1/*
2 * This implements the various checks for CONFIG_HARDENED_USERCOPY*,
3 * which are designed to protect kernel memory from needless exposure
4 * and overwrite under many unintended conditions. This code is based
5 * on PAX_USERCOPY, which is:
6 *
7 * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
8 * Security Inc.
9 *
10 * This program is free software; you can redistribute it and/or modify
11 * it under the terms of the GNU General Public License version 2 as
12 * published by the Free Software Foundation.
13 *
14 */
15#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
16
17#include <linux/mm.h>
18#include <linux/slab.h>
19#include <linux/sched.h>
20#include <linux/sched/task.h>
21#include <linux/sched/task_stack.h>
22#include <linux/thread_info.h>
23#include <linux/atomic.h>
24#include <linux/jump_label.h>
25#include <asm/sections.h>
26
27/*
28 * Checks if a given pointer and length is contained by the current
29 * stack frame (if possible).
30 *
31 * Returns:
32 * NOT_STACK: not at all on the stack
33 * GOOD_FRAME: fully within a valid stack frame
34 * GOOD_STACK: fully on the stack (when can't do frame-checking)
35 * BAD_STACK: error condition (invalid stack position or bad stack frame)
36 */
37static noinline int check_stack_object(const void *obj, unsigned long len)
38{
39 const void * const stack = task_stack_page(current);
40 const void * const stackend = stack + THREAD_SIZE;
41 int ret;
42
43 /* Object is not on the stack at all. */
44 if (obj + len <= stack || stackend <= obj)
45 return NOT_STACK;
46
47 /*
48 * Reject: object partially overlaps the stack (passing the
49 * the check above means at least one end is within the stack,
50 * so if this check fails, the other end is outside the stack).
51 */
52 if (obj < stack || stackend < obj + len)
53 return BAD_STACK;
54
55 /* Check if object is safely within a valid frame. */
56 ret = arch_within_stack_frames(stack, stackend, obj, len);
57 if (ret)
58 return ret;
59
60 return GOOD_STACK;
61}
62
63/*
64 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
65 * an unexpected state during a copy_from_user() or copy_to_user() call.
66 * There are several checks being performed on the buffer by the
67 * __check_object_size() function. Normal stack buffer usage should never
68 * trip the checks, and kernel text addressing will always trip the check.
69 * For cache objects, it is checking that only the whitelisted range of
70 * bytes for a given cache is being accessed (via the cache's usersize and
71 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
72 * kmem_cache_create_usercopy() function to create the cache (and
73 * carefully audit the whitelist range).
74 */
75void usercopy_warn(const char *name, const char *detail, bool to_user,
76 unsigned long offset, unsigned long len)
77{
78 WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
79 to_user ? "exposure" : "overwrite",
80 to_user ? "from" : "to",
81 name ? : "unknown?!",
82 detail ? " '" : "", detail ? : "", detail ? "'" : "",
83 offset, len);
84}
85
86void __noreturn usercopy_abort(const char *name, const char *detail,
87 bool to_user, unsigned long offset,
88 unsigned long len)
89{
90 pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
91 to_user ? "exposure" : "overwrite",
92 to_user ? "from" : "to",
93 name ? : "unknown?!",
94 detail ? " '" : "", detail ? : "", detail ? "'" : "",
95 offset, len);
96
97 /*
98 * For greater effect, it would be nice to do do_group_exit(),
99 * but BUG() actually hooks all the lock-breaking and per-arch
100 * Oops code, so that is used here instead.
101 */
102 BUG();
103}
104
105/* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
106static bool overlaps(const unsigned long ptr, unsigned long n,
107 unsigned long low, unsigned long high)
108{
109 const unsigned long check_low = ptr;
110 unsigned long check_high = check_low + n;
111
112 /* Does not overlap if entirely above or entirely below. */
113 if (check_low >= high || check_high <= low)
114 return false;
115
116 return true;
117}
118
119/* Is this address range in the kernel text area? */
120static inline void check_kernel_text_object(const unsigned long ptr,
121 unsigned long n, bool to_user)
122{
123 unsigned long textlow = (unsigned long)_stext;
124 unsigned long texthigh = (unsigned long)_etext;
125 unsigned long textlow_linear, texthigh_linear;
126
127 if (overlaps(ptr, n, textlow, texthigh))
128 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
129
130 /*
131 * Some architectures have virtual memory mappings with a secondary
132 * mapping of the kernel text, i.e. there is more than one virtual
133 * kernel address that points to the kernel image. It is usually
134 * when there is a separate linear physical memory mapping, in that
135 * __pa() is not just the reverse of __va(). This can be detected
136 * and checked:
137 */
138 textlow_linear = (unsigned long)lm_alias(textlow);
139 /* No different mapping: we're done. */
140 if (textlow_linear == textlow)
141 return;
142
143 /* Check the secondary mapping... */
144 texthigh_linear = (unsigned long)lm_alias(texthigh);
145 if (overlaps(ptr, n, textlow_linear, texthigh_linear))
146 usercopy_abort("linear kernel text", NULL, to_user,
147 ptr - textlow_linear, n);
148}
149
150static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
151 bool to_user)
152{
153 /* Reject if object wraps past end of memory. */
154 if (ptr + n < ptr)
155 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
156
157 /* Reject if NULL or ZERO-allocation. */
158 if (ZERO_OR_NULL_PTR(ptr))
159 usercopy_abort("null address", NULL, to_user, ptr, n);
160}
161
162/* Checks for allocs that are marked in some way as spanning multiple pages. */
163static inline void check_page_span(const void *ptr, unsigned long n,
164 struct page *page, bool to_user)
165{
166#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
167 const void *end = ptr + n - 1;
168 struct page *endpage;
169 bool is_reserved, is_cma;
170
171 /*
172 * Sometimes the kernel data regions are not marked Reserved (see
173 * check below). And sometimes [_sdata,_edata) does not cover
174 * rodata and/or bss, so check each range explicitly.
