xref: /linux/mm/usercopy.c (revision aec499c75cf8e0b599be4d559e6922b613085f8f)
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/atomic.h>
21 #include <linux/jump_label.h>
22 #include <asm/sections.h>
23 #include "slab.h"
24 
25 /*
26  * Checks if a given pointer and length is contained by the current
27  * stack frame (if possible).
28  *
29  * Returns:
30  *	NOT_STACK: not at all on the stack
31  *	GOOD_FRAME: fully within a valid stack frame
32  *	GOOD_STACK: fully on the stack (when can't do frame-checking)
33  *	BAD_STACK: error condition (invalid stack position or bad stack frame)
34  */
35 static noinline int check_stack_object(const void *obj, unsigned long len)
36 {
37 	const void * const stack = task_stack_page(current);
38 	const void * const stackend = stack + THREAD_SIZE;
39 	int ret;
40 
41 	/* Object is not on the stack at all. */
42 	if (obj + len <= stack || stackend <= obj)
43 		return NOT_STACK;
44 
45 	/*
46 	 * Reject: object partially overlaps the stack (passing the
47 	 * check above means at least one end is within the stack,
48 	 * so if this check fails, the other end is outside the stack).
49 	 */
50 	if (obj < stack || stackend < obj + len)
51 		return BAD_STACK;
52 
53 	/* Check if object is safely within a valid frame. */
54 	ret = arch_within_stack_frames(stack, stackend, obj, len);
55 	if (ret)
56 		return ret;
57 
58 	return GOOD_STACK;
59 }
60 
61 /*
62  * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
63  * an unexpected state during a copy_from_user() or copy_to_user() call.
64  * There are several checks being performed on the buffer by the
65  * __check_object_size() function. Normal stack buffer usage should never
66  * trip the checks, and kernel text addressing will always trip the check.
67  * For cache objects, it is checking that only the whitelisted range of
68  * bytes for a given cache is being accessed (via the cache's usersize and
69  * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
70  * kmem_cache_create_usercopy() function to create the cache (and
71  * carefully audit the whitelist range).
72  */
73 void usercopy_warn(const char *name, const char *detail, bool to_user,
74 		   unsigned long offset, unsigned long len)
75 {
76 	WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
77 		 to_user ? "exposure" : "overwrite",
78 		 to_user ? "from" : "to",
79 		 name ? : "unknown?!",
80 		 detail ? " '" : "", detail ? : "", detail ? "'" : "",
81 		 offset, len);
82 }
83 
84 void __noreturn usercopy_abort(const char *name, const char *detail,
85 			       bool to_user, unsigned long offset,
86 			       unsigned long len)
87 {
88 	pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
89 		 to_user ? "exposure" : "overwrite",
90 		 to_user ? "from" : "to",
91 		 name ? : "unknown?!",
92 		 detail ? " '" : "", detail ? : "", detail ? "'" : "",
93 		 offset, len);
94 
95 	/*
96 	 * For greater effect, it would be nice to do do_group_exit(),
97 	 * but BUG() actually hooks all the lock-breaking and per-arch
98 	 * Oops code, so that is used here instead.
99 	 */
100 	BUG();
101 }
102 
103 /* Returns true if any portion of [ptr,ptr+n) over laps with [low,high). */
104 static bool overlaps(const unsigned long ptr, unsigned long n,
105 		     unsigned long low, unsigned long high)
106 {
107 	const unsigned long check_low = ptr;
108 	unsigned long check_high = check_low + n;
109 
110 	/* Does not overlap if entirely above or entirely below. */
111 	if (check_low >= high || check_high <= low)
112 		return false;
113 
114 	return true;
115 }
116 
117 /* Is this address range in the kernel text area? */
118 static inline void check_kernel_text_object(const unsigned long ptr,
119 					    unsigned long n, bool to_user)
120 {
121 	unsigned long textlow = (unsigned long)_stext;
122 	unsigned long texthigh = (unsigned long)_etext;
123 	unsigned long textlow_linear, texthigh_linear;
124 
125 	if (overlaps(ptr, n, textlow, texthigh))
126 		usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
127 
128 	/*
129 	 * Some architectures have virtual memory mappings with a secondary
130 	 * mapping of the kernel text, i.e. there is more than one virtual
131 	 * kernel address that points to the kernel image. It is usually
132 	 * when there is a separate linear physical memory mapping, in that
133 	 * __pa() is not just the reverse of __va(). This can be detected
134 	 * and checked:
135 	 */
136 	textlow_linear = (unsigned long)lm_alias(textlow);
137 	/* No different mapping: we're done. */
138 	if (textlow_linear == textlow)
139 		return;
140 
141 	/* Check the secondary mapping... */
142 	texthigh_linear = (unsigned long)lm_alias(texthigh);
143 	if (overlaps(ptr, n, textlow_linear, texthigh_linear))
144 		usercopy_abort("linear kernel text", NULL, to_user,
145 			       ptr - textlow_linear, n);
146 }
147 
148 static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
149 				       bool to_user)
150 {
151 	/* Reject if object wraps past end of memory. */
152 	if (ptr + (n - 1) < ptr)
153 		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
154 
155 	/* Reject if NULL or ZERO-allocation. */
156 	if (ZERO_OR_NULL_PTR(ptr))
157 		usercopy_abort("null address", NULL, to_user, ptr, n);
158 }
159 
160 /* Checks for allocs that are marked in some way as spanning multiple pages. */
161 static inline void check_page_span(const void *ptr, unsigned long n,
162 				   struct page *page, bool to_user)
163 {
164 #ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
165 	const void *end = ptr + n - 1;
166 	struct page *endpage;
167 	bool is_reserved, is_cma;
168 
169 	/*
170 	 * Sometimes the kernel data regions are not marked Reserved (see
171 	 * check below). And sometimes [_sdata,_edata) does not cover
172 	 * rodata and/or bss, so check each range explicitly.
