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