1/*
  2 * This implements the various checks for CONFIG_HARDENED_USERCOPY*,
  3 * which are designed to protect kernel memory from needless exposure
  4 * and overwrite under many unintended conditions. This code is based
  5 * on PAX_USERCOPY, which is:
  6 *
  7 * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
  8 * Security Inc.
  9 *
 10 * This program is free software; you can redistribute it and/or modify
 11 * it under the terms of the GNU General Public License version 2 as
 12 * published by the Free Software Foundation.
 13 *
 14 */
 15#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 16
 17#include <linux/mm.h>
 18#include <linux/slab.h>
 19#include <linux/sched.h>
 20#include <linux/sched/task.h>
 21#include <linux/sched/task_stack.h>
 22#include <linux/thread_info.h>
 23#include <asm/sections.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 */
 35static 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	 * the 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 */
 73void 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
 84void __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). */
104static 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? */
118static 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
148static 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 < 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. */
161static 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
224static inline void check_heap_object(const void *ptr, unsigned long n,
225				     bool to_user)
226{
227	struct page *page;
228
229	if (!virt_addr_valid(ptr))
230		return;
231
232	page = virt_to_head_page(ptr);
233
234	if (PageSlab(page)) {
235		/* Check slab allocator for flags and size. */
236		__check_heap_object(ptr, n, page, to_user);
237	} else {
238		/* Verify object does not incorrectly span multiple pages. */
239		check_page_span(ptr, n, page, to_user);
240	}
241}
242
243/*
244 * Validates that the given object is:
245 * - not bogus address
246 * - known-safe heap or stack object
247 * - not in kernel text
248 */
249void __check_object_size(const void *ptr, unsigned long n, bool to_user)
250{
251	/* Skip all tests if size is zero. */
252	if (!n)
253		return;
254
255	/* Check for invalid addresses. */
256	check_bogus_address((const unsigned long)ptr, n, to_user);
257
258	/* Check for bad heap object. */
259	check_heap_object(ptr, n, to_user);
260
261	/* Check for bad stack object. */
262	switch (check_stack_object(ptr, n)) {
263	case NOT_STACK:
264		/* Object is not touching the current process stack. */
265		break;
266	case GOOD_FRAME:
267	case GOOD_STACK:
268		/*
269		 * Object is either in the correct frame (when it
270		 * is possible to check) or just generally on the
271		 * process stack (when frame checking not available).
272		 */
273		return;
274	default:
275		usercopy_abort("process stack", NULL, to_user, 0, n);
276	}
277
278	/* Check for object in kernel to avoid text exposure. */
279	check_kernel_text_object((const unsigned long)ptr, n, to_user);
280}
281EXPORT_SYMBOL(__check_object_size);