usercopy.c

  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/ucopysize.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 page *page;
168	struct slab *slab;
169
170	if (is_kmap_addr(ptr)) {
171		offset = offset_in_page(ptr);
172		if (n > PAGE_SIZE - offset)
173			usercopy_abort("kmap", NULL, to_user, offset, n);
174		return;
175	}
176
177	if (is_vmalloc_addr(ptr) && !pagefault_disabled()) {
178		struct vmap_area *area = find_vmap_area(addr);
179
180		if (!area)
181			usercopy_abort("vmalloc", "no area", to_user, 0, n);
182
183		if (n > area->va_end - addr) {
184			offset = addr - area->va_start;
185			usercopy_abort("vmalloc", NULL, to_user, offset, n);
186		}
187		return;
188	}
189
190	if (!virt_addr_valid(ptr))
191		return;
192
193	page = virt_to_page(ptr);
194	slab = page_slab(page);
195	if (slab) {
196		/* Check slab allocator for flags and size. */
197		__check_heap_object(ptr, n, slab, to_user);
198	} else if (PageCompound(page)) {
199		page = compound_head(page);
200		offset = ptr - page_address(page);
201		if (n > page_size(page) - offset)
202			usercopy_abort("page alloc", NULL, to_user, offset, n);
203	}
204
205	/*
206	 * We cannot check non-compound pages.  They might be part of
207	 * a large allocation, in which case crossing a page boundary
208	 * is fine.
209	 */
210}
211
212DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_HARDENED_USERCOPY_DEFAULT_ON,
213			   validate_usercopy_range);
214EXPORT_SYMBOL(validate_usercopy_range);
215
216/*
217 * Validates that the given object is:
218 * - not bogus address
219 * - fully contained by stack (or stack frame, when available)
220 * - fully within SLAB object (or object whitelist area, when available)
221 * - not in kernel text
222 */
223void __check_object_size(const void *ptr, unsigned long n, bool to_user)
224{
225	/* Skip all tests if size is zero. */
226	if (!n)
227		return;
228
229	/* Check for invalid addresses. */
230	check_bogus_address((const unsigned long)ptr, n, to_user);
231
232	/* Check for bad stack object. */
233	switch (check_stack_object(ptr, n)) {
234	case NOT_STACK:
235		/* Object is not touching the current process stack. */
236		break;
237	case GOOD_FRAME:
238	case GOOD_STACK:
239		/*
240		 * Object is either in the correct frame (when it
241		 * is possible to check) or just generally on the
242		 * process stack (when frame checking not available).
243		 */
244		return;
245	default:
246		usercopy_abort("process stack", NULL, to_user,
247#ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
248			IS_ENABLED(CONFIG_STACK_GROWSUP) ?
249				ptr - (void *)current_stack_pointer :
250				(void *)current_stack_pointer - ptr,
251#else
252			0,
253#endif
254			n);
255	}
256
257	/* Check for bad heap object. */
258	check_heap_object(ptr, n, to_user);
259
260	/* Check for object in kernel to avoid text exposure. */
261	check_kernel_text_object((const unsigned long)ptr, n, to_user);
262}
263EXPORT_SYMBOL(__check_object_size);
264
265static bool enable_checks __initdata =
266		IS_ENABLED(CONFIG_HARDENED_USERCOPY_DEFAULT_ON);
267
268static int __init parse_hardened_usercopy(char *str)
269{
270	if (kstrtobool(str, &enable_checks))
271		pr_warn("Invalid option string for hardened_usercopy: '%s'\n",
272			str);
273	return 1;
274}
275
276__setup("hardened_usercopy=", parse_hardened_usercopy);
277
278static int __init set_hardened_usercopy(void)
279{
280	if (enable_checks)
281		static_branch_enable(&validate_usercopy_range);
282	else
283		static_branch_disable(&validate_usercopy_range);
284	return 1;
285}
286
287late_initcall(set_hardened_usercopy);