aboutsummaryrefslogtreecommitdiffstats
path: root/arch/x86/mm/kmmio.c
blob: c0f6198565eb63592a3cbebaf7c8e538e987b157 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
/* Support for MMIO probes.
 * Benfit many code from kprobes
 * (C) 2002 Louis Zhuang <louis.zhuang@intel.com>.
 *     2007 Alexander Eichner
 *     2008 Pekka Paalanen <pq@iki.fi>
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/list.h>
#include <linux/rculist.h>
#include <linux/spinlock.h>
#include <linux/hash.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/uaccess.h>
#include <linux/ptrace.h>
#include <linux/preempt.h>
#include <linux/percpu.h>
#include <linux/kdebug.h>
#include <linux/mutex.h>
#include <linux/io.h>
#include <asm/cacheflush.h>
#include <asm/tlbflush.h>
#include <linux/errno.h>
#include <asm/debugreg.h>
#include <linux/mmiotrace.h>

#define KMMIO_PAGE_HASH_BITS 4
#define KMMIO_PAGE_TABLE_SIZE (1 << KMMIO_PAGE_HASH_BITS)

struct kmmio_fault_page {
	struct list_head list;
	struct kmmio_fault_page *release_next;
	unsigned long page; /* location of the fault page */
	pteval_t old_presence; /* page presence prior to arming */
	bool armed;

	/*
	 * Number of times this page has been registered as a part
	 * of a probe. If zero, page is disarmed and this may be freed.
	 * Used only by writers (RCU) and post_kmmio_handler().
	 * Protected by kmmio_lock, when linked into kmmio_page_table.
	 */
	int count;
};

struct kmmio_delayed_release {
	struct rcu_head rcu;
	struct kmmio_fault_page *release_list;
};

struct kmmio_context {
	struct kmmio_fault_page *fpage;
	struct kmmio_probe *probe;
	unsigned long saved_flags;
	unsigned long addr;
	int active;
};

static DEFINE_SPINLOCK(kmmio_lock);

/* Protected by kmmio_lock */
unsigned int kmmio_count;

/* Read-protected by RCU, write-protected by kmmio_lock. */
static struct list_head kmmio_page_table[KMMIO_PAGE_TABLE_SIZE];
static LIST_HEAD(kmmio_probes);

static struct list_head *kmmio_page_list(unsigned long page)
{
	return &kmmio_page_table[hash_long(page, KMMIO_PAGE_HASH_BITS)];
}

/* Accessed per-cpu */
static DEFINE_PER_CPU(struct kmmio_context, kmmio_ctx);

/*
 * this is basically a dynamic stabbing problem:
 * Could use the existing prio tree code or
 * Possible better implementations:
 * The Interval Skip List: A Data Structure for Finding All Intervals That
 * Overlap a Point (might be simple)
 * Space Efficient Dynamic Stabbing with Fast Queries - Mikkel Thorup
 */
/* Get the kmmio at this addr (if any). You must be holding RCU read lock. */
static struct kmmio_probe *get_kmmio_probe(unsigned long addr)
{
	struct kmmio_probe *p;
	list_for_each_entry_rcu(p, &kmmio_probes, list) {
		if (addr >= p->addr && addr < (p->addr + p->len))
			return p;
	}
	return NULL;
}

/* You must be holding RCU read lock. */
static struct kmmio_fault_page *get_kmmio_fault_page(unsigned long page)
{
	struct list_head *head;
	struct kmmio_fault_page *f;

	page &= PAGE_MASK;
	head = kmmio_page_list(page);
	list_for_each_entry_rcu(f, head, list) {
		if (f->page == page)
			return f;
	}
	return NULL;
}

static void clear_pmd_presence(pmd_t *pmd, bool clear, pmdval_t *old)
{
	pmdval_t v = pmd_val(*pmd);
	if (clear) {
		*old = v & _PAGE_PRESENT;
		v &= ~_PAGE_PRESENT;
	} else	/* presume this has been called with clear==true previously */
		v |= *old;
	set_pmd(pmd, __pmd(v));
}

static void clear_pte_presence(pte_t *pte, bool clear, pteval_t *old)
{
	pteval_t v = pte_val(*pte);
	if (clear) {
		*old = v & _PAGE_PRESENT;
		v &= ~_PAGE_PRESENT;
	} else	/* presume this has been called with clear==true previously */
		v |= *old;
	set_pte_atomic(pte, __pte(v));
}

static int clear_page_presence(struct kmmio_fault_page *f, bool clear)
{
	unsigned int level;
	pte_t *pte = lookup_address(f->page, &level);

	if (!pte) {
		pr_err("no pte for page 0x%08lx\n", f->page);
		return -1;
	}

	switch (level) {
	case PG_LEVEL_2M:
		clear_pmd_presence((pmd_t *)pte, clear, &f->old_presence);
		break;
	case PG_LEVEL_4K:
		clear_pte_presence(pte, clear, &f->old_presence);
		break;
	default:
		pr_err("unexpected page level 0x%x.\n", level);
		return -1;
	}

