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1d094cefc3
The watchpoint encoding masks for size and address were off-by-one bit each, with the size mask using 1 unnecessary bit and the address mask missing 1 bit. However, due to the way the size is shifted into the encoded watchpoint, we were effectively wasting and never using the extra bit. For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write bit, 14 bits for the size bits, and then 49 bits left for the address. Prior to this fix we would end up with this usage: [ write<1> | size<14> | wasted<1> | address<48> ] Fix it by subtracting 1 bit from the GENMASK() end and start ranges of size and address respectively. The added static_assert()s verify that the masks are as expected. With the fixed version, we get the expected usage: [ write<1> | size<14> | address<49> ] Functionally no change is expected, since that extra address bit is insignificant for enabled architectures. Acked-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Marco Elver <elver@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
98 lines
3 KiB
C
98 lines
3 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _KERNEL_KCSAN_ENCODING_H
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#define _KERNEL_KCSAN_ENCODING_H
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#include <linux/bits.h>
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#include <linux/log2.h>
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#include <linux/mm.h>
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#include "kcsan.h"
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#define SLOT_RANGE PAGE_SIZE
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#define INVALID_WATCHPOINT 0
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#define CONSUMED_WATCHPOINT 1
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/*
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* The maximum useful size of accesses for which we set up watchpoints is the
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* max range of slots we check on an access.
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*/
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#define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
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/*
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* Number of bits we use to store size info.
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*/
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#define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
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/*
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* This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
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* however, most 64-bit architectures do not use the full 64-bit address space.
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* Also, in order for a false positive to be observable 2 things need to happen:
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*
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* 1. different addresses but with the same encoded address race;
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* 2. and both map onto the same watchpoint slots;
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*
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* Both these are assumed to be very unlikely. However, in case it still
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* happens, the report logic will filter out the false positive (see report.c).
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*/
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#define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
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/* Bitmasks for the encoded watchpoint access information. */
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#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
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#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
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#define WATCHPOINT_ADDR_MASK GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
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static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
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static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
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static inline bool check_encodable(unsigned long addr, size_t size)
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{
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/*
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* While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
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* pointer deref inside KCSAN.
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*/
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return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
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}
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static inline long
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encode_watchpoint(unsigned long addr, size_t size, bool is_write)
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{
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return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
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(size << WATCHPOINT_ADDR_BITS) |
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(addr & WATCHPOINT_ADDR_MASK));
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}
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static __always_inline bool decode_watchpoint(long watchpoint,
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unsigned long *addr_masked,
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size_t *size,
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bool *is_write)
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{
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if (watchpoint == INVALID_WATCHPOINT ||
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watchpoint == CONSUMED_WATCHPOINT)
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return false;
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*addr_masked = (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
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*size = ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >> WATCHPOINT_ADDR_BITS;
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*is_write = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
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return true;
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}
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/*
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* Return watchpoint slot for an address.
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*/
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static __always_inline int watchpoint_slot(unsigned long addr)
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{
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return (addr / PAGE_SIZE) % CONFIG_KCSAN_NUM_WATCHPOINTS;
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}
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static __always_inline bool matching_access(unsigned long addr1, size_t size1,
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unsigned long addr2, size_t size2)
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{
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unsigned long end_range1 = addr1 + size1 - 1;
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unsigned long end_range2 = addr2 + size2 - 1;
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return addr1 <= end_range2 && addr2 <= end_range1;
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}
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#endif /* _KERNEL_KCSAN_ENCODING_H */
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