arch/x86/mm/kmmio.c

   1 /* Support for MMIO probes.
   2  * Benfit many code from kprobes
   3  * (C) 2002 Louis Zhuang <louis.zhuang@intel.com>.
   4  *     2007 Alexander Eichner
   5  *     2008 Pekka Paalanen <pq@iki.fi>
   6  */
   7
   8 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
   9
  10 #include <linux/list.h>
  11 #include <linux/rculist.h>
  12 #include <linux/spinlock.h>
  13 #include <linux/hash.h>
  14 #include <linux/init.h>
  15 #include <linux/module.h>
  16 #include <linux/kernel.h>
  17 #include <linux/uaccess.h>
  18 #include <linux/ptrace.h>
  19 #include <linux/preempt.h>
  20 #include <linux/percpu.h>
  21 #include <linux/kdebug.h>
  22 #include <linux/mutex.h>
  23 #include <linux/io.h>
  24 #include <linux/slab.h>
  25 #include <asm/cacheflush.h>
  26 #include <asm/tlbflush.h>
  27 #include <linux/errno.h>
  28 #include <asm/debugreg.h>
  29 #include <linux/mmiotrace.h>
  30
  31 #define KMMIO_PAGE_HASH_BITS 4
  32 #define KMMIO_PAGE_TABLE_SIZE (1 << KMMIO_PAGE_HASH_BITS)
  33
  34 struct kmmio_fault_page {
  35         struct list_head list;
  36         struct kmmio_fault_page *release_next;
  37         unsigned long page; /* location of the fault page */
  38         pteval_t old_presence; /* page presence prior to arming */
  39         bool armed;
  40
  41         /*
  42          * Number of times this page has been registered as a part
  43          * of a probe. If zero, page is disarmed and this may be freed.
  44          * Used only by writers (RCU) and post_kmmio_handler().
  45          * Protected by kmmio_lock, when linked into kmmio_page_table.
  46          */
  47         int count;
  48
  49         bool scheduled_for_release;
  50 };
  51
  52 struct kmmio_delayed_release {
  53         struct rcu_head rcu;
  54         struct kmmio_fault_page *release_list;
  55 };
  56
  57 struct kmmio_context {
  58         struct kmmio_fault_page *fpage;
  59         struct kmmio_probe *probe;
  60         unsigned long saved_flags;
  61         unsigned long addr;
  62         int active;
  63 };
  64
  65 static DEFINE_SPINLOCK(kmmio_lock);
  66
  67 /* Protected by kmmio_lock */
  68 unsigned int kmmio_count;
  69
  70 /* Read-protected by RCU, write-protected by kmmio_lock. */
  71 static struct list_head kmmio_page_table[KMMIO_PAGE_TABLE_SIZE];
  72 static LIST_HEAD(kmmio_probes);
  73
  74 static struct list_head *kmmio_page_list(unsigned long page)
  75 {
  76         return &kmmio_page_table[hash_long(page, KMMIO_PAGE_HASH_BITS)];
  77 }
  78
  79 /* Accessed per-cpu */
  80 static DEFINE_PER_CPU(struct kmmio_context, kmmio_ctx);
  81
  82 /*
  83  * this is basically a dynamic stabbing problem:
  84  * Could use the existing prio tree code or
  85  * Possible better implementations:
  86  * The Interval Skip List: A Data Structure for Finding All Intervals That
  87  * Overlap a Point (might be simple)
  88  * Space Efficient Dynamic Stabbing with Fast Queries - Mikkel Thorup
  89  */
  90 /* Get the kmmio at this addr (if any). You must be holding RCU read lock. */
  91 static struct kmmio_probe *get_kmmio_probe(unsigned long addr)
  92 {
  93         struct kmmio_probe *p;
  94         list_for_each_entry_rcu(p, &kmmio_probes, list) {
  95                 if (addr >= p->addr && addr < (p->addr + p->len))
  96                         return p;
  97         }
  98         return NULL;
  99 }
 100
 101 /* You must be holding RCU read lock. */
 102 static struct kmmio_fault_page *get_kmmio_fault_page(unsigned long page)
 103 {
 104         struct list_head *head;
 105         struct kmmio_fault_page *f;
 106
 107         page &= PAGE_MASK;
 108         head = kmmio_page_list(page);
 109         list_for_each_entry_rcu(f, head, list) {
 110                 if (f->page == page)
 111                         return f;
 112         }
 113         return NULL;
 114 }
 115
 116 static void clear_pmd_presence(pmd_t *pmd, bool clear, pmdval_t *old)
 117 {
 118         pmdval_t v = pmd_val(*pmd);
 119         if (clear) {
 120                 *old = v & _PAGE_PRESENT;
 121                 v &= ~_PAGE_PRESENT;
