1// SPDX-License-Identifier: GPL-2.0
2#include <linux/compiler.h>
3#include <linux/export.h>
4#include <linux/fault-inject-usercopy.h>
5#include <linux/kasan-checks.h>
6#include <linux/thread_info.h>
7#include <linux/uaccess.h>
8#include <linux/kernel.h>
9#include <linux/errno.h>
10#include <linux/mm.h>
11
12#include <asm/byteorder.h>
13#include <asm/word-at-a-time.h>
14
15#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
16#define IS_UNALIGNED(src, dst)	0
17#else
18#define IS_UNALIGNED(src, dst)	\
19	(((long) dst | (long) src) & (sizeof(long) - 1))
20#endif
21
22/*
23 * Do a strncpy, return length of string without final '\0'.
24 * 'count' is the user-supplied count (return 'count' if we
25 * hit it), 'max' is the address space maximum (and we return
26 * -EFAULT if we hit it).
27 */
28static inline long do_strncpy_from_user(char *dst, const char __user *src,
29					unsigned long count, unsigned long max)
30{
31	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
32	unsigned long res = 0;
33
34	if (IS_UNALIGNED(src, dst))
35		goto byte_at_a_time;
36
37	while (max >= sizeof(unsigned long)) {
38		unsigned long c, data, mask;
39
40		/* Fall back to byte-at-a-time if we get a page fault */
41		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
42
43		/*
44		 * Note that we mask out the bytes following the NUL. This is
45		 * important to do because string oblivious code may read past
46		 * the NUL. For those routines, we don't want to give them
47		 * potentially random bytes after the NUL in `src`.
48		 *
49		 * One example of such code is BPF map keys. BPF treats map keys
50		 * as an opaque set of bytes. Without the post-NUL mask, any BPF
51		 * maps keyed by strings returned from strncpy_from_user() may
52		 * have multiple entries for semantically identical strings.
53		 */
54		if (has_zero(c, &data, &constants)) {
55			data = prep_zero_mask(c, data, &constants);
56			data = create_zero_mask(data);
57			mask = zero_bytemask(data);
58			*(unsigned long *)(dst+res) = c & mask;
59			return res + find_zero(data);
60		}
61
62		*(unsigned long *)(dst+res) = c;
63
64		res += sizeof(unsigned long);
65		max -= sizeof(unsigned long);
66	}
67
68byte_at_a_time:
69	while (max) {
70		char c;
71
72		unsafe_get_user(c,src+res, efault);
73		dst[res] = c;
74		if (!c)
75			return res;
76		res++;
77		max--;
78	}
79
80	/*
81	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
82	 * too? If so, that's ok - we got as much as the user asked for.
83	 */
84	if (res >= count)
85		return res;
86
87	/*
88	 * Nope: we hit the address space limit, and we still had more
89	 * characters the caller would have wanted. That's an EFAULT.
90	 */
91efault:
92	return -EFAULT;
93}
94
95/**
96 * strncpy_from_user: - Copy a NUL terminated string from userspace.
97 * @dst:   Destination address, in kernel space.  This buffer must be at
98 *         least @count bytes long.
99 * @src:   Source address, in user space.
100 * @count: Maximum number of bytes to copy, including the trailing NUL.
101 *
102 * Copies a NUL-terminated string from userspace to kernel space.
103 *
104 * On success, returns the length of the string (not including the trailing
105 * NUL).
106 *
107 * If access to userspace fails, returns -EFAULT (some data may have been
108 * copied).
109 *
110 * If @count is smaller than the length of the string, copies @count bytes
111 * and returns @count.
112 */
113long strncpy_from_user(char *dst, const char __user *src, long count)
114{
115	unsigned long max_addr, src_addr;
116
117	might_fault();
118	if (should_fail_usercopy())
119		return -EFAULT;
120	if (unlikely(count <= 0))
121		return 0;
122
123	max_addr = user_addr_max();
124	src_addr = (unsigned long)untagged_addr(src);
125	if (likely(src_addr < max_addr)) {
126		unsigned long max = max_addr - src_addr;
127		long retval;
128
129		/*
130		 * Truncate 'max' to the user-specified limit, so that
131		 * we only have one limit we need to check in the loop
132		 */
133		if (max > count)
134			max = count;
135
136		kasan_check_write(dst, count);
137		check_object_size(dst, count, false);
138		if (user_read_access_begin(src, max)) {
139			retval = do_strncpy_from_user(dst, src, count, max);
140			user_read_access_end();
141			return retval;
142		}
143	}
144	return -EFAULT;
145}
146EXPORT_SYMBOL(strncpy_from_user);
147