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         unsigned long align, res = 0;
  32         unsigned long c;
  33
  34         /*
  35          * Do everything aligned. But that means that we
  36          * need to also expand the maximum..
  37          */
  38         align = (sizeof(unsigned long) - 1) & (unsigned long)src;
  39         src -= align;
  40         max += align;
  41
  42         unsafe_get_user(c, (unsigned long __user *)src, efault);
  43         c |= aligned_byte_mask(align);
  44
  45         for (;;) {
  46                 unsigned long data;
  47                 if (has_zero(c, &data, &constants)) {
  48                         data = prep_zero_mask(c, data, &constants);
  49                         data = create_zero_mask(data);
  50                         return res + find_zero(data) + 1 - align;
  51                 }
  52                 res += sizeof(unsigned long);
  53                 /* We already handled 'unsigned long' bytes. Did we do it all ? */
  54                 if (unlikely(max <= sizeof(unsigned long)))
  55                         break;
  56                 max -= sizeof(unsigned long);
  57                 unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
  58         }
  59         res -= align;
  60
  61         /*
  62          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  63          * too? If so, return the marker for "too long".
  64          */
  65         if (res >= count)
  66                 return count+1;
  67
  68         /*
  69          * Nope: we hit the address space limit, and we still had more
  70          * characters the caller would have wanted. That's 0.
  71          */
  72 efault:
  73         return 0;
  74 }
  75
  76 /**
  77  * strnlen_user: - Get the size of a user string INCLUDING final NUL.
  78  * @str: The string to measure.
  79  * @count: Maximum count (including NUL character)
  80  *
  81  * Context: User context only. This function may sleep if pagefaults are
  82  *          enabled.
  83  *
  84  * Get the size of a NUL-terminated string in user space.
  85  *
  86  * Returns the size of the string INCLUDING the terminating NUL.
  87  * If the string is too long, returns a number larger than @count. User
  88  * has to check the return value against "> count".
  89  * On exception (or invalid count), returns 0.
  90  *
  91  * NOTE! You should basically never use this function. There is
  92  * almost never any valid case for using the length of a user space
  93  * string, since the string can be changed at any time by other
  94  * threads. Use "strncpy_from_user()" instead to get a stable copy
  95  * of the string.
  96  */
  97 long strnlen_user(const char __user *str, long count)
  98 {
  99         unsigned long max_addr, src_addr;
 100
 101         if (unlikely(count <= 0))
 102                 return 0;
 103
 104         max_addr = user_addr_max();
 105         src_addr = (unsigned long)str;
 106         if (likely(src_addr < max_addr)) {
 107                 unsigned long max = max_addr - src_addr;
 108                 long retval;
 109
 110                 /*
 111                  * Truncate 'max' to the user-specified limit, so that
 112                  * we only have one limit we need to check in the loop
 113                  */
 114                 if (max > count)
 115                         max = count;
 116
 117                 if (user_access_begin(VERIFY_READ, 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);