 122         } else  /* presume this has been called with clear==true previously */
 123                 v |= *old;
 124         set_pmd(pmd, __pmd(v));
 125 }
 126
 127 static void clear_pte_presence(pte_t *pte, bool clear, pteval_t *old)
 128 {
 129         pteval_t v = pte_val(*pte);
 130         if (clear) {
 131                 *old = v & _PAGE_PRESENT;
 132                 v &= ~_PAGE_PRESENT;
 133         } else  /* presume this has been called with clear==true previously */
 134                 v |= *old;
 135         set_pte_atomic(pte, __pte(v));
 136 }
 137
 138 static int clear_page_presence(struct kmmio_fault_page *f, bool clear)
 139 {
 140         unsigned int level;
 141         pte_t *pte = lookup_address(f->page, &level);
 142
 143         if (!pte) {
 144                 pr_err("no pte for page 0x%08lx\n", f->page);
 145                 return -1;
 146         }
 147
 148         switch (level) {
 149         case PG_LEVEL_2M:
 150                 clear_pmd_presence((pmd_t *)pte, clear, &f->old_presence);
 151                 break;
 152         case PG_LEVEL_4K:
 153                 clear_pte_presence(pte, clear, &f->old_presence);
 154                 break;
 155         default:
 156                 pr_err("unexpected page level 0x%x.\n", level);
 157                 return -1;
 158         }
 159
 160         __flush_tlb_one(f->page);
 161         return 0;
 162 }
 163
 164 /*
 165  * Mark the given page as not present. Access to it will trigger a fault.
 166  *
 167  * Struct kmmio_fault_page is protected by RCU and kmmio_lock, but the
 168  * protection is ignored here. RCU read lock is assumed held, so the struct
 169  * will not disappear unexpectedly. Furthermore, the caller must guarantee,
 170  * that double arming the same virtual address (page) cannot occur.
 171  *
 172  * Double disarming on the other hand is allowed, and may occur when a fault
 173  * and mmiotrace shutdown happen simultaneously.
 174  */
 175 static int arm_kmmio_fault_page(struct kmmio_fault_page *f)
 176 {
 177         int ret;
 178         WARN_ONCE(f->armed, KERN_ERR pr_fmt("kmmio page already armed.\n"));
 179         if (f->armed) {
 180                 pr_warning("double-arm: page 0x%08lx, ref %d, old %d\n",
 181                            f->page, f->count, !!f->old_presence);
 182         }
 183         ret = clear_page_presence(f, true);
 184         WARN_ONCE(ret < 0, KERN_ERR pr_fmt("arming 0x%08lx failed.\n"),
 185                   f->page);
 186         f->armed = true;
 187         return ret;
 188 }
 189
 190 /** Restore the given page to saved presence state. */
 191 static void disarm_kmmio_fault_page(struct kmmio_fault_page *f)
 192 {
 193         int ret = clear_page_presence(f, false);
 194         WARN_ONCE(ret < 0,
 195                         KERN_ERR "kmmio disarming 0x%08lx failed.\n", f->page);
 196         f->armed = false;
 197 }
 198
 199 /*
 200  * This is being called from do_page_fault().
 201  *
 202  * We may be in an interrupt or a critical section. Also prefecthing may
 203  * trigger a page fault. We may be in the middle of process switch.
 204  * We cannot take any locks, because we could be executing especially
 205  * within a kmmio critical section.
 206  *
 207  * Local interrupts are disabled, so preemption cannot happen.
 208  * Do not enable interrupts, do not sleep, and watch out for other CPUs.
 209  */
 210 /*
 211  * Interrupts are disabled on entry as trap3 is an interrupt gate
 212  * and they remain disabled throughout this function.
 213  */
 214 int kmmio_handler(struct pt_regs *regs, unsigned long addr)
 215 {
 216         struct kmmio_context *ctx;
 217         struct kmmio_fault_page *faultpage;
 218         int ret = 0; /* default to fault not handled */
 219
 220         /*
 221          * Preemption is now disabled to prevent process switch during
 222          * single stepping. We can only handle one active kmmio trace
 223          * per cpu, so ensure that we finish it before something else
 224          * gets to run. We also hold the RCU read lock over single
 225          * stepping to avoid looking up the probe and kmmio_fault_page
 226          * again.
