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