lib/strnlen_user.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/kernel.h>
   3 #include <linux/export.h>
   4 #include <linux/uaccess.h>
   5 #include <linux/mm.h>
   6 #include <linux/bitops.h>
   7
   8 #include <asm/word-at-a-time.h>
   9
  10 /*
  11  * Do a strnlen, return length of string *with* final '\0'.
  12  * 'count' is the user-supplied count, while 'max' is the
  13  * address space maximum.
  14  *
  15  * Return 0 for exceptions (which includes hitting the address
  16  * space maximum), or 'count+1' if hitting the user-supplied
  17  * maximum count.
  18  *
  19  * NOTE! We can sometimes overshoot the user-supplied maximum
  20  * if it fits in a aligned 'long'. The caller needs to check
  21  * the return value against "> max".
  22  */
  23 static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
  24 {
  25         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  26         unsigned long align, res = 0;
  27         unsigned long c;
  28
  29         /*
  30          * Truncate 'max' to the user-specified limit, so that
  31          * we only have one limit we need to check in the loop
  32          */
  33         if (max > count)
  34                 max = count;
  35
  36         /*
  37          * Do everything aligned. But that means that we
  38          * need to also expand the maximum..
  39          */
  40         align = (sizeof(unsigned long) - 1) & (unsigned long)src;
  41         src -= align;
  42         max += align;
  43
  44         unsafe_get_user(c, (unsigned long __user *)src, efault);
  45         c |= aligned_byte_mask(align);
  46
  47         for (;;) {
  48                 unsigned long data;
  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) + 1 - align;
  53                 }
  54                 res += sizeof(unsigned long);
  55                 /* We already handled 'unsigned long' bytes. Did we do it all ? */
  56                 if (unlikely(max <= sizeof(unsigned long)))
  57                         break;
  58                 max -= sizeof(unsigned long);
  59                 unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
  60         }
  61         res -= align;
  62
  63         /*
  64          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  65          * too? If so, return the marker for "too long".
  66          */
  67         if (res >= count)
  68                 return count+1;
  69
  70         /*
  71          * Nope: we hit the address space limit, and we still had more
  72          * characters the caller would have wanted. That's 0.
  73          */
  74 efault:
  75         return 0;
  76 }
  77
  78 /**
  79  * strnlen_user: - Get the size of a user string INCLUDING final NUL.
  80  * @str: The string to measure.
  81  * @count: Maximum count (including NUL character)
  82  *
  83  * Context: User context only. This function may sleep if pagefaults are
  84  *          enabled.
  85  *
  86  * Get the size of a NUL-terminated string in user space.
  87  *
  88  * Returns the size of the string INCLUDING the terminating NUL.
  89  * If the string is too long, returns a number larger than @count. User
  90  * has to check the return value against "> count".
  91  * On exception (or invalid count), returns 0.
  92  *
  93  * NOTE! You should basically never use this function. There is
  94  * almost never any valid case for using the length of a user space
  95  * string, since the string can be changed at any time by other
  96  * threads. Use "strncpy_from_user()" instead to get a stable copy
  97  * of the string.
  98  */
  99 long strnlen_user(const char __user *str, long count)
 100 {
 101         unsigned long max_addr, src_addr;
 102
 103         if (unlikely(count <= 0))
 104                 return 0;
 105
 106         max_addr = user_addr_max();
 107         src_addr = (unsigned long)untagged_addr(str);
 108         if (likely(src_addr < max_addr)) {
 109                 unsigned long max = max_addr - src_addr;
 110                 long retval;
 111
 112                 if (user_access_begin(str, max)) {
 113                         retval = do_strnlen_user(str, count, max);
 114                         user_access_end();
 115                         return retval;
 116                 }
 117         }
 118         return 0;
 119 }
 120 EXPORT_SYMBOL(strnlen_user);