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 #include <linux/mm.h>
  10
  11 #include <asm/byteorder.h>
  12 #include <asm/word-at-a-time.h>
  13
  14 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  15 #define IS_UNALIGNED(src, dst)  0
  16 #else
  17 #define IS_UNALIGNED(src, dst)  \
  18         (((long) dst | (long) src) & (sizeof(long) - 1))
  19 #endif
  20
  21 /*
  22  * Do a strncpy, return length of string without final '\0'.
  23  * 'count' is the user-supplied count (return 'count' if we
  24  * hit it), 'max' is the address space maximum (and we return
  25  * -EFAULT if we hit it).
  26  */
  27 static inline long do_strncpy_from_user(char *dst, const char __user *src,
  28                                         unsigned long count, unsigned long max)
  29 {
  30         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  31         unsigned long res = 0;
  32
  33         /*
  34          * Truncate 'max' to the user-specified limit, so that
  35          * we only have one limit we need to check in the loop
  36          */
  37         if (max > count)
  38                 max = count;
  39
  40         if (IS_UNALIGNED(src, dst))
  41                 goto byte_at_a_time;
  42
  43         while (max >= sizeof(unsigned long)) {
  44                 unsigned long c, data;
  45
  46                 /* Fall back to byte-at-a-time if we get a page fault */
  47                 unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  48
  49                 *(unsigned long *)(dst+res) = c;
  50                 if (has_zero(c, &data, &constants)) {
  51                         data = prep_zero_mask(c, data, &constants);
  52                         data = create_zero_mask(data);
  53                         return res + find_zero(data);
  54                 }
  55                 res += sizeof(unsigned long);
  56                 max -= sizeof(unsigned long);
  57         }
  58
  59 byte_at_a_time:
  60         while (max) {
  61                 char c;
  62
  63                 unsafe_get_user(c,src+res, efault);
  64                 dst[res] = c;
  65                 if (!c)
  66                         return res;
  67                 res++;
  68                 max--;
  69         }
  70
  71         /*
  72          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  73          * too? If so, that's ok - we got as much as the user asked for.
  74          */
  75         if (res >= count)
  76                 return res;
  77
  78         /*
  79          * Nope: we hit the address space limit, and we still had more
  80          * characters the caller would have wanted. That's an EFAULT.
  81          */
  82 efault:
  83         return -EFAULT;
  84 }
  85
  86 /**
  87  * strncpy_from_user: - Copy a NUL terminated string from userspace.
  88  * @dst:   Destination address, in kernel space.  This buffer must be at
  89  *         least @count bytes long.
  90  * @src:   Source address, in user space.
  91  * @count: Maximum number of bytes to copy, including the trailing NUL.
  92  *
  93  * Copies a NUL-terminated string from userspace to kernel space.
  94  *
  95  * On success, returns the length of the string (not including the trailing
  96  * NUL).
  97  *
  98  * If access to userspace fails, returns -EFAULT (some data may have been
  99  * copied).
 100  *
 101  * If @count is smaller than the length of the string, copies @count bytes
 102  * and returns @count.
 103  */
 104 long strncpy_from_user(char *dst, const char __user *src, 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)untagged_addr(src);
 113         if (likely(src_addr < max_addr)) {
 114                 unsigned long max = max_addr - src_addr;
 115                 long retval;
 116
 117                 kasan_check_write(dst, count);
 118                 check_object_size(dst, count, false);
 119                 if (user_access_begin(src, max)) {
 120                         retval = do_strncpy_from_user(dst, src, count, max);
 121                         user_access_end();
 122                         return retval;
 123                 }
 124         }
 125         return -EFAULT;
 126 }
 127 EXPORT_SYMBOL(strncpy_from_user);