LXR linux/lib/strncpy_from

   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 */
  28static 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
  68byte_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         */
  91efault:
  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 */
 113long 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}
 146EXPORT_SYMBOL(strncpy_from_user);
 147