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, low: textlow, high: texthigh)) |
128 | usercopy_abort(name: "kernel text" , NULL, to_user, offset: ptr - textlow, len: 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, low: textlow_linear, high: texthigh_linear)) |
146 | usercopy_abort(name: "linear kernel text" , NULL, to_user, |
147 | offset: ptr - textlow_linear, len: 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(name: "wrapped address" , NULL, to_user, offset: 0, len: ptr + n); |
156 | |
157 | /* Reject if NULL or ZERO-allocation. */ |
158 | if (ZERO_OR_NULL_PTR(ptr)) |
159 | usercopy_abort(name: "null address" , NULL, to_user, offset: ptr, len: 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(x: ptr)) { |
170 | offset = offset_in_page(ptr); |
171 | if (n > PAGE_SIZE - offset) |
172 | usercopy_abort(name: "kmap" , NULL, to_user, offset, len: n); |
173 | return; |
174 | } |
175 | |
176 | if (is_vmalloc_addr(x: ptr) && !pagefault_disabled()) { |
177 | struct vmap_area *area = find_vmap_area(addr); |
178 | |
179 | if (!area) |
180 | usercopy_abort(name: "vmalloc" , detail: "no area" , to_user, offset: 0, len: n); |
181 | |
182 | if (n > area->va_end - addr) { |
183 | offset = addr - area->va_start; |
184 | usercopy_abort(name: "vmalloc" , NULL, to_user, offset, len: n); |
185 | } |
186 | return; |
187 | } |
188 | |
189 | if (!virt_addr_valid(ptr)) |
190 | return; |
191 | |
192 | folio = virt_to_folio(x: 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(name: "page alloc" , NULL, to_user, offset, len: 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(ptr: (const unsigned long)ptr, n, to_user); |
224 | |
225 | /* Check for bad stack object. */ |
226 | switch (check_stack_object(obj: ptr, len: 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(name: "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 | len: 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(ptr: (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(s: str, res: &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 | |