety error, taint the kernel! */
 278         add_taint(TAINT_BAD_PAGE, LOCKDEP_STILL_OK);
 279 }
 280
 281 #ifdef CONFIG_PRINTK
 282 static void kfence_to_kp_stack(const struct kfence_track *track, void **kp_stack)
 283 {
 284         int i, j;
 285
 286         i = get_stack_skipnr(track->stack_entries, track->num_stack_entries, NULL);
 287         for (j = 0; i < track->num_stack_entries && j < KS_ADDRS_COUNT; ++i, ++j)
 288                 kp_stack[j] = (void *)track->stack_entries[i];
 289         if (j < KS_ADDRS_COUNT)
 290                 kp_stack[j] = NULL;
 291 }
 292
 293 bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab)
 294 {
 295         struct kfence_metadata *meta = addr_to_metadata((unsigned long)object);
 296         unsigned long flags;
 297
 298         if (!meta)
 299                 return false;
 300
 301         /*
 302          * If state is UNUSED at least show the pointer requested; the rest
 303          * would be garbage data.
 304          */
 305         kpp->kp_ptr = object;
 306
 307         /* Requesting info an a never-used object is almost certainly a bug. */
 308         if (WARN_ON(meta->state == KFENCE_OBJECT_UNUSED))
 309                 return true;
 310
 311         raw_spin_lock_irqsave(&meta->lock, flags);
 312
 313         kpp->kp_slab = slab;
 314         kpp->kp_slab_cache = meta->cache;
 315         kpp->kp_objp = (void *)meta->addr;
 316         kfence_to_kp_stack(&meta->alloc_track, kpp->kp_stack);
 317         if (meta->state == KFENCE_OBJECT_FREED)
 318                 kfence_to_kp_stack(&meta->free_track, kpp->kp_free_stack);
 319         /* get_stack_skipnr() ensures the first entry is outside allocator. */
 320         kpp->kp_ret = kpp->kp_stack[0];
 321
 322         raw_spin_unlock_irqrestore(&meta->lock, flags);
 323
 324         return true;
 325 }
 326 #endif