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/slab.h>
16 #include <linux/sched.h>
17 #include <linux/sched/task.h>
18 #include <linux/sched/task_stack.h>
19 #include <linux/thread_info.h>
20 #include <linux/vmalloc.h>
21 #include <linux/atomic.h>
22 #include <linux/jump_label.h>
23 #include <asm/sections.h>
24 #include "slab.h"
25
26 /*
27 * Checks if a given pointer and length is contained by the current
28 * stack frame (if possible).
29 *
30 * Returns:
31 * NOT_STACK: not at all on the stack
32 * GOOD_FRAME: fully within a valid stack frame
33 * GOOD_STACK: within the current stack (when can't frame-check exactly)
34 * BAD_STACK: error condition (invalid stack position or bad stack frame)
35 */
check_stack_object(const void * obj,unsigned long len)36 static noinline int check_stack_object(const void *obj, unsigned long len)
37 {
38 const void * const stack = task_stack_page(current);
39 const void * const stackend = stack + THREAD_SIZE;
40 int ret;
41
42 /* Object is not on the stack at all. */
43 if (obj + len <= stack || stackend <= obj)
44 return NOT_STACK;
45
46 /*
47 * Reject: object partially overlaps the stack (passing the
48 * check above means at least one end is within the stack,
49 * so if this check fails, the other end is outside the stack).
50 */
51 if (obj < stack || stackend < obj + len)
52 return BAD_STACK;
53
54 /* Check if object is safely within a valid frame. */
55 ret = arch_within_stack_frames(stack, stackend, obj, len);
56 if (ret)
57 return ret;
58
59 /* Finally, check stack depth if possible. */
60 #ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
61 if (IS_ENABLED(CONFIG_STACK_GROWSUP)) {
62 if ((void *)current_stack_pointer < obj + len)
63 return BAD_STACK;
64 } else {
65 if (obj < (void *)current_stack_pointer)
66 return BAD_STACK;
67 }
68 #endif
69
70 return GOOD_STACK;
71 }
72
73 /*
74 * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
75 * an unexpected state during a copy_from_user() or copy_to_user() call.
76 * There are several checks being performed on the buffer by the
77 * __check_object_size() function. Normal stack buffer usage should never
78 * trip the checks, and kernel text addressing will always trip the check.
79 * For cache objects, it is checking that only the whitelisted range of
80 * bytes for a given cache is being accessed (via the cache's usersize and
81 * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
82 * kmem_cache_create_usercopy() function to create the cache (and
83 * carefully audit the whitelist range).
84 */
usercopy_abort(const char * name,const char * detail,bool to_user,unsigned long offset,unsigned long len)85 void __noreturn usercopy_abort(const char *name, const char *detail,
86 bool to_user, unsigned long offset,
87 unsigned long len)
88 {
89 pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
90 to_user ? "exposure" : "overwrite",
91 to_user ? "from" : "to",
92 name ? : "unknown?!",
93 detail ? " '" : "", detail ? : "", detail ? "'" : "",
94 offset, len);
95
96 /*
97 * For greater effect, it would be nice to do do_group_exit(),
98 * but BUG() actually hooks all the lock-breaking and per-arch
99 * Oops code, so that is used here instead.
100 */
101 BUG();
102 }
103
104 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
overlaps(const unsigned long ptr,unsigned long n,unsigned long low,unsigned long high)105 static bool overlaps(const unsigned long ptr, unsigned long n,
106 unsigned long low, unsigned long high)
107 {
108 const unsigned long check_low = ptr;
109 unsigned long check_high = check_low + n;
110
111 /* Does not overlap if entirely above or entirely below. */
112 if (check_low >= high || check_high <= low)
113 return false;
114
115 return true;
116 }
117
118 /* Is this address range in the kernel text area? */
check_kernel_text_object(const unsigned long ptr,unsigned long n,bool to_user)119 static inline void check_kernel_text_object(const unsigned long ptr,
120 unsigned long n, bool to_user)
121 {
122 unsigned long textlow = (unsigned long)_stext;
123 unsigned long texthigh = (unsigned long)_etext;
124 unsigned long textlow_linear, texthigh_linear;
125
126 if (overlaps(ptr, n, textlow, texthigh))
127 usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
128
129 /*
130 * Some architectures have virtual memory mappings with a secondary
131 * mapping of the kernel text, i.e. there is more than one virtual
132 * kernel address that points to the kernel image. It is usually