 227          */
 228         preempt_disable();
 229         rcu_read_lock();
 230
 231         faultpage = get_kmmio_fault_page(addr);
 232         if (!faultpage) {
 233                 /*
 234                  * Either this page fault is not caused by kmmio, or
 235                  * another CPU just pulled the kmmio probe from under
 236                  * our feet. The latter case should not be possible.
 237                  */
 238                 goto no_kmmio;
 239         }
 240
 241         ctx = &get_cpu_var(kmmio_ctx);
 242         if (ctx->active) {
 243                 if (addr == ctx->addr) {
 244                         /*
 245                          * A second fault on the same page means some other
 246                          * condition needs handling by do_page_fault(), the
 247                          * page really not being present is the most common.
 248                          */
 249                         pr_debug("secondary hit for 0x%08lx CPU %d.\n",
 250                                  addr, smp_processor_id());
 251
 252                         if (!faultpage->old_presence)
 253                                 pr_info("unexpected secondary hit for address 0x%08lx on CPU %d.\n",
 254                                         addr, smp_processor_id());
 255                 } else {
 256                         /*
 257                          * Prevent overwriting already in-flight context.
 258                          * This should not happen, let's hope disarming at
 259                          * least prevents a panic.
 260                          */
 261                         pr_emerg("recursive probe hit on CPU %d, for address 0x%08lx. Ignoring.\n",
 262                                  smp_processor_id(), addr);
 263                         pr_emerg("previous hit was at 0x%08lx.\n", ctx->addr);
 264                         disarm_kmmio_fault_page(faultpage);
 265                 }
 266                 goto no_kmmio_ctx;
 267         }
 268         ctx->active++;
 269
 270         ctx->fpage = faultpage;
 271         ctx->probe = get_kmmio_probe(addr);
 272         ctx->saved_flags = (regs->flags & (X86_EFLAGS_TF | X86_EFLAGS_IF));
 273         ctx->addr = addr;
 274
 275         if (ctx->probe && ctx->probe->pre_handler)
 276                 ctx->probe->pre_handler(ctx->probe, regs, addr);
 277
 278         /*
 279          * Enable single-stepping and disable interrupts for the faulting
 280          * context. Local interrupts must not get enabled during stepping.
 281          */
 282         regs->flags |= X86_EFLAGS_TF;
 283         regs->flags &= ~X86_EFLAGS_IF;
 284
 285         /* Now we set present bit in PTE and single step. */
 286         disarm_kmmio_fault_page(ctx->fpage);
 287
 288         /*
 289          * If another cpu accesses the same page while we are stepping,
 290          * the access will not be caught. It will simply succeed and the
 291          * only downside is we lose the event. If this becomes a problem,
 292          * the user should drop to single cpu before tracing.
 293          */
 294
 295         put_cpu_var(kmmio_ctx);
 296         return 1; /* fault handled */
 297
 298 no_kmmio_ctx:
 299         put_cpu_var(kmmio_ctx);
 300 no_kmmio:
 301         rcu_read_unlock();
 302         preempt_enable_no_resched();
 303         return ret;
 304 }
 305
 306 /*
 307  * Interrupts are disabled on entry as trap1 is an interrupt gate
 308  * and they remain disabled throughout this function.
 309  * This must always get called as the pair to kmmio_handler().
 310  */
 311 static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
 312 {
 313         int ret = 0;
 314         struct kmmio_context *ctx = &get_cpu_var(kmmio_ctx);
 315
 316         if (!ctx->active) {
 317                 /*
 318                  * debug traps without an active context are due to either
 319                  * something external causing them (f.e. using a debugger while
 320                  * mmio tracing enabled), or erroneous behaviour
 321                  */
 322                 pr_warning("unexpected debug trap on CPU %d.\n",
 323                            smp_processor_id());
 324                 goto out;
 325         }
 326
 327         if (ctx->probe && ctx->probe->post_handler)
 328                 ctx->probe->post_handler(ctx->probe, condition, regs);
 329
 330         /* Prevent racing against release_kmmio_fault_page(). */
 331         spin_lock(&kmmio_lock);
 332         if (ctx->fpage->count)
 333                 arm_kmmio_fault_page(ctx->fpage);
 334         spin_unlock(&kmmio_lock);
 335
 336         regs->flags &= ~X86_EFLAGS_TF;
 337         regs->flags |= ctx->saved_flags;
 338
 339         /* These were acquired in kmmio_handler(). */
 340         ctx->active--;
 341         BUG_ON(ctx->active);
 342         rcu_read_unlock();
 343         preempt_enable_no_resched();
 344
 345         /*
 346          * if somebody else is singlestepping across a probe point, flags
 347          * will have TF set, in which case, continue the remaining processing
 348          * of do_debug, as if this is not a probe hit.
