xref: /openbmc/linux/lib/strncpy_from_user.c (revision 550987be)
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 
10 #include <asm/byteorder.h>
11 #include <asm/word-at-a-time.h>
12 
13 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
14 #define IS_UNALIGNED(src, dst)	0
15 #else
16 #define IS_UNALIGNED(src, dst)	\
17 	(((long) dst | (long) src) & (sizeof(long) - 1))
18 #endif
19 
20 /*
21  * Do a strncpy, return length of string without final '\0'.
22  * 'count' is the user-supplied count (return 'count' if we
23  * hit it), 'max' is the address space maximum (and we return
24  * -EFAULT if we hit it).
25  */
26 static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
27 {
28 	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
29 	long res = 0;
30 
31 	/*
32 	 * Truncate 'max' to the user-specified limit, so that
33 	 * we only have one limit we need to check in the loop
34 	 */
35 	if (max > count)
36 		max = count;
37 
38 	if (IS_UNALIGNED(src, dst))
39 		goto byte_at_a_time;
40 
41 	while (max >= sizeof(unsigned long)) {
42 		unsigned long c, data;
43 
44 		/* Fall back to byte-at-a-time if we get a page fault */
45 		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
46 
47 		*(unsigned long *)(dst+res) = c;
48 		if (has_zero(c, &data, &constants)) {
49 			data = prep_zero_mask(c, data, &constants);
50 			data = create_zero_mask(data);
51 			return res + find_zero(data);
52 		}
53 		res += sizeof(unsigned long);
54 		max -= sizeof(unsigned long);
55 	}
56 
57 byte_at_a_time:
58 	while (max) {
59 		char c;
60 
61 		unsafe_get_user(c,src+res, efault);
62 		dst[res] = c;
63 		if (!c)
64 			return res;
65 		res++;
66 		max--;
67 	}
68 
69 	/*
70 	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
71 	 * too? If so, that's ok - we got as much as the user asked for.
72 	 */
73 	if (res >= count)
74 		return res;
75 
76 	/*
77 	 * Nope: we hit the address space limit, and we still had more
78 	 * characters the caller would have wanted. That's an EFAULT.
79 	 */
80 efault:
81 	return -EFAULT;
82 }
83 
84 /**
85  * strncpy_from_user: - Copy a NUL terminated string from userspace.
86  * @dst:   Destination address, in kernel space.  This buffer must be at
87  *         least @count bytes long.
88  * @src:   Source address, in user space.
89  * @count: Maximum number of bytes to copy, including the trailing NUL.
90  *
91  * Copies a NUL-terminated string from userspace to kernel space.
92  *
93  * On success, returns the length of the string (not including the trailing
94  * NUL).
95  *
96  * If access to userspace fails, returns -EFAULT (some data may have been
97  * copied).
98  *
99  * If @count is smaller than the length of the string, copies @count bytes
100  * and returns @count.
101  */
102 long strncpy_from_user(char *dst, const char __user *src, long count)
103 {
104 	unsigned long max_addr, src_addr;
105 
106 	if (unlikely(count <= 0))
107 		return 0;
108 
109 	max_addr = user_addr_max();
110 	src_addr = (unsigned long)src;
111 	if (likely(src_addr < max_addr)) {
112 		unsigned long max = max_addr - src_addr;
113 		long retval;
114 
115 		kasan_check_write(dst, count);
116 		check_object_size(dst, count, false);
117 		user_access_begin();
118 		retval = do_strncpy_from_user(dst, src, count, max);
119 		user_access_end();
120 		return retval;
121 	}
122 	return -EFAULT;
123 }
124 EXPORT_SYMBOL(strncpy_from_user);
125