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 */
28 static __always_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
68 byte_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 */
91 efault:
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 */
113 long 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 = TASK_SIZE_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 }
146 EXPORT_SYMBOL(strncpy_from_user);
147
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