	__flush_tlb_one(f->page);
	return 0;
}

/*
 * Mark the given page as not present. Access to it will trigger a fault.
 *
 * Struct kmmio_fault_page is protected by RCU and kmmio_lock, but the
 * protection is ignored here. RCU read lock is assumed held, so the struct
 * will not disappear unexpectedly. Furthermore, the caller must guarantee,
 * that double arming the same virtual address (page) cannot occur.
 *
 * Double disarming on the other hand is allowed, and may occur when a fault
 * and mmiotrace shutdown happen simultaneously.
 */
static int arm_kmmio_fault_page(struct kmmio_fault_page *f)
{
	int ret;
	WARN_ONCE(f->armed, KERN_ERR pr_fmt("kmmio page already armed.\n"));
	if (f->armed) {
		pr_warning("double-arm: page 0x%08lx, ref %d, old %d\n",
			   f->page, f->count, !!f->old_presence);
	}
	ret = clear_page_presence(f, true);
	WARN_ONCE(ret < 0, KERN_ERR pr_fmt("arming 0x%08lx failed.\n"),
		  f->page);
	f->armed = true;
	return ret;
}

/** Restore the given page to saved presence state. */
static void disarm_kmmio_fault_page(struct kmmio_fault_page *f)
{
	int ret = clear_page_presence(f, false);
	WARN_ONCE(ret < 0,
			KERN_ERR "kmmio disarming 0x%08lx failed.\n", f->page);
	f->armed = false;
}

/*
 * This is being called from do_page_fault().
 *
 * We may be in an interrupt or a critical section. Also prefecthing may
 * trigger a page fault. We may be in the middle of process switch.
 * We cannot take any locks, because we could be executing especially
 * within a kmmio critical section.
 *
 * Local interrupts are disabled, so preemption cannot happen.
 * Do not enable interrupts, do not sleep, and watch out for other CPUs.
 */
/*
 * Interrupts are disabled on entry as trap3 is an interrupt gate
 * and they remain disabled throughout this function.
 */
int kmmio_handler(struct pt_regs *regs, unsigned long addr)
{
	struct kmmio_context *ctx;
	struct kmmio_fault_page *faultpage;
	int ret = 0; /* default to fault not handled */

	/*
	 * Preemption is now disabled to prevent process switch during
	 * single stepping. We can only handle one active kmmio trace
	 * per cpu, so ensure that we finish it before something else
	 * gets to run. We also hold the RCU read lock over single
	 * stepping to avoid looking up the probe and kmmio_fault_page
	 * again.
	 */
	preempt_disable();
	rcu_read_lock();

	faultpage = get_kmmio_fault_page(addr);
	if (!faultpage) {
		/*
		 * Either this page fault is not caused by kmmio, or
		 * another CPU just pulled the kmmio probe from under
		 * our feet. The latter case should not be possible.
		 */
		goto no_kmmio;
	}

	ctx = &get_cpu_var(kmmio_ctx);
	if (ctx->active) {
		if (addr == ctx->addr) {
			/*
			 * A second fault on the same page means some other
			 * condition needs handling by do_page_fault(), the
			 * page really not being present is the most common.
			 */
			pr_debug("secondary hit for 0x%08lx CPU %d.\n",
				 addr, smp_processor_id());

			if (!faultpage->old_presence)
				pr_info("unexpected secondary hit for address 0x%08lx on CPU %d.\n",
					addr, smp_processor_id());
		} else {
			/*
			 * Prevent overwriting already in-flight context.
			 * This should not happen, let's hope disarming at
			 * least prevents a panic.
			 */
			pr_emerg("recursive probe hit on CPU %d, for address 0x%08lx. Ignoring.\n",
				 smp_processor_id(), addr);
			pr_emerg("previous hit was at 0x%08lx.\n", ctx->addr);
			disarm_kmmio_fault_page(faultpage);
		}
		goto no_kmmio_ctx;
	}
	ctx->active++;

	ctx->fpage = faultpage;
	ctx->probe = get_kmmio_probe(addr);
	ctx->saved_flags = (regs->flags & (X86_EFLAGS_TF | X86_EFLAGS_IF));
	ctx->addr = addr;

	if (ctx->probe && ctx->probe->pre_handler)
		ctx->probe->pre_handler(ctx->probe, regs, addr);

