xref: /linux/mm/kfence/report.c (revision 9c5968db9e625019a0ee5226c7eebef5519d366a)
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 <linux/string_choices.h>
20 #include <linux/sched/clock.h>
21 #include <trace/events/error_report.h>
22 
23 #include <asm/kfence.h>
24 
25 #include "kfence.h"
26 
27 /* May be overridden by <asm/kfence.h>. */
28 #ifndef ARCH_FUNC_PREFIX
29 #define ARCH_FUNC_PREFIX ""
30 #endif
31 
32 /* Helper function to either print to a seq_file or to console. */
33 __printf(2, 3)
34 static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)
35 {
36 	va_list args;
37 
38 	va_start(args, fmt);
39 	if (seq)
40 		seq_vprintf(seq, fmt, args);
41 	else
42 		vprintk(fmt, args);
43 	va_end(args);
44 }
45 
46 /*
47  * Get the number of stack entries to skip to get out of MM internals. @type is
48  * optional, and if set to NULL, assumes an allocation or free stack.
49  */
50 static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries,
51 			    const enum kfence_error_type *type)
52 {
53 	char buf[64];
54 	int skipnr, fallback = 0;
55 
56 	if (type) {
57 		/* Depending on error type, find different stack entries. */
58 		switch (*type) {
59 		case KFENCE_ERROR_UAF:
60 		case KFENCE_ERROR_OOB:
61 		case KFENCE_ERROR_INVALID:
62 			/*
63 			 * kfence_handle_page_fault() may be called with pt_regs
64 			 * set to NULL; in that case we'll simply show the full
65 			 * stack trace.
66 			 */
67 			return 0;
68 		case KFENCE_ERROR_CORRUPTION:
69 		case KFENCE_ERROR_INVALID_FREE:
70 			break;
71 		}
72 	}
73 
74 	for (skipnr = 0; skipnr < num_entries; skipnr++) {
75 		int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);
76 
77 		if (str_has_prefix(buf, ARCH_FUNC_PREFIX "kfence_") ||
78 		    str_has_prefix(buf, ARCH_FUNC_PREFIX "__kfence_") ||
79 		    str_has_prefix(buf, ARCH_FUNC_PREFIX "__kmem_cache_free") ||
80 		    !strncmp(buf, ARCH_FUNC_PREFIX "__slab_free", len)) {
81 			/*
82 			 * In case of tail calls from any of the below to any of
83 			 * the above, optimized by the compiler such that the
84 			 * stack trace would omit the initial entry point below.
85 			 */
86 			fallback = skipnr + 1;
87 		}
88 
89 		/*
90 		 * The below list should only include the initial entry points
91 		 * into the slab allocators. Includes the *_bulk() variants by
92 		 * checking prefixes.
93 		 */
94 		if (str_has_prefix(buf, ARCH_FUNC_PREFIX "kfree") ||
95 		    str_has_prefix(buf, ARCH_FUNC_PREFIX "kmem_cache_free") ||
96 		    str_has_prefix(buf, ARCH_FUNC_PREFIX "__kmalloc") ||
97 		    str_has_prefix(buf, ARCH_FUNC_PREFIX "kmem_cache_alloc"))
98 			goto found;
99 	}
100 	if (fallback < num_entries)
101 		return fallback;
102 found:
103 	skipnr++;
104 	return skipnr < num_entries ? skipnr : 0;
105 }
106 
107 static void kfence_print_stack(struct seq_file *seq, const struct kfence_metadata *meta,
108 			       bool show_alloc)
109 {
110 	const struct kfence_track *track = show_alloc ? &meta->alloc_track : &meta->free_track;
111 	u64 ts_sec = track->ts_nsec;
112 	unsigned long rem_nsec = do_div(ts_sec, NSEC_PER_SEC);
113 	u64 interval_nsec = local_clock() - track->ts_nsec;
114 	unsigned long rem_interval_nsec = do_div(interval_nsec, NSEC_PER_SEC);
115 
116 	/* Timestamp matches printk timestamp format. */
117 	seq_con_printf(seq, "%s by task %d on cpu %d at %lu.%06lus (%lu.%06lus ago):\n",
118 		       show_alloc ? "allocated" : meta->state == KFENCE_OBJECT_RCU_FREEING ?