 349          */
 350         if (!(regs->flags & X86_EFLAGS_TF))
 351                 ret = 1;
 352 out:
 353         put_cpu_var(kmmio_ctx);
 354         return ret;
 355 }
 356
 357 /* You must be holding kmmio_lock. */
 358 static int add_kmmio_fault_page(unsigned long page)
 359 {
 360         struct kmmio_fault_page *f;
 361
 362         page &= PAGE_MASK;
 363         f = get_kmmio_fault_page(page);
 364         if (f) {
 365                 if (!f->count)
 366                         arm_kmmio_fault_page(f);
 367                 f->count++;
 368                 return 0;
 369         }
 370
 371         f = kzalloc(sizeof(*f), GFP_ATOMIC);
 372         if (!f)
 373                 return -1;
 374
 375         f->count = 1;
 376         f->page = page;
 377
 378         if (arm_kmmio_fault_page(f)) {
 379                 kfree(f);
 380                 return -1;
 381         }
 382
 383         list_add_rcu(&f->list, kmmio_page_list(f->page));
 384
 385         return 0;
 386 }
 387
 388 /* You must be holding kmmio_lock. */
 389 static void release_kmmio_fault_page(unsigned long page,
 390                                 struct kmmio_fault_page **release_list)
 391 {
 392         struct kmmio_fault_page *f;
 393
 394         page &= PAGE_MASK;
 395         f = get_kmmio_fault_page(page);
 396         if (!f)
 397                 return;
 398
 399         f->count--;
 400         BUG_ON(f->count < 0);
 401         if (!f->count) {
 402                 disarm_kmmio_fault_page(f);
 403                 if (!f->scheduled_for_release) {
 404                         f->release_next = *release_list;
 405                         *release_list = f;
 406                         f->scheduled_for_release = true;
 407                 }
 408         }
 409 }
 410
 411 /*
 412  * With page-unaligned ioremaps, one or two armed pages may contain
 413  * addresses from outside the intended mapping. Events for these addresses
 414  * are currently silently dropped. The events may result only from programming
 415  * mistakes by accessing addresses before the beginning or past the end of a
 416  * mapping.
 417  */
 418 int register_kmmio_probe(struct kmmio_probe *p)
 419 {
 420         unsigned long flags;
 421         int ret = 0;
 422         unsigned long size = 0;
 423         const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
 424
 425         spin_lock_irqsave(&kmmio_lock, flags);
 426         if (get_kmmio_probe(p->addr)) {
 427                 ret = -EEXIST;
 428                 goto out;
 429         }
 430         kmmio_count++;
 431         list_add_rcu(&p->list, &kmmio_probes);
 432         while (size < size_lim) {
 433                 if (add_kmmio_fault_page(p->addr + size))
 434                         pr_err("Unable to set page fault.\n");
 435                 size += PAGE_SIZE;
 436         }
 437 out:
 438         spin_unlock_irqrestore(&kmmio_lock, flags);
 439         /*
 440          * XXX: What should I do here?
 441          * Here was a call to global_flush_tlb(), but it does not exist
 442          * anymore. It seems it's not needed after all.
 443          */
 444         return ret;
 445 }
 446 EXPORT_SYMBOL(register_kmmio_probe);
 447
 448 static void rcu_free_kmmio_fault_pages(struct rcu_head *head)
 449 {
 450         struct kmmio_delayed_release *dr = container_of(
 451                                                 head,
 452                                                 struct kmmio_delayed_release,
 453                                                 rcu);
 454         struct kmmio_fault_page *f = dr->release_list;
 455         while (f) {
 456                 struct kmmio_fault_page *next = f->release_next;
 457                 BUG_ON(f->count);
 458                 kfree(f);
 459                 f = next;
 460         }
 461         kfree(dr);
 462 }
 463
 464 static void remove_kmmio_fault_pages(struct rcu_head *head)
 465 {
 466         struct kmmio_delayed_release *dr =
 467                 container_of(head, struct kmmio_delayed_release, rcu);
 468         struct kmmio_fault_page *f = dr->release_list;
 469         struct kmmio_fault_page **prevp = &dr->release_list;
 470         unsigned long flags;
 471
 472         spin_lock_irqsave(&kmmio_lock, flags);
 473         while (f) {
 474                 if (!f->count) {
 475                         list_del_rcu(&f->list);
 476                         prevp = &f->release_next;
 477                 } else {
 478                         *prevp = f->release_next;
 479                         f->release_next = NULL;
 480                         f->scheduled_for_release = false;
 481                 }
 482                 f = *prevp;
 483         }
 484         spin_unlock_irqrestore(&kmmio_lock, flags);
 485
 486         /* This is the real RCU destroy call. */
 487         call_rcu(&dr->rcu, rcu_free_kmmio_fault_pages);
 488 }
 489
 490 /*
 491  * Remove a kmmio probe. You have to synchronize_rcu() before you can be
 492  * sure that the callbacks will not be called anymore. Only after that
 493  * you may actually release your struct kmmio_probe.