	/*
	 * Enable single-stepping and disable interrupts for the faulting
	 * context. Local interrupts must not get enabled during stepping.
	 */
	regs->flags |= X86_EFLAGS_TF;
	regs->flags &= ~X86_EFLAGS_IF;

	/* Now we set present bit in PTE and single step. */
	disarm_kmmio_fault_page(ctx->fpage);

	/*
	 * If another cpu accesses the same page while we are stepping,
	 * the access will not be caught. It will simply succeed and the
	 * only downside is we lose the event. If this becomes a problem,
	 * the user should drop to single cpu before tracing.
	 */

	put_cpu_var(kmmio_ctx);
	return 1; /* fault handled */

no_kmmio_ctx:
	put_cpu_var(kmmio_ctx);
no_kmmio:
	rcu_read_unlock();
	preempt_enable_no_resched();
	return ret;
}

/*
 * Interrupts are disabled on entry as trap1 is an interrupt gate
 * and they remain disabled throughout this function.
 * This must always get called as the pair to kmmio_handler().
 */
static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
{
	int ret = 0;
	struct kmmio_context *ctx = &get_cpu_var(kmmio_ctx);

	if (!ctx->active) {
		/*
		 * debug traps without an active context are due to either
		 * something external causing them (f.e. using a debugger while
		 * mmio tracing enabled), or erroneous behaviour
		 */
		pr_warning("unexpected debug trap on CPU %d.\n",
			   smp_processor_id());
		goto out;
	}

	if (ctx->probe && ctx->probe->post_handler)
		ctx->probe->post_handler(ctx->probe, condition, regs);

	/* Prevent racing against release_kmmio_fault_page(). */
	spin_lock(&kmmio_lock);
	if (ctx->fpage->count)
		arm_kmmio_fault_page(ctx->fpage);
	spin_unlock(&kmmio_lock);

	regs->flags &= ~X86_EFLAGS_TF;
	regs->flags |= ctx->saved_flags;

	/* These were acquired in kmmio_handler(). */
	ctx->active--;
	BUG_ON(ctx->active);
	rcu_read_unlock();
	preempt_enable_no_resched();

	/*
	 * if somebody else is singlestepping across a probe point, flags
	 * will have TF set, in which case, continue the remaining processing
	 * of do_debug, as if this is not a probe hit.
	 */
	if (!(regs->flags & X86_EFLAGS_TF))
		ret = 1;
out:
	put_cpu_var(kmmio_ctx);
	return ret;
}

/* You must be holding kmmio_lock. */
static int add_kmmio_fault_page(unsigned long page)
{
	struct kmmio_fault_page *f;

	page &= PAGE_MASK;
	f = get_kmmio_fault_page(page);
	if (f) {
		if (!f->count)
			arm_kmmio_fault_page(f);
		f->count++;
		return 0;
	}

	f = kzalloc(sizeof(*f), GFP_ATOMIC);
	if (!f)
		return -1;

	f->count = 1;
	f->page = page;

	if (arm_kmmio_fault_page(f)) {
		kfree(f);
		return -1;
	}

	list_add_rcu(&f->list, kmmio_page_list(f->page));

	return 0;
}

/* You must be holding kmmio_lock. */
static void release_kmmio_fault_page(unsigned long page,
				struct kmmio_fault_page **release_list)
{
	struct kmmio_fault_page *f;

	page &= PAGE_MASK;
	f = get_kmmio_fault_page(page);
	if (!f)
		return;

	f->count--;
	BUG_ON(f->count < 0);
	if (!f->count) {
		disarm_kmmio_fault_page(f);
		f->release_next = *release_list;
		*release_list = f;
	}
}

/*
 * With page-unaligned ioremaps, one or two armed pages may contain
 * addresses from outside the intended mapping. Events for these addresses
 * are currently silently dropped. The events may result only from programming
 * mistakes by accessing addresses before the beginning or past the end of a
 * mapping.
 */
int register_kmmio_probe(struct kmmio_probe *p)
{
	unsigned long flags;
	int ret = 0;
	unsigned long size = 0;
	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);

	spin_lock_irqsave(&kmmio_lock, flags);
	if (get_kmmio_probe(p->addr)) {
		ret = -EEXIST;
		goto out;
	}
	kmmio_count++;
	list_add_rcu(&p->list, &kmmio_probes);
	while (size < size_lim) {
		if (add_kmmio_fault_page(p->addr + size))
			pr_err("Unable to set page fault.\n");
		size += PAGE_SIZE;
	}
out:
	spin_unlock_irqrestore(&kmmio_lock, flags);
	/*
	 * XXX: What should I do here?
	 * Here was a call to global_flush_tlb(), but it does not exist
	 * anymore. It seems it's not needed after all.
	 */
	return ret;
}
EXPORT_SYMBOL(register_kmmio_probe);

