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Linux/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  * @src:   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 }
108 

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