119 		       "rcu freeing" : "freed", track->pid,
120 		       track->cpu, (unsigned long)ts_sec, rem_nsec / 1000,
121 		       (unsigned long)interval_nsec, rem_interval_nsec / 1000);
122 
123 	if (track->num_stack_entries) {
124 		/* Skip allocation/free internals stack. */
125 		int i = get_stack_skipnr(track->stack_entries, track->num_stack_entries, NULL);
126 
127 		/* stack_trace_seq_print() does not exist; open code our own. */
128 		for (; i < track->num_stack_entries; i++)
129 			seq_con_printf(seq, " %pS\n", (void *)track->stack_entries[i]);
130 	} else {
131 		seq_con_printf(seq, " no %s stack\n", show_alloc ? "allocation" : "deallocation");
132 	}
133 }
134 
135 void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta)
136 {
137 	const int size = abs(meta->size);
138 	const unsigned long start = meta->addr;
139 	const struct kmem_cache *const cache = meta->cache;
140 
141 	lockdep_assert_held(&meta->lock);
142 
143 	if (meta->state == KFENCE_OBJECT_UNUSED) {
144 		seq_con_printf(seq, "kfence-#%td unused\n", meta - kfence_metadata);
145 		return;
146 	}
147 
148 	seq_con_printf(seq, "kfence-#%td: 0x%p-0x%p, size=%d, cache=%s\n\n",
149 		       meta - kfence_metadata, (void *)start, (void *)(start + size - 1),
150 		       size, (cache && cache->name) ? cache->name : "<destroyed>");
151 
152 	kfence_print_stack(seq, meta, true);
153 
154 	if (meta->state == KFENCE_OBJECT_FREED || meta->state == KFENCE_OBJECT_RCU_FREEING) {
155 		seq_con_printf(seq, "\n");
156 		kfence_print_stack(seq, meta, false);
157 	}
158 }
159 
160 /*
161  * Show bytes at @addr that are different from the expected canary values, up to
162  * @max_bytes.
163  */
164 static void print_diff_canary(unsigned long address, size_t bytes_to_show,
165 			      const struct kfence_metadata *meta)
166 {
167 	const unsigned long show_until_addr = address + bytes_to_show;
168 	const u8 *cur, *end;
169 
170 	/* Do not show contents of object nor read into following guard page. */
171 	end = (const u8 *)(address < meta->addr ? min(show_until_addr, meta->addr)
172 						: min(show_until_addr, PAGE_ALIGN(address)));
173 
174 	pr_cont("[");
175 	for (cur = (const u8 *)address; cur < end; cur++) {
176 		if (*cur == KFENCE_CANARY_PATTERN_U8(cur))
177 			pr_cont(" .");
178 		else if (no_hash_pointers)
179 			pr_cont(" 0x%02x", *cur);
180 		else /* Do not leak kernel memory in non-debug builds. */
181 			pr_cont(" !");
182 	}
183 	pr_cont(" ]");
184 }
185 
186 static const char *get_access_type(bool is_write)
187 {
188 	return str_write_read(is_write);
189 }
190 
191 void kfence_report_error(unsigned long address, bool is_write, struct pt_regs *regs,
192 			 const struct kfence_metadata *meta, enum kfence_error_type type)
193 {
194 	unsigned long stack_entries[KFENCE_STACK_DEPTH] = { 0 };
195 	const ptrdiff_t object_index = meta ? meta - kfence_metadata : -1;
196 	int num_stack_entries;
197 	int skipnr = 0;
198 
199 	if (regs) {
200 		num_stack_entries = stack_trace_save_regs(regs, stack_entries, KFENCE_STACK_DEPTH, 0);
201 	} else {
202 		num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 1);
203 		skipnr = get_stack_skipnr(stack_entries, num_stack_entries, &type);
204 	}
205 
206 	/* Require non-NULL meta, except if KFENCE_ERROR_INVALID. */
207 	if (WARN_ON(type != KFENCE_ERROR_INVALID && !meta))
208 		return;
209 
210 	if (meta)
211 		lockdep_assert_held(&meta->lock);
212 	/*
213 	 * Because we may generate reports in printk-unfriendly parts of the
214 	 * kernel, such as scheduler code, the use of printk() could deadlock.
215 	 * Until such time that all printing code here is safe in all parts of
216 	 * the kernel, accept the risk, and just get our message out (given the
217 	 * system might already behave unpredictably due to the memory error).
218 	 * As such, also disable lockdep to hide warnings, and avoid disabling
219 	 * lockdep for the rest of the kernel.