static void rcu_free_kmmio_fault_pages(struct rcu_head *head)
{
	struct kmmio_delayed_release *dr = container_of(
						head,
						struct kmmio_delayed_release,
						rcu);
	struct kmmio_fault_page *f = dr->release_list;
	while (f) {
		struct kmmio_fault_page *next = f->release_next;
		BUG_ON(f->count);
		kfree(f);
		f = next;
	}
	kfree(dr);
}

static void remove_kmmio_fault_pages(struct rcu_head *head)
{
	struct kmmio_delayed_release *dr =
		container_of(head, struct kmmio_delayed_release, rcu);
	struct kmmio_fault_page *f = dr->release_list;
	struct kmmio_fault_page **prevp = &dr->release_list;
	unsigned long flags;

	spin_lock_irqsave(&kmmio_lock, flags);
	while (f) {
		if (!f->count) {
			list_del_rcu(&f->list);
			prevp = &f->release_next;
		} else {
			*prevp = f->release_next;
		}
		f = f->release_next;
	}
	spin_unlock_irqrestore(&kmmio_lock, flags);

	/* This is the real RCU destroy call. */
	call_rcu(&dr->rcu, rcu_free_kmmio_fault_pages);
}

/*
 * Remove a kmmio probe. You have to synchronize_rcu() before you can be
 * sure that the callbacks will not be called anymore. Only after that
 * you may actually release your struct kmmio_probe.
 *
 * Unregistering a kmmio fault page has three steps:
 * 1. release_kmmio_fault_page()
 *    Disarm the page, wait a grace period to let all faults finish.
 * 2. remove_kmmio_fault_pages()
 *    Remove the pages from kmmio_page_table.
 * 3. rcu_free_kmmio_fault_pages()
 *    Actually free the kmmio_fault_page structs as with RCU.
 */
void unregister_kmmio_probe(struct kmmio_probe *p)
{
	unsigned long flags;
	unsigned long size = 0;
	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
	struct kmmio_fault_page *release_list = NULL;
	struct kmmio_delayed_release *drelease;

	spin_lock_irqsave(&kmmio_lock, flags);
	while (size < size_lim) {
		release_kmmio_fault_page(p->addr + size, &release_list);
		size += PAGE_SIZE;
	}
	list_del_rcu(&p->list);
	kmmio_count--;
	spin_unlock_irqrestore(&kmmio_lock, flags);

	drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
	if (!drelease) {
		pr_crit("leaking kmmio_fault_page objects.\n");
		return;
	}
	drelease->release_list = release_list;

	/*
	 * This is not really RCU here. We have just disarmed a set of
	 * pages so that they cannot trigger page faults anymore. However,
	 * we cannot remove the pages from kmmio_page_table,
	 * because a probe hit might be in flight on another CPU. The
	 * pages are collected into a list, and they will be removed from
	 * kmmio_page_table when it is certain that no probe hit related to
	 * these pages can be in flight. RCU grace period sounds like a
	 * good choice.
	 *
	 * If we removed the pages too early, kmmio page fault handler might
	 * not find the respective kmmio_fault_page and determine it's not
	 * a kmmio fault, when it actually is. This would lead to madness.
	 */
	call_rcu(&drelease->rcu, remove_kmmio_fault_pages);
}
EXPORT_SYMBOL(unregister_kmmio_probe);

static int
kmmio_die_notifier(struct notifier_block *nb, unsigned long val, void *args)
{
	struct die_args *arg = args;

	if (val == DIE_DEBUG && (arg->err & DR_STEP))
		if (post_kmmio_handler(arg->err, arg->regs) == 1) {
			/*
			 * Reset the BS bit in dr6 (pointed by args->err) to
			 * denote completion of processing
			 */
			(*(unsigned long *)ERR_PTR(arg->err)) &= ~DR_STEP;
			return NOTIFY_STOP;
		}

	return NOTIFY_DONE;
}

static struct notifier_block nb_die = {
	.notifier_call = kmmio_die_notifier
};

int kmmio_init(void)
{
	int i;

	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++)
		INIT_LIST_HEAD(&kmmio_page_table[i]);

	return register_die_notifier(&nb_die);
}

void kmmio_cleanup(void)
{
	int i;

	unregister_die_notifier(&nb_die);
	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++) {
		WARN_ONCE(!list_empty(&kmmio_page_table[i]),
			KERN_ERR "kmmio_page_table not empty at cleanup, any further tracing will leak memory.\n");
	}
}