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