1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * This implements the various checks for CONFIG_HARDENED_USERCOPY*,
4 * which are designed to protect kernel memory from needless exposure
5 * and overwrite under many unintended conditions. This code is based
6 * on PAX_USERCOPY, which is:
7 *
8 * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
9 * Security Inc.
10 */
11 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
12
13 #include <linux/mm.h>
14 #include <linux/highmem.h>
15 #include <linux/kstrtox.h>
16 #include <linux/slab.h>
17 #include <linux/sched.h>
18 #include <linux/sched/task.h>
19 #include <linux/sched/task_stack.h>
20 #include <linux/thread_info.h>
21 #include <linux/vmalloc.h>
22 #include <linux/atomic.h>
23 #include <linux/jump_label.h>
24 #include <asm/sections.h>
25 #include "slab.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: within the current stack (when can't frame-check exactly)
35 * BAD_STACK: error condition (invalid stack position or bad stack frame)
36 */
37 static 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 * 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 /* Finally, check stack depth if possible. */
61 #ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
62 if (IS_ENABLED(CONFIG_STACK_GROWSUP)) {
63 if ((void *)current_stack_pointer < obj + len)
64 return BAD_STACK;
65 } else {
66 if (obj < (void *)current_stack_pointer)
67 return BAD_STACK;
68 }
69 #endif
70
71 return GOOD_STACK;
72 }
73
74 /*
75 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
76 * an unexpected state during a copy_from_user() or copy_to_user() call.
77 * There are several checks being performed on the buffer by the
78 * __check_object_size() function. Normal stack buffer usage should never
79 * trip the checks, and kernel text addressing will always trip the check.
80 * For cache objects, it is checking that only the whitelisted range of
81 * bytes for a given cache is being accessed (via the cache's usersize and
82 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
83 * kmem_cache_create_usercopy() function to create the cache (and
84 * carefully audit the whitelist range).
85 */
86 void __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). */
106 static 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? */
120 static 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
150 static 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 - 1) < 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 static inline void check_heap_object(const void *ptr, unsigned long n,
163 bool to_user)
164 {
165 unsigned long addr = (unsigned long)ptr;
166 unsigned long offset;
167 struct folio *folio;
168
169 if (is_kmap_addr(ptr)) {
170 offset = offset_in_page(ptr);
171 if (n > PAGE_SIZE - offset)
172 usercopy_abort("kmap", NULL, to_user, offset, n);
173 return;
174 }
175
176 if (is_vmalloc_addr(ptr) && !pagefault_disabled()) {
177 struct vmap_area *area = find_vmap_area(addr);
178
179 if (!area)
180 usercopy_abort("vmalloc", "no area", to_user, 0, n);
181
182 if (n > area->va_end - addr) {
183 offset = addr - area->va_start;
184 usercopy_abort("vmalloc", NULL, to_user, offset, n);
185 }
186 return;
187 }
188
189 if (!virt_addr_valid(ptr))
190 return;
191
192 folio = virt_to_folio(ptr);
193
194 if (folio_test_slab(folio)) {
195 /* Check slab allocator for flags and size. */
196 __check_heap_object(ptr, n, folio_slab(folio), to_user);
197 } else if (folio_test_large(folio)) {
198 offset = ptr - folio_address(folio);
199 if (n > folio_size(folio) - offset)
200 usercopy_abort("page alloc", NULL, to_user, offset, n);
201 }
202 }
203
204 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
205
206 /*
207 * Validates that the given object is:
208 * - not bogus address
209 * - fully contained by stack (or stack frame, when available)
210 * - fully within SLAB object (or object whitelist area, when available)
211 * - not in kernel text
212 */
213 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
214 {
215 if (static_branch_unlikely(&bypass_usercopy_checks))
216 return;
217
218 /* Skip all tests if size is zero. */
219 if (!n)
220 return;
221
222 /* Check for invalid addresses. */
223 check_bogus_address((const unsigned long)ptr, n, to_user);
224
225 /* Check for bad stack object. */
226 switch (check_stack_object(ptr, n)) {
227 case NOT_STACK:
228 /* Object is not touching the current process stack. */
229 break;
230 case GOOD_FRAME:
231 case GOOD_STACK:
232 /*
233 * Object is either in the correct frame (when it
234 * is possible to check) or just generally on the
235 * process stack (when frame checking not available).
236 */
237 return;
238 default:
239 usercopy_abort("process stack", NULL, to_user,
240 #ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
241 IS_ENABLED(CONFIG_STACK_GROWSUP) ?
242 ptr - (void *)current_stack_pointer :
243 (void *)current_stack_pointer - ptr,
244 #else
245 0,
246 #endif
247 n);
248 }
249
250 /* Check for bad heap object. */
251 check_heap_object(ptr, n, to_user);
252
253 /* Check for object in kernel to avoid text exposure. */
254 check_kernel_text_object((const unsigned long)ptr, n, to_user);
255 }
256 EXPORT_SYMBOL(__check_object_size);
257
258 static bool enable_checks __initdata = true;
259
260 static int __init parse_hardened_usercopy(char *str)
261 {
262 if (kstrtobool(str, &enable_checks))
263 pr_warn("Invalid option string for hardened_usercopy: '%s'\n",
264 str);
265 return 1;
266 }
267
268 __setup("hardened_usercopy=", parse_hardened_usercopy);
269
270 static int __init set_hardened_usercopy(void)
271 {
272 if (enable_checks == false)
273 static_branch_enable(&bypass_usercopy_checks);
274 return 1;
275 }
276
277 late_initcall(set_hardened_usercopy);
278
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