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