xref: /openbmc/linux/mm/usercopy.c (revision 4d906098)
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