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
__probe_kernel_read(void * dst,const void * src,size_t size)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
__probe_kernel_write(void * dst,const void * src,size_t size)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 */
strncpy_from_unsafe(char * dst,const void * unsafe_addr,long count)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
