kernel/kcsan/encoding.h

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