kernel/kcsan/encoding.h

   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 */