133 * when there is a separate linear physical memory mapping, in that
134 * __pa() is not just the reverse of __va(). This can be detected
135 * and checked:
136 */
137 textlow_linear = (unsigned long)lm_alias(textlow);
138 /* No different mapping: we're done. */
139 if (textlow_linear == textlow)
140 return;
141
142 /* Check the secondary mapping... */
143 texthigh_linear = (unsigned long)lm_alias(texthigh);
144 if (overlaps(ptr, n, textlow_linear, texthigh_linear))
145 usercopy_abort("linear kernel text", NULL, to_user,
146 ptr - textlow_linear, n);
147 }
148
check_bogus_address(const unsigned long ptr,unsigned long n,bool to_user)149 static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
150 bool to_user)
151 {
152 /* Reject if object wraps past end of memory. */
153 if (ptr + (n - 1) < ptr)
154 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
155
156 /* Reject if NULL or ZERO-allocation. */
157 if (ZERO_OR_NULL_PTR(ptr))
158 usercopy_abort("null address", NULL, to_user, ptr, n);
159 }
160
check_heap_object(const void * ptr,unsigned long n,bool to_user)161 static inline void check_heap_object(const void *ptr, unsigned long n,
162 bool to_user)
163 {
164 uintptr_t addr = (uintptr_t)ptr;
165 unsigned long offset;
166 struct folio *folio;
167
168 if (is_kmap_addr(ptr)) {
169 offset = offset_in_page(ptr);
170 if (n > PAGE_SIZE - offset)
171 usercopy_abort("kmap", NULL, to_user, offset, n);
172 return;
173 }
174
175 if (is_vmalloc_addr(ptr)) {
176 struct vmap_area *area = find_vmap_area(addr);
177
178 if (!area)
179 usercopy_abort("vmalloc", "no area", to_user, 0, n);
180
181 if (n > area->va_end - addr) {
182 offset = addr - area->va_start;
183 usercopy_abort("vmalloc", NULL, to_user, offset, n);
184 }
185 return;
186 }
187
188 if (!virt_addr_valid(ptr))
189 return;
190
191 folio = virt_to_folio(ptr);
192
193 if (folio_test_slab(folio)) {
194 /* Check slab allocator for flags and size. */
195 __check_heap_object(ptr, n, folio_slab(folio), to_user);
196 } else if (folio_test_large(folio)) {
197 offset = ptr - folio_address(folio);
198 if (n > folio_size(folio) - offset)
199 usercopy_abort("page alloc", NULL, to_user, offset, n);
200 }
201 }
202
203 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
204
205 /*
206 * Validates that the given object is:
207 * - not bogus address
208 * - fully contained by stack (or stack frame, when available)
209 * - fully within SLAB object (or object whitelist area, when available)
210 * - not in kernel text
211 */
__check_object_size(const void * ptr,unsigned long n,bool to_user)212 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
213 {
214 if (static_branch_unlikely(&bypass_usercopy_checks))
215 return;
216
217 /* Skip all tests if size is zero. */
218 if (!n)
219 return;
220
221 /* Check for invalid addresses. */
222 check_bogus_address((const unsigned long)ptr, n, to_user);
223
224 /* Check for bad stack object. */
225 switch (check_stack_object(ptr, n)) {
226 case NOT_STACK:
227 /* Object is not touching the current process stack. */
228 break;
229 case GOOD_FRAME:
230 case GOOD_STACK:
231 /*
232 * Object is either in the correct frame (when it
233 * is possible to check) or just generally on the
234 * process stack (when frame checking not available).
235 */
236 return;
237 default:
238 usercopy_abort("process stack", NULL, to_user,
239 #ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
240 IS_ENABLED(CONFIG_STACK_GROWSUP) ?
241 ptr - (void *)current_stack_pointer :
242 (void *)current_stack_pointer - ptr,
243 #else
244 0,
245 #endif
246 n);
247 }
248
249 /* Check for bad heap object. */
250 check_heap_object(ptr, n, to_user);
251
252 /* Check for object in kernel to avoid text exposure. */
253 check_kernel_text_object((const unsigned long)ptr, n, to_user);
254 }
255 EXPORT_SYMBOL(__check_object_size);
256
257 static bool enable_checks __initdata = true;
258
parse_hardened_usercopy(char * str)259 static int __init parse_hardened_usercopy(char *str)
260 {
261 if (strtobool(str, &enable_checks))
262 pr_warn("Invalid option string for hardened_usercopy: '%s'\n",
263 str);
264 return 1;
265 }
266
267 __setup("hardened_usercopy=", parse_hardened_usercopy);
268
set_hardened_usercopy(void)269 static int __init set_hardened_usercopy(void)
270 {
271 if (enable_checks == false)
272 static_branch_enable(&bypass_usercopy_checks);
273 return 1;
274 }
275
276 late_initcall(set_hardened_usercopy);
277