kernel/kcsan/encoding.h

0001 /* SPDX-License-Identifier: GPL-2.0 */
0002 /*
0003  * KCSAN watchpoint encoding.
0004  *
0005  * Copyright (C) 2019, Google LLC.
0006  */
0007
0008 #ifndef _KERNEL_KCSAN_ENCODING_H
0009 #define _KERNEL_KCSAN_ENCODING_H
0010
0011 #include <linux/bits.h>
0012 #include <linux/log2.h>
0013 #include <linux/mm.h>
0014
0015 #include "kcsan.h"
0016
0017 #define SLOT_RANGE PAGE_SIZE
0018
0019 #define INVALID_WATCHPOINT  0
0020 #define CONSUMED_WATCHPOINT 1
0021
0022 /*
0023  * The maximum useful size of accesses for which we set up watchpoints is the
0024  * max range of slots we check on an access.
0025  */
0026 #define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
0027
0028 /*
0029  * Number of bits we use to store size info.
0030  */
0031 #define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
0032 /*
0033  * This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
0034  * however, most 64-bit architectures do not use the full 64-bit address space.
0035  * Also, in order for a false positive to be observable 2 things need to happen:
0036  *
0037  *  1. different addresses but with the same encoded address race;
0038  *  2. and both map onto the same watchpoint slots;
0039  *
0040  * Both these are assumed to be very unlikely. However, in case it still
0041  * happens, the report logic will filter out the false positive (see report.c).
0042  */
0043 #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
0044
0045 /* Bitmasks for the encoded watchpoint access information. */
0046 #define WATCHPOINT_WRITE_MASK   BIT(BITS_PER_LONG-1)
0047 #define WATCHPOINT_SIZE_MASK    GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
0048 #define WATCHPOINT_ADDR_MASK    GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
0049 static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
0050 static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
0051
0052 static inline bool check_encodable(unsigned long addr, size_t size)
0053 {
0054     /*
0055      * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
0056      * pointer deref inside KCSAN.
0057      */
0058     return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
0059 }
0060
0061 static inline long
0062 encode_watchpoint(unsigned long addr, size_t size, bool is_write)
0063 {
0064     return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
0065               (size << WATCHPOINT_ADDR_BITS) |
0066               (addr & WATCHPOINT_ADDR_MASK));
0067 }
0068
0069 static __always_inline bool decode_watchpoint(long watchpoint,
0070                           unsigned long *addr_masked,
0071                           size_t *size,
0072                           bool *is_write)
0073 {
0074     if (watchpoint == INVALID_WATCHPOINT ||
0075         watchpoint == CONSUMED_WATCHPOINT)
0076         return false;
0077
0078     *addr_masked =    (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
0079     *size        =   ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >> WATCHPOINT_ADDR_BITS;
0080     *is_write    = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
0081
0082     return true;
0083 }
0084
0085 /*
0086  * Return watchpoint slot for an address.
0087  */
0088 static __always_inline int watchpoint_slot(unsigned long addr)
0089 {
0090     return (addr / PAGE_SIZE) % CONFIG_KCSAN_NUM_WATCHPOINTS;
0091 }
0092
0093 static __always_inline bool matching_access(unsigned long addr1, size_t size1,
0094                         unsigned long addr2, size_t size2)
0095 {
0096     unsigned long end_range1 = addr1 + size1 - 1;
0097     unsigned long end_range2 = addr2 + size2 - 1;
0098
0099     return addr1 <= end_range2 && addr2 <= end_range1;
0100 }
0101
0102 #endif /* _KERNEL_KCSAN_ENCODING_H */