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