220 	 */
221 	lockdep_off();
222 
223 	pr_err("==================================================================\n");
224 	/* Print report header. */
225 	switch (type) {
226 	case KFENCE_ERROR_OOB: {
227 		const bool left_of_object = address < meta->addr;
228 
229 		pr_err("BUG: KFENCE: out-of-bounds %s in %pS\n\n", get_access_type(is_write),
230 		       (void *)stack_entries[skipnr]);
231 		pr_err("Out-of-bounds %s at 0x%p (%luB %s of kfence-#%td):\n",
232 		       get_access_type(is_write), (void *)address,
233 		       left_of_object ? meta->addr - address : address - meta->addr,
234 		       left_of_object ? "left" : "right", object_index);
235 		break;
236 	}
237 	case KFENCE_ERROR_UAF:
238 		pr_err("BUG: KFENCE: use-after-free %s in %pS\n\n", get_access_type(is_write),
239 		       (void *)stack_entries[skipnr]);
240 		pr_err("Use-after-free %s at 0x%p (in kfence-#%td):\n",
241 		       get_access_type(is_write), (void *)address, object_index);
242 		break;
243 	case KFENCE_ERROR_CORRUPTION:
244 		pr_err("BUG: KFENCE: memory corruption in %pS\n\n", (void *)stack_entries[skipnr]);
245 		pr_err("Corrupted memory at 0x%p ", (void *)address);
246 		print_diff_canary(address, 16, meta);
247 		pr_cont(" (in kfence-#%td):\n", object_index);
248 		break;
249 	case KFENCE_ERROR_INVALID:
250 		pr_err("BUG: KFENCE: invalid %s in %pS\n\n", get_access_type(is_write),
251 		       (void *)stack_entries[skipnr]);
252 		pr_err("Invalid %s at 0x%p:\n", get_access_type(is_write),
253 		       (void *)address);
254 		break;
255 	case KFENCE_ERROR_INVALID_FREE:
256 		pr_err("BUG: KFENCE: invalid free in %pS\n\n", (void *)stack_entries[skipnr]);
257 		pr_err("Invalid free of 0x%p (in kfence-#%td):\n", (void *)address,
258 		       object_index);
259 		break;
260 	}
261 
262 	/* Print stack trace and object info. */
263 	stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr, 0);
264 
265 	if (meta) {
266 		pr_err("\n");
267 		kfence_print_object(NULL, meta);
268 	}
269 
270 	/* Print report footer. */
271 	pr_err("\n");
272 	if (no_hash_pointers && regs)
273 		show_regs(regs);
274 	else
275 		dump_stack_print_info(KERN_ERR);
276 	trace_error_report_end(ERROR_DETECTOR_KFENCE, address);
277 	pr_err("==================================================================\n");
278 
279 	lockdep_on();
280 
281 	check_panic_on_warn("KFENCE");
282 
283 	/* We encountered a memory safety error, taint the kernel! */
284 	add_taint(TAINT_BAD_PAGE, LOCKDEP_STILL_OK);
285 }
286 
287 #ifdef CONFIG_PRINTK
288 static void kfence_to_kp_stack(const struct kfence_track *track, void **kp_stack)
289 {
290 	int i, j;
291 
292 	i = get_stack_skipnr(track->stack_entries, track->num_stack_entries, NULL);
293 	for (j = 0; i < track->num_stack_entries && j < KS_ADDRS_COUNT; ++i, ++j)
294 		kp_stack[j] = (void *)track->stack_entries[i];
295 	if (j < KS_ADDRS_COUNT)
296 		kp_stack[j] = NULL;
297 }
298 
299 bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab)
300 {
301 	struct kfence_metadata *meta = addr_to_metadata((unsigned long)object);
302 	unsigned long flags;
303 
304 	if (!meta)
305 		return false;
306 
307 	/*
308 	 * If state is UNUSED at least show the pointer requested; the rest
309 	 * would be garbage data.
310 	 */
311 	kpp->kp_ptr = object;
312 
313 	/* Requesting info an a never-used object is almost certainly a bug. */
314 	if (WARN_ON(meta->state == KFENCE_OBJECT_UNUSED))
315 		return true;
316 
317 	raw_spin_lock_irqsave(&meta->lock, flags);
318 
319 	kpp->kp_slab = slab;
320 	kpp->kp_slab_cache = meta->cache;
321 	kpp->kp_objp = (void *)meta->addr;
322 	kfence_to_kp_stack(&meta->alloc_track, kpp->kp_stack);
323 	if (meta->state == KFENCE_OBJECT_FREED || meta->state == KFENCE_OBJECT_RCU_FREEING)
324 		kfence_to_kp_stack(&meta->free_track, kpp->kp_free_stack);
325 	/* get_stack_skipnr() ensures the first entry is outside allocator. */
326 	kpp->kp_ret = kpp->kp_stack[0];
327 
328 	raw_spin_unlock_irqrestore(&meta->lock, flags);
329 
330 	return true;
331 }
332 #endif
333