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
   6 #include <asm/word-at-a-time.h>
   7
   8 /* Set bits in the first 'n' bytes when loaded from memory */
   9 #ifdef __LITTLE_ENDIAN
  10 #  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
  11 #else
  12 #  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
  13 #endif
  14
  15 /*
  16  * Do a strnlen, return length of string *with* final '\0'.
  17  * 'count' is the user-supplied count, while 'max' is the
  18  * address space maximum.
  19  *
  20  * Return 0 for exceptions (which includes hitting the address
  21  * space maximum), or 'count+1' if hitting the user-supplied
  22  * maximum count.
  23  *
  24  * NOTE! We can sometimes overshoot the user-supplied maximum
  25  * if it fits in a aligned 'long'. The caller needs to check
  26  * the return value against "> max".
  27  */
  28 static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
  29 {
  30         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  31         long align, res = 0;
  32         unsigned long c;
  33
  34         /*
  35          * Truncate 'max' to the user-specified limit, so that
  36          * we only have one limit we need to check in the loop
  37          */
  38         if (max > count)
  39                 max = count;
  40
  41         /*
  42          * Do everything aligned. But that means that we
  43          * need to also expand the maximum..
  44          */
  45         align = (sizeof(long) - 1) & (unsigned long)src;
  46         src -= align;
  47         max += align;
  48
  49         unsafe_get_user(c, (unsigned long __user *)src, efault);
  50         c |= aligned_byte_mask(align);
  51
  52         for (;;) {
  53                 unsigned long data;
  54                 if (has_zero(c, &data, &constants)) {
  55                         data = prep_zero_mask(c, data, &constants);
  56                         data = create_zero_mask(data);
  57                         return res + find_zero(data) + 1 - align;
  58                 }
  59                 res += sizeof(unsigned long);
  60                 /* We already handled 'unsigned long' bytes. Did we do it all ? */
  61                 if (unlikely(max <= sizeof(unsigned long)))
  62                         break;
  63                 max -= sizeof(unsigned long);
  64                 unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
  65         }
  66         res -= align;
  67
  68         /*
  69          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  70          * too? If so, return the marker for "too long".
  71          */
  72         if (res >= count)
  73                 return count+1;
  74
  75         /*
  76          * Nope: we hit the address space limit, and we still had more
  77          * characters the caller would have wanted. That's 0.
  78          */
  79 efault:
  80         return 0;
  81 }
  82
  83 /**
  84  * strnlen_user: - Get the size of a user string INCLUDING final NUL.
  85  * @str: The string to measure.
  86  * @count: Maximum count (including NUL character)
  87  *
  88  * Context: User context only. This function may sleep if pagefaults are
  89  *          enabled.
  90  *
  91  * Get the size of a NUL-terminated string in user space.
  92  *
  93  * Returns the size of the string INCLUDING the terminating NUL.
  94  * If the string is too long, returns a number larger than @count. User
  95  * has to check the return value against "> count".
  96  * On exception (or invalid count), returns 0.
  97  *
  98  * NOTE! You should basically never use this function. There is
  99  * almost never any valid case for using the length of a user space
 100  * string, since the string can be changed at any time by other
 101  * threads. Use "strncpy_from_user()" instead to get a stable copy
 102  * of the string.
 103  */
 104 long strnlen_user(const char __user *str, long count)
 105 {
 106         unsigned long max_addr, src_addr;
 107
 108         if (unlikely(count <= 0))
 109                 return 0;
 110
 111         max_addr = user_addr_max();
 112         src_addr = (unsigned long)str;
 113         if (likely(src_addr < max_addr)) {
 114                 unsigned long max = max_addr - src_addr;
 115                 long retval;
 116
 117                 if (user_access_begin(str, max)) {
 118                         retval = do_strnlen_user(str, count, max);
 119                         user_access_end();
 120                         return retval;
 121                 }
 122         }
 123         return 0;
 124 }
 125 EXPORT_SYMBOL(strnlen_user);