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