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)
49static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
50static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
51
52static 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
61static inline long
62encode_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
69static __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 */
88static __always_inline int watchpoint_slot(unsigned long addr)
89{
90	return (addr / PAGE_SIZE) % CONFIG_KCSAN_NUM_WATCHPOINTS;
91}
92
93static __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