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 */
37static 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 */
86void __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). */
106static 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? */
120static 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
150static 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
162static 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
204static 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 */
213void __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}
256EXPORT_SYMBOL(__check_object_size);
257
258static bool enable_checks __initdata = true;
259
260static 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
270static int __init set_hardened_usercopy(void)
271{
272	if (enable_checks == false)
273		static_branch_enable(&bypass_usercopy_checks);
274	return 1;
275}
276
277late_initcall(set_hardened_usercopy);
278