lib/strncpy_from_user.c

   1 #include <linux/compiler.h>
   2 #include <linux/export.h>
   3 #include <linux/kasan-checks.h>
   4 #include <linux/thread_info.h>
   5 #include <linux/uaccess.h>
   6 #include <linux/kernel.h>
   7 #include <linux/errno.h>
   8
   9 #include <asm/byteorder.h>
  10 #include <asm/word-at-a-time.h>
  11
  12 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  13 #define IS_UNALIGNED(src, dst)  0
  14 #else
  15 #define IS_UNALIGNED(src, dst)  \
  16         (((long) dst | (long) src) & (sizeof(long) - 1))
  17 #endif
  18
  19 /*
  20  * Do a strncpy, return length of string without final '\0'.
  21  * 'count' is the user-supplied count (return 'count' if we
  22  * hit it), 'max' is the address space maximum (and we return
  23  * -EFAULT if we hit it).
  24  */
  25 static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
  26 {
  27         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  28         long res = 0;
  29
  30         /*
  31          * Truncate 'max' to the user-specified limit, so that
  32          * we only have one limit we need to check in the loop
  33          */
  34         if (max > count)
  35                 max = count;
  36
  37         if (IS_UNALIGNED(src, dst))
  38                 goto byte_at_a_time;
  39
  40         while (max >= sizeof(unsigned long)) {
  41                 unsigned long c, data;
  42
  43                 /* Fall back to byte-at-a-time if we get a page fault */
  44                 unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  45
  46                 *(unsigned long *)(dst+res) = c;
  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);
  51                 }
  52                 res += sizeof(unsigned long);
  53                 max -= sizeof(unsigned long);
  54         }
  55
  56 byte_at_a_time:
  57         while (max) {
  58                 char c;
  59
  60                 unsafe_get_user(c,src+res, efault);
  61                 dst[res] = c;
  62                 if (!c)
  63                         return res;
  64                 res++;
  65                 max--;
  66         }
  67
  68         /*
  69          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  70          * too? If so, that's ok - we got as much as the user asked for.
  71          */
  72         if (res >= count)
  73                 return res;
  74
  75         /*
  76          * Nope: we hit the address space limit, and we still had more
  77          * characters the caller would have wanted. That's an EFAULT.
  78          */
  79 efault:
  80         return -EFAULT;
  81 }
  82
  83 /**
  84  * strncpy_from_user: - Copy a NUL terminated string from userspace.
  85  * @dst:   Destination address, in kernel space.  This buffer must be at
  86  *         least @count bytes long.
  87  * @src:   Source address, in user space.
  88  * @count: Maximum number of bytes to copy, including the trailing NUL.
  89  *
  90  * Copies a NUL-terminated string from userspace to kernel space.
  91  *
  92  * On success, returns the length of the string (not including the trailing
  93  * NUL).
  94  *
  95  * If access to userspace fails, returns -EFAULT (some data may have been
  96  * copied).
  97  *
  98  * If @count is smaller than the length of the string, copies @count bytes
  99  * and returns @count.
 100  */
 101 long strncpy_from_user(char *dst, const char __user *src, long count)
 102 {
 103         unsigned long max_addr, src_addr;
 104
 105         if (unlikely(count <= 0))
 106                 return 0;
 107
 108         max_addr = user_addr_max();
 109         src_addr = (unsigned long)src;
 110         if (likely(src_addr < max_addr)) {
 111                 unsigned long max = max_addr - src_addr;
 112                 long retval;
 113
 114                 kasan_check_write(dst, count);
 115                 check_object_size(dst, count, false);
 116                 user_access_begin();
 117                 retval = do_strncpy_from_user(dst, src, count, max);
 118                 user_access_end();
 119                 return retval;
 120         }
 121         return -EFAULT;
 122 }
 123 EXPORT_SYMBOL(strncpy_from_user);