xref: /openbmc/linux/mm/kfence/report.c (revision 66553609)
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)
seq_con_printf(struct seq_file * seq,const char * fmt,...)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  */
get_stack_skipnr(const unsigned long stack_entries[],int num_entries,const enum kfence_error_type * type)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 
kfence_print_stack(struct seq_file * seq,const struct kfence_metadata * meta,bool show_alloc)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 
kfence_print_object(struct seq_file * seq,const struct kfence_metadata * meta)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  */
print_diff_canary(unsigned long address,size_t bytes_to_show,const struct kfence_metadata * meta)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 
get_access_type(bool is_write)180 static const char *get_access_type(bool is_write)
181 {
182 	return is_write ? "write" : "read";
183 }
184 
kfence_report_error(unsigned long address,bool is_write,struct pt_regs * regs,const struct kfence_metadata * meta,enum kfence_error_type type)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
kfence_to_kp_stack(const struct kfence_track * track,void ** kp_stack)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 
__kfence_obj_info(struct kmem_obj_info * kpp,void * object,struct slab * slab)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
327