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/fault-inject-usercopy.h>
   5 #include <linux/kasan-checks.h>
   6 #include <linux/thread_info.h>
   7 #include <linux/uaccess.h>
   8 #include <linux/kernel.h>
   9 #include <linux/errno.h>
  10 #include <linux/mm.h>
  11
  12 #include <asm/byteorder.h>
  13 #include <asm/word-at-a-time.h>
  14
  15 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  16 #define IS_UNALIGNED(src, dst)  0
  17 #else
  18 #define IS_UNALIGNED(src, dst)  \
  19         (((long) dst | (long) src) & (sizeof(long) - 1))
  20 #endif
  21
  22 /*
  23  * Do a strncpy, return length of string without final '\0'.
  24  * 'count' is the user-supplied count (return 'count' if we
  25  * hit it), 'max' is the address space maximum (and we return
  26  * -EFAULT if we hit it).
  27  */
  28 static inline long do_strncpy_from_user(char *dst, const char __user *src,
  29                                         unsigned long count, unsigned long max)
  30 {
  31         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  32         unsigned long res = 0;
  33
  34         if (IS_UNALIGNED(src, dst))
  35                 goto byte_at_a_time;
  36
  37         while (max >= sizeof(unsigned long)) {
  38                 unsigned long c, data, mask;
  39
  40                 /* Fall back to byte-at-a-time if we get a page fault */
  41                 unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  42
  43                 /*
  44                  * Note that we mask out the bytes following the NUL. This is
  45                  * important to do because string oblivious code may read past
  46                  * the NUL. For those routines, we don't want to give them
  47                  * potentially random bytes after the NUL in `src`.
  48                  *
  49                  * One example of such code is BPF map keys. BPF treats map keys
  50                  * as an opaque set of bytes. Without the post-NUL mask, any BPF
  51                  * maps keyed by strings returned from strncpy_from_user() may
  52                  * have multiple entries for semantically identical strings.
  53                  */
  54                 if (has_zero(c, &data, &constants)) {
  55                         data = prep_zero_mask(c, data, &constants);
  56                         data = create_zero_mask(data);
  57                         mask = zero_bytemask(data);
  58                         *(unsigned long *)(dst+res) = c & mask;
  59                         return res + find_zero(data);
  60                 }
  61
  62                 *(unsigned long *)(dst+res) = c;
  63
  64                 res += sizeof(unsigned long);
  65                 max -= sizeof(unsigned long);
  66         }
  67
  68 byte_at_a_time:
  69         while (max) {
  70                 char c;
  71
  72                 unsafe_get_user(c,src+res, efault);
  73                 dst[res] = c;
  74                 if (!c)
  75                         return res;
  76                 res++;
  77                 max--;
  78         }
  79
  80         /*
  81          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  82          * too? If so, that's ok - we got as much as the user asked for.
  83          */
  84         if (res >= count)
  85                 return res;
  86
  87         /*
  88          * Nope: we hit the address space limit, and we still had more
  89          * characters the caller would have wanted. That's an EFAULT.
  90          */
  91 efault:
  92         return -EFAULT;
  93 }
  94
  95 /**
  96  * strncpy_from_user: - Copy a NUL terminated string from userspace.
  97  * @dst:   Destination address, in kernel space.  This buffer must be at
  98  *         least @count bytes long.
  99  * @src:   Source address, in user space.
 100  * @count: Maximum number of bytes to copy, including the trailing NUL.
 101  *
 102  * Copies a NUL-terminated string from userspace to kernel space.
 103  *
 104  * On success, returns the length of the string (not including the trailing
 105  * NUL).
 106  *
 107  * If access to userspace fails, returns -EFAULT (some data may have been
 108  * copied).
 109  *
 110  * If @count is smaller than the length of the string, copies @count bytes
 111  * and returns @count.
 112  */
 113 long strncpy_from_user(char *dst, const char __user *src, long count)
 114 {
 115         unsigned long max_addr, src_addr;
 116
 117         might_fault();
 118         if (should_fail_usercopy())
 119                 return -EFAULT;
 120         if (unlikely(count <= 0))
 121                 return 0;
 122
 123         max_addr = user_addr_max();
 124         src_addr = (unsigned long)untagged_addr(src);
 125         if (likely(src_addr < max_addr)) {
 126                 unsigned long max = max_addr - src_addr;
 127                 long retval;
 128
 129                 /*
 130                  * Truncate 'max' to the user-specified limit, so that
 131                  * we only have one limit we need to check in the loop
 132                  */
 133                 if (max > count)
 134                         max = count;
 135
 136                 kasan_check_write(dst, count);
 137                 check_object_size(dst, count, false);
 138                 if (user_read_access_begin(src, max)) {
 139                         retval = do_strncpy_from_user(dst, src, count, max);
 140                         user_read_access_end();
 141                         return retval;
 142                 }
 143         }
 144         return -EFAULT;
 145 }
 146 EXPORT_SYMBOL(strncpy_from_user);