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 */