aboutsummaryrefslogtreecommitdiff
path: root/mm/usercopy.c
blob: d0d268135d96d6494949bb5eec702f7365ddc86a (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
// SPDX-License-Identifier: GPL-2.0-only
/*
 * This implements the various checks for CONFIG_HARDENED_USERCOPY*,
 * which are designed to protect kernel memory from needless exposure
 * and overwrite under many unintended conditions. This code is based
 * on PAX_USERCOPY, which is:
 *
 * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
 * Security Inc.
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/mm.h>
#include <linux/highmem.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/sched/task_stack.h>
#include <linux/thread_info.h>
#include <linux/atomic.h>
#include <linux/jump_label.h>
#include <asm/sections.h>
#include "slab.h"

/*
 * Checks if a given pointer and length is contained by the current
 * stack frame (if possible).
 *
 * Returns:
 *	NOT_STACK: not at all on the stack
 *	GOOD_FRAME: fully within a valid stack frame
 *	GOOD_STACK: fully on the stack (when can't do frame-checking)
 *	BAD_STACK: error condition (invalid stack position or bad stack frame)
 */
static noinline int check_stack_object(const void *obj, unsigned long len)
{
	const void * const stack = task_stack_page(current);
	const void * const stackend = stack + THREAD_SIZE;
	int ret;

	/* Object is not on the stack at all. */
	if (obj + len <= stack || stackend <= obj)
		return NOT_STACK;

	/*
	 * Reject: object partially overlaps the stack (passing the
	 * check above means at least one end is within the stack,
	 * so if this check fails, the other end is outside the stack).
	 */
	if (obj < stack || stackend < obj + len)
		return BAD_STACK;

	/* Check if object is safely within a valid frame. */
	ret = arch_within_stack_frames(stack, stackend, obj, len);
	if (ret)
		return ret;

	return GOOD_STACK;
}

/*
 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
 * an unexpected state during a copy_from_user() or copy_to_user() call.
 * There are several checks being performed on the buffer by the
 * __check_object_size() function. Normal stack buffer usage should never
 * trip the checks, and kernel text addressing will always trip the check.
 * For cache objects, it is checking that only the whitelisted range of
 * bytes for a given cache is being accessed (via the cache's usersize and
 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
 * kmem_cache_create_usercopy() function to create the cache (and
 * carefully audit the whitelist range).
 */
void usercopy_warn(const char *name, const char *detail, bool to_user,
		   unsigned long offset, unsigned long len)
{
	WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
		 to_user ? "exposure" : "overwrite",
		 to_user ? "from" : "to",
		 name ? : "unknown?!",
		 detail ? " '" : "", detail ? : "", detail ? "'" : "",
		 offset, len);
}

void __noreturn usercopy_abort(const char *name, const char *detail,
			       bool to_user, unsigned long offset,
			       unsigned long len)
{
	pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
		 to_user ? "exposure" : "overwrite",
		 to_user ? "from" : "to",
		 name ? : "unknown?!",
		 detail ? " '" : "", detail ? : "", detail ? "'" : "",
		 offset, len);

	/*
	 * For greater effect, it would be nice to do do_group_exit(),
	 * but BUG() actually hooks all the lock-breaking and per-arch
	 * Oops code, so that is used here instead.
	 */
	BUG();
}

/* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
static bool overlaps(const unsigned long ptr, unsigned long n,
		     unsigned long low, unsigned long high)
{
	const unsigned long check_low = ptr;
	unsigned long check_high = check_low + n;

	/* Does not overlap if entirely above or entirely below. */
	if (check_low >= high || check_high <= low)
		return false;

	return true;
}

/* Is this address range in the kernel text area? */
static inline void check_kernel_text_object(const unsigned long ptr,
					    unsigned long n, bool to_user)
{
	unsigned long textlow = (unsigned long)_stext;
	unsigned long texthigh = (unsigned long)_etext;
	unsigned long textlow_linear, texthigh_linear;

	if (overlaps(ptr, n, textlow, texthigh))
		usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);

	/*
	 * Some architectures have virtual memory mappings with a secondary
	 * mapping of the kernel text, i.e. there is more than one virtual
	 * kernel address that points to the kernel image. It is usually
	 * when there is a separate linear physical memory mapping, in that
	 * __pa() is not just the reverse of __va(). This can be detected
	 * and checked:
	 */
	textlow_linear = (unsigned long)lm_alias(textlow);
	/* No different mapping: we're done. */
	if (textlow_linear == textlow)
		return;

	/* Check the secondary mapping... */
	texthigh_linear = (unsigned long)lm_alias(texthigh);
	if (overlaps(ptr, n, textlow_linear, texthigh_linear))
		usercopy_abort("linear kernel text", NULL, to_user,
			       ptr - textlow_linear, n);
}

static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
				       bool to_user)
{
	/* Reject if object wraps past end of memory. */
	if (ptr + (n - 1) < ptr)
		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);

