lib/strncpy_from_user.c

   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,
  27                                         unsigned long count, unsigned long max)
  28 {
  29         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  30         unsigned long res = 0;
  31
  32         if (IS_UNALIGNED(src, dst))
  33                 goto byte_at_a_time;
  34
  35         while (max >= sizeof(unsigned long)) {
  36                 unsigned long c, data;
  37
  38                 /* Fall back to byte-at-a-time if we get a page fault */
  39                 unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  40
  41                 *(unsigned long *)(dst+res) = c;
  42                 if (has_zero(c, &data, &constants)) {
  43                         data = prep_zero_mask(c, data, &constants);
  44                         data = create_zero_mask(data);
  45                         return res + find_zero(data);
  46                 }
  47                 res += sizeof(unsigned long);
  48                 max -= sizeof(unsigned long);
  49         }
  50
  51 byte_at_a_time:
  52         while (max) {
  53                 char c;
  54
  55                 unsafe_get_user(c,src+res, efault);
  56                 dst[res] = c;
  57                 if (!c)
  58                         return res;
  59                 res++;
  60                 max--;
  61         }
  62
  63         /*
  64          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  65          * too? If so, that's ok - we got as much as the user asked for.
  66          */
  67         if (res >= count)
  68                 return res;
  69
  70         /*
  71          * Nope: we hit the address space limit, and we still had more
  72          * characters the caller would have wanted. That's an EFAULT.
  73          */
  74 efault:
  75         return -EFAULT;
  76 }
  77
  78 /**
  79  * strncpy_from_user: - Copy a NUL terminated string from userspace.
  80  * @dst:   Destination address, in kernel space.  This buffer must be at
  81  *         least @count bytes long.
  82  * @src:   Source address, in user space.
  83  * @count: Maximum number of bytes to copy, including the trailing NUL.
  84  *
  85  * Copies a NUL-terminated string from userspace to kernel space.
  86  *
  87  * On success, returns the length of the string (not including the trailing
  88  * NUL).
  89  *
  90  * If access to userspace fails, returns -EFAULT (some data may have been
  91  * copied).
  92  *
  93  * If @count is smaller than the length of the string, copies @count bytes
  94  * and returns @count.
  95  */
  96 long strncpy_from_user(char *dst, const char __user *src, long count)
  97 {
  98         unsigned long max_addr, src_addr;
  99
 100         if (unlikely(count <= 0))
 101                 return 0;
 102
 103         max_addr = user_addr_max();
 104         src_addr = (unsigned long)src;
 105         if (likely(src_addr < max_addr)) {
 106                 unsigned long max = max_addr - src_addr;
 107                 long retval;
 108
 109                 /*
 110                  * Truncate 'max' to the user-specified limit, so that
 111                  * we only have one limit we need to check in the loop
 112                  */
 113                 if (max > count)
 114                         max = count;
 115
 116                 kasan_check_write(dst, count);
 117                 check_object_size(dst, count, false);
 118                 if (user_access_begin(VERIFY_READ, src, max)) {
 119                         retval = do_strncpy_from_user(dst, src, count, max);
 120                         user_access_end();
 121                         return retval;
 122                 }
 123         }
 124         return -EFAULT;
 125 }
 126 EXPORT_SYMBOL(strncpy_from_user);