 494  *
 495  * Unregistering a kmmio fault page has three steps:
 496  * 1. release_kmmio_fault_page()
 497  *    Disarm the page, wait a grace period to let all faults finish.
 498  * 2. remove_kmmio_fault_pages()
 499  *    Remove the pages from kmmio_page_table.
 500  * 3. rcu_free_kmmio_fault_pages()
 501  *    Actually free the kmmio_fault_page structs as with RCU.
 502  */
 503 void unregister_kmmio_probe(struct kmmio_probe *p)
 504 {
 505         unsigned long flags;
 506         unsigned long size = 0;
 507         const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
 508         struct kmmio_fault_page *release_list = NULL;
 509         struct kmmio_delayed_release *drelease;
 510
 511         spin_lock_irqsave(&kmmio_lock, flags);
 512         while (size < size_lim) {
 513                 release_kmmio_fault_page(p->addr + size, &release_list);
 514                 size += PAGE_SIZE;
 515         }
 516         list_del_rcu(&p->list);
 517         kmmio_count--;
 518         spin_unlock_irqrestore(&kmmio_lock, flags);
 519
 520         if (!release_list)
 521                 return;
 522
 523         drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
 524         if (!drelease) {
 525                 pr_crit("leaking kmmio_fault_page objects.\n");
 526                 return;
 527         }
 528         drelease->release_list = release_list;
 529
 530         /*
 531          * This is not really RCU here. We have just disarmed a set of
 532          * pages so that they cannot trigger page faults anymore. However,
 533          * we cannot remove the pages from kmmio_page_table,
 534          * because a probe hit might be in flight on another CPU. The
 535          * pages are collected into a list, and they will be removed from
 536          * kmmio_page_table when it is certain that no probe hit related to
 537          * these pages can be in flight. RCU grace period sounds like a
 538          * good choice.
 539          *
 540          * If we removed the pages too early, kmmio page fault handler might
 541          * not find the respective kmmio_fault_page and determine it's not
 542          * a kmmio fault, when it actually is. This would lead to madness.
 543          */
 544         call_rcu(&drelease->rcu, remove_kmmio_fault_pages);
 545 }
 546 EXPORT_SYMBOL(unregister_kmmio_probe);
 547
 548 static int
 549 kmmio_die_notifier(struct notifier_block *nb, unsigned long val, void *args)
 550 {
 551         struct die_args *arg = args;
 552         unsigned long* dr6_p = (unsigned long *)ERR_PTR(arg->err);
 553
 554         if (val == DIE_DEBUG && (*dr6_p & DR_STEP))
 555                 if (post_kmmio_handler(*dr6_p, arg->regs) == 1) {
 556                         /*
 557                          * Reset the BS bit in dr6 (pointed by args->err) to
 558                          * denote completion of processing
 559                          */
 560                         *dr6_p &= ~DR_STEP;
 561                         return NOTIFY_STOP;
 562                 }
 563
 564         return NOTIFY_DONE;
 565 }
 566
 567 static struct notifier_block nb_die = {
 568         .notifier_call = kmmio_die_notifier
 569 };
 570
 571 int kmmio_init(void)
 572 {
 573         int i;
 574
 575         for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++)
 576                 INIT_LIST_HEAD(&kmmio_page_table[i]);
 577
 578         return register_die_notifier(&nb_die);
 579 }
 580
 581 void kmmio_cleanup(void)
 582 {
 583         int i;
 584
 585         unregister_die_notifier(&nb_die);
 586         for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++) {
 587                 WARN_ONCE(!list_empty(&kmmio_page_table[i]),
 588                         KERN_ERR "kmmio_page_table not empty at cleanup, any further tracing will leak memory.\n");
 589         }
 590 }