xref: /linux/lib/strncpy_from_user.c (revision bb9ae1a66c85eeb626864efd812c62026e126ec0)
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 	if (can_do_masked_user_access()) {
124 		long retval;
125 
126 		src = masked_user_access_begin(src);
127 		retval = do_strncpy_from_user(dst, src, count, count);
128 		user_read_access_end();
129 		return retval;
130 	}
131 
132 	max_addr = TASK_SIZE_MAX;
133 	src_addr = (unsigned long)untagged_addr(src);
134 	if (likely(src_addr < max_addr)) {
135 		unsigned long max = max_addr - src_addr;
136 		long retval;
137 
138 		/*
139 		 * Truncate 'max' to the user-specified limit, so that
140 		 * we only have one limit we need to check in the loop
141 		 */
142 		if (max > count)
143 			max = count;
144 
145 		kasan_check_write(dst, count);
146 		check_object_size(dst, count, false);
147 		if (user_read_access_begin(src, max)) {
148 			retval = do_strncpy_from_user(dst, src, count, max);
149 			user_read_access_end();
150 			return retval;
151 		}
152 	}
153 	return -EFAULT;
154 }
155 EXPORT_SYMBOL(strncpy_from_user);
156