	/* Reject if NULL or ZERO-allocation. */
	if (ZERO_OR_NULL_PTR(ptr))
		usercopy_abort("null address", NULL, to_user, ptr, n);
}

/* Checks for allocs that are marked in some way as spanning multiple pages. */
static inline void check_page_span(const void *ptr, unsigned long n,
				   struct page *page, bool to_user)
{
#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
	const void *end = ptr + n - 1;
	struct page *endpage;
	bool is_reserved, is_cma;

	/*
	 * Sometimes the kernel data regions are not marked Reserved (see
	 * check below). And sometimes [_sdata,_edata) does not cover
	 * rodata and/or bss, so check each range explicitly.
	 */

	/* Allow reads of kernel rodata region (if not marked as Reserved). */
	if (ptr >= (const void *)__start_rodata &&
	    end <= (const void *)__end_rodata) {
		if (!to_user)
			usercopy_abort("rodata", NULL, to_user, 0, n);
		return;
	}

	/* Allow kernel data region (if not marked as Reserved). */
	if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
		return;

	/* Allow kernel bss region (if not marked as Reserved). */
	if (ptr >= (const void *)__bss_start &&
	    end <= (const void *)__bss_stop)
		return;

	/* Is the object wholly within one base page? */
	if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
		   ((unsigned long)end & (unsigned long)PAGE_MASK)))
		return;

	/* Allow if fully inside the same compound (__GFP_COMP) page. */
	endpage = virt_to_head_page(end);
	if (likely(endpage == page))
		return;

	/*
	 * Reject if range is entirely either Reserved (i.e. special or
	 * device memory), or CMA. Otherwise, reject since the object spans
	 * several independently allocated pages.
	 */
	is_reserved = PageReserved(page);
	is_cma = is_migrate_cma_page(page);
	if (!is_reserved && !is_cma)
		usercopy_abort("spans multiple pages", NULL, to_user, 0, n);

	for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
		page = virt_to_head_page(ptr);
		if (is_reserved && !PageReserved(page))
			usercopy_abort("spans Reserved and non-Reserved pages",
				       NULL, to_user, 0, n);
		if (is_cma && !is_migrate_cma_page(page))
			usercopy_abort("spans CMA and non-CMA pages", NULL,
				       to_user, 0, n);
	}
#endif
}

static inline void check_heap_object(const void *ptr, unsigned long n,
				     bool to_user)
{
	struct folio *folio;

	if (!virt_addr_valid(ptr))
		return;

	/*
	 * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
	 * highmem page or fallback to virt_to_page(). The following
	 * is effectively a highmem-aware virt_to_slab().
	 */
	folio = page_folio(kmap_to_page((void *)ptr));

	if (folio_test_slab(folio)) {
		/* Check slab allocator for flags and size. */
		__check_heap_object(ptr, n, folio_slab(folio), to_user);
	} else {
		/* Verify object does not incorrectly span multiple pages. */
		check_page_span(ptr, n, folio_page(folio, 0), to_user);
	}
}

static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);

/*
 * Validates that the given object is:
 * - not bogus address
 * - fully contained by stack (or stack frame, when available)
 * - fully within SLAB object (or object whitelist area, when available)
 * - not in kernel text
 */
void __check_object_size(const void *ptr, unsigned long n, bool to_user)
{
	if (static_branch_unlikely(&bypass_usercopy_checks))
		return;

	/* Skip all tests if size is zero. */
	if (!n)
		return;

	/* Check for invalid addresses. */
	check_bogus_address((const unsigned long)ptr, n, to_user);

	/* Check for bad stack object. */
	switch (check_stack_object(ptr, n)) {
	case NOT_STACK:
		/* Object is not touching the current process stack. */
		break;
	case GOOD_FRAME:
	case GOOD_STACK:
		/*
		 * Object is either in the correct frame (when it
		 * is possible to check) or just generally on the
		 * process stack (when frame checking not available).
		 */
		return;
	default:
		usercopy_abort("process stack", NULL, to_user, 0, n);
	}

	/* Check for bad heap object. */
	check_heap_object(ptr, n, to_user);

	/* Check for object in kernel to avoid text exposure. */
	check_kernel_text_object((const unsigned long)ptr, n, to_user);
}
EXPORT_SYMBOL(__check_object_size);

static bool enable_checks __initdata = true;

static int __init parse_hardened_usercopy(char *str)
{
	return strtobool(str, &enable_checks);
}

__setup("hardened_usercopy=", parse_hardened_usercopy);

static int __init set_hardened_usercopy(void)
{
	if (enable_checks == false)
		static_branch_enable(&bypass_usercopy_checks);
	return 1;
}

late_initcall(set_hardened_usercopy);