173 	 */
174 
175 	/* Allow reads of kernel rodata region (if not marked as Reserved). */
176 	if (ptr >= (const void *)__start_rodata &&
177 	    end <= (const void *)__end_rodata) {
178 		if (!to_user)
179 			usercopy_abort("rodata", NULL, to_user, 0, n);
180 		return;
181 	}
182 
183 	/* Allow kernel data region (if not marked as Reserved). */
184 	if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
185 		return;
186 
187 	/* Allow kernel bss region (if not marked as Reserved). */
188 	if (ptr >= (const void *)__bss_start &&
189 	    end <= (const void *)__bss_stop)
190 		return;
191 
192 	/* Is the object wholly within one base page? */
193 	if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
194 		   ((unsigned long)end & (unsigned long)PAGE_MASK)))
195 		return;
196 
197 	/* Allow if fully inside the same compound (__GFP_COMP) page. */
198 	endpage = virt_to_head_page(end);
199 	if (likely(endpage == page))
200 		return;
201 
202 	/*
203 	 * Reject if range is entirely either Reserved (i.e. special or
204 	 * device memory), or CMA. Otherwise, reject since the object spans
205 	 * several independently allocated pages.
206 	 */
207 	is_reserved = PageReserved(page);
208 	is_cma = is_migrate_cma_page(page);
209 	if (!is_reserved && !is_cma)
210 		usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
211 
212 	for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
213 		page = virt_to_head_page(ptr);
214 		if (is_reserved && !PageReserved(page))
215 			usercopy_abort("spans Reserved and non-Reserved pages",
216 				       NULL, to_user, 0, n);
217 		if (is_cma && !is_migrate_cma_page(page))
218 			usercopy_abort("spans CMA and non-CMA pages", NULL,
219 				       to_user, 0, n);
220 	}
221 #endif
222 }
223 
224 static inline void check_heap_object(const void *ptr, unsigned long n,
225 				     bool to_user)
226 {
227 	struct folio *folio;
228 
229 	if (!virt_addr_valid(ptr))
230 		return;
231 
232 	/*
233 	 * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
234 	 * highmem page or fallback to virt_to_page(). The following
235 	 * is effectively a highmem-aware virt_to_slab().
236 	 */
237 	folio = page_folio(kmap_to_page((void *)ptr));
238 
239 	if (folio_test_slab(folio)) {
240 		/* Check slab allocator for flags and size. */
241 		__check_heap_object(ptr, n, folio_slab(folio), to_user);
242 	} else {
243 		/* Verify object does not incorrectly span multiple pages. */
244 		check_page_span(ptr, n, folio_page(folio, 0), to_user);
245 	}
246 }
247 
248 static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
249 
250 /*
251  * Validates that the given object is:
252  * - not bogus address
253  * - fully contained by stack (or stack frame, when available)
254  * - fully within SLAB object (or object whitelist area, when available)
255  * - not in kernel text
256  */
257 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
258 {
259 	if (static_branch_unlikely(&bypass_usercopy_checks))
260 		return;
261 
262 	/* Skip all tests if size is zero. */
263 	if (!n)
264 		return;
265 
266 	/* Check for invalid addresses. */
267 	check_bogus_address((const unsigned long)ptr, n, to_user);
268 
269 	/* Check for bad stack object. */
270 	switch (check_stack_object(ptr, n)) {
271 	case NOT_STACK:
272 		/* Object is not touching the current process stack. */
273 		break;
274 	case GOOD_FRAME:
275 	case GOOD_STACK:
276 		/*
277 		 * Object is either in the correct frame (when it
278 		 * is possible to check) or just generally on the
279 		 * process stack (when frame checking not available).
280 		 */
281 		return;
282 	default:
283 		usercopy_abort("process stack", NULL, to_user, 0, n);
284 	}
285 
286 	/* Check for bad heap object. */
287 	check_heap_object(ptr, n, to_user);
288 
289 	/* Check for object in kernel to avoid text exposure. */
290 	check_kernel_text_object((const unsigned long)ptr, n, to_user);
291 }
292 EXPORT_SYMBOL(__check_object_size);
293 
294 static bool enable_checks __initdata = true;
295 
296 static int __init parse_hardened_usercopy(char *str)
297 {
298 	return strtobool(str, &enable_checks);
299 }
300 
301 __setup("hardened_usercopy=", parse_hardened_usercopy);
302 
303 static int __init set_hardened_usercopy(void)
304 {
305 	if (enable_checks == false)
306 		static_branch_enable(&bypass_usercopy_checks);
307 	return 1;
308 }
309 
310 late_initcall(set_hardened_usercopy);
311