mm/maccess.c

   1 /*
   2  * Access kernel memory without faulting.
   3  */
   4 #include <linux/export.h>
   5 #include <linux/mm.h>
   6 #include <linux/uaccess.h>
   7
   8 /**
   9  * probe_kernel_read(): safely attempt to read from a location
  10  * @dst: pointer to the buffer that shall take the data
  11  * @src: address to read from
  12  * @size: size of the data chunk
  13  *
  14  * Safely read from address @src to the buffer at @dst.  If a kernel fault
  15  * happens, handle that and return -EFAULT.
  16  *
  17  * We ensure that the copy_from_user is executed in atomic context so that
  18  * do_page_fault() doesn't attempt to take mmap_sem.  This makes
  19  * probe_kernel_read() suitable for use within regions where the caller
  20  * already holds mmap_sem, or other locks which nest inside mmap_sem.
  21  */
  22
  23 long __weak probe_kernel_read(void *dst, const void *src, size_t size)
  24     __attribute__((alias("__probe_kernel_read")));
  25
  26 long __probe_kernel_read(void *dst, const void *src, size_t size)
  27 {
  28         long ret;
  29         mm_segment_t old_fs = get_fs();
  30
  31         set_fs(KERNEL_DS);
  32         pagefault_disable();
  33         ret = __copy_from_user_inatomic(dst,
  34                         (__force const void __user *)src, size);
  35         pagefault_enable();
  36         set_fs(old_fs);
  37
  38         return ret ? -EFAULT : 0;
  39 }
  40 EXPORT_SYMBOL_GPL(probe_kernel_read);
  41
  42 /**
  43  * probe_kernel_write(): safely attempt to write to a location
  44  * @dst: address to write to
  45  * @src: pointer to the data that shall be written
  46  * @size: size of the data chunk
  47  *
  48  * Safely write to address @dst from the buffer at @src.  If a kernel fault
  49  * happens, handle that and return -EFAULT.
  50  */
  51 long __weak probe_kernel_write(void *dst, const void *src, size_t size)
  52     __attribute__((alias("__probe_kernel_write")));
  53
  54 long __probe_kernel_write(void *dst, const void *src, size_t size)
  55 {
  56         long ret;
  57         mm_segment_t old_fs = get_fs();
  58
  59         set_fs(KERNEL_DS);
  60         pagefault_disable();
  61         ret = __copy_to_user_inatomic((__force void __user *)dst, src, size);
  62         pagefault_enable();
  63         set_fs(old_fs);
  64
  65         return ret ? -EFAULT : 0;
  66 }
  67 EXPORT_SYMBOL_GPL(probe_kernel_write);
  68
  69 /**
  70  * strncpy_from_unsafe: - Copy a NUL terminated string from unsafe address.
  71  * @dst:   Destination address, in kernel space.  This buffer must be at
  72  *         least @count bytes long.
  73  * @unsafe_addr: Unsafe address.
  74  * @count: Maximum number of bytes to copy, including the trailing NUL.
  75  *
  76  * Copies a NUL-terminated string from unsafe address to kernel buffer.
  77  *
  78  * On success, returns the length of the string INCLUDING the trailing NUL.
  79  *
  80  * If access fails, returns -EFAULT (some data may have been copied
  81  * and the trailing NUL added).
  82  *
  83  * If @count is smaller than the length of the string, copies @count-1 bytes,
  84  * sets the last byte of @dst buffer to NUL and returns @count.
  85  */
  86 long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
  87 {
  88         mm_segment_t old_fs = get_fs();
  89         const void *src = unsafe_addr;
  90         long ret;
  91
  92         if (unlikely(count <= 0))
  93                 return 0;
  94
  95         set_fs(KERNEL_DS);
  96         pagefault_disable();
  97
  98         do {
  99                 ret = __get_user(*dst++, (const char __user __force *)src++);
 100         } while (dst[-1] && ret == 0 && src - unsafe_addr < count);
 101
 102         dst[-1] = '\0';
 103         pagefault_enable();
 104         set_fs(old_fs);
 105
 106         return ret ? -EFAULT : src - unsafe_addr;
 107 }