1 /* SPDX-License-Identifier: GPL-2.0 */
2 /*
3 * KCSAN watchpoint encoding.
4 *
5 * Copyright (C) 2019, Google LLC.
6 */
7
8 #ifndef _KERNEL_KCSAN_ENCODING_H
9 #define _KERNEL_KCSAN_ENCODING_H
10
11 #include <linux/bits.h>
12 #include <linux/log2.h>
13 #include <linux/mm.h>
14
15 #include "kcsan.h"
16
17 #define SLOT_RANGE PAGE_SIZE
18
19 #define INVALID_WATCHPOINT 0
20 #define CONSUMED_WATCHPOINT 1
21
22 /*
23 * The maximum useful size of accesses for which we set up watchpoints is the
24 * max range of slots we check on an access.
25 */
26 #define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
27
28 /*
29 * Number of bits we use to store size info.
30 */
31 #define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
32 /*
33 * This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
34 * however, most 64-bit architectures do not use the full 64-bit address space.
35 * Also, in order for a false positive to be observable 2 things need to happen:
36 *
37 * 1. different addresses but with the same encoded address race;
38 * 2. and both map onto the same watchpoint slots;
39 *
40 * Both these are assumed to be very unlikely. However, in case it still
41 * happens, the report logic will filter out the false positive (see report.c).
42 */
43 #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
44
45 /* Bitmasks for the encoded watchpoint access information. */
46 #define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
47 #define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
48 #define WATCHPOINT_ADDR_MASK GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
49 static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
50 static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
51
52 static inline bool check_encodable(unsigned long addr, size_t size)
53 {
54 /*
55 * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
56 * pointer deref inside KCSAN.
57 */
58 return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
59 }
60
61 static inline long
62 encode_watchpoint(unsigned long addr, size_t size, bool is_write)
63 {
64 return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
65 (size << WATCHPOINT_ADDR_BITS) |
66 (addr & WATCHPOINT_ADDR_MASK));
67 }
68
69 static __always_inline bool decode_watchpoint(long watchpoint,
70 unsigned long *addr_masked,
71 size_t *size,
72 bool *is_write)
73 {
74 if (watchpoint == INVALID_WATCHPOINT ||
75 watchpoint == CONSUMED_WATCHPOINT)
76 return false;
77
78 *addr_masked = (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
79 *size = ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >> WATCHPOINT_ADDR_BITS;
80 *is_write = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
81
82 return true;
83 }
84
85 /*
86 * Return watchpoint slot for an address.
87 */
88 static __always_inline int watchpoint_slot(unsigned long addr)
89 {
90 return (addr / PAGE_SIZE) % CONFIG_KCSAN_NUM_WATCHPOINTS;
91 }
92
93 static __always_inline bool matching_access(unsigned long addr1, size_t size1,
94 unsigned long addr2, size_t size2)
95 {
96 unsigned long end_range1 = addr1 + size1 - 1;
97 unsigned long end_range2 = addr2 + size2 - 1;
98
99 return addr1 <= end_range2 && addr2 <= end_range1;
100 }
101
102 #endif /* _KERNEL_KCSAN_ENCODING_H */
103
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