175 */
176
177 /* Allow reads of kernel rodata region (if not marked as Reserved). */
178 if (ptr >= (const void *)__start_rodata &&
179 end <= (const void *)__end_rodata) {
180 if (!to_user)
181 usercopy_abort("rodata", NULL, to_user, 0, n);
182 return;
183 }
184
185 /* Allow kernel data region (if not marked as Reserved). */
186 if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
187 return;
188
189 /* Allow kernel bss region (if not marked as Reserved). */
190 if (ptr >= (const void *)__bss_start &&
191 end <= (const void *)__bss_stop)
192 return;
193
194 /* Is the object wholly within one base page? */
195 if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
196 ((unsigned long)end & (unsigned long)PAGE_MASK)))
197 return;
198
199 /* Allow if fully inside the same compound (__GFP_COMP) page. */
200 endpage = virt_to_head_page(end);
201 if (likely(endpage == page))
202 return;
203
204 /*
205 * Reject if range is entirely either Reserved (i.e. special or
206 * device memory), or CMA. Otherwise, reject since the object spans
207 * several independently allocated pages.
208 */
209 is_reserved = PageReserved(page);
210 is_cma = is_migrate_cma_page(page);
211 if (!is_reserved && !is_cma)
212 usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
213
214 for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
215 page = virt_to_head_page(ptr);
216 if (is_reserved && !PageReserved(page))
217 usercopy_abort("spans Reserved and non-Reserved pages",
218 NULL, to_user, 0, n);
219 if (is_cma && !is_migrate_cma_page(page))
220 usercopy_abort("spans CMA and non-CMA pages", NULL,
221 to_user, 0, n);
222 }
223#endif
224}
225
226static inline void check_heap_object(const void *ptr, unsigned long n,
227 bool to_user)
228{
229 struct page *page;
230
231 if (!virt_addr_valid(ptr))
232 return;
233
234 page = virt_to_head_page(ptr);
235
236 if (PageSlab(page)) {
237 /* Check slab allocator for flags and size. */
238 __check_heap_object(ptr, n, page, to_user);
239 } else {
240 /* Verify object does not incorrectly span multiple pages. */
241 check_page_span(ptr, n, page, to_user);
242 }
243}
244
245static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
246
247/*
248 * Validates that the given object is:
249 * - not bogus address
250 * - fully contained by stack (or stack frame, when available)
251 * - fully within SLAB object (or object whitelist area, when available)
252 * - not in kernel text
253 */
254void __check_object_size(const void *ptr, unsigned long n, bool to_user)
255{
256 if (static_branch_unlikely(&bypass_usercopy_checks))
257 return;
258
259 /* Skip all tests if size is zero. */
260 if (!n)
261 return;
262
263 /* Check for invalid addresses. */
264 check_bogus_address((const unsigned long)ptr, n, to_user);
265
266 /* Check for bad stack object. */
267 switch (check_stack_object(ptr, n)) {
268 case NOT_STACK:
269 /* Object is not touching the current process stack. */
270 break;
271 case GOOD_FRAME:
272 case GOOD_STACK:
273 /*
274 * Object is either in the correct frame (when it
275 * is possible to check) or just generally on the
276 * process stack (when frame checking not available).
277 */
278 return;
279 default:
280 usercopy_abort("process stack", NULL, to_user, 0, n);
281 }
282
283 /* Check for bad heap object. */
284 check_heap_object(ptr, n, to_user);
285
286 /* Check for object in kernel to avoid text exposure. */
287 check_kernel_text_object((const unsigned long)ptr, n, to_user);
288}
289EXPORT_SYMBOL(__check_object_size);
290
291static bool enable_checks __initdata = true;
292
293static int __init parse_hardened_usercopy(char *str)
294{
295 return strtobool(str, &enable_checks);
296}
297
298__setup("hardened_usercopy=", parse_hardened_usercopy);
299
300static int __init set_hardened_usercopy(void)
301{
302 if (enable_checks == false)
303 static_branch_enable(&bypass_usercopy_checks);
304 return 1;
305}
306
307late_initcall(set_hardened_usercopy);
308