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    Subject[PATCH 00/32] kasan: switch tag-based modes to stack ring from per-object metadata
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    From: Andrey Konovalov <andreyknvl@google.com>

    This series makes the tag-based KASAN modes use a ring buffer for storing
    stack depot handles for alloc/free stack traces for slab objects instead
    of per-object metadata. This ring buffer is referred to as the stack ring.

    On each alloc/free of a slab object, the tagged address of the object and
    the current stack trace are recorded in the stack ring.

    On each bug report, if the accessed address belongs to a slab object, the
    stack ring is scanned for matching entries. The newest entries are used to
    print the alloc/free stack traces in the report: one entry for alloc and
    one for free.

    The ring buffer is lock-free.

    The advantages of this approach over storing stack trace handles in
    per-object metadata with the tag-based KASAN modes:

    - Allows to find relevant stack traces for use-after-free bugs without
    using quarantine for freed memory. (Currently, if the object was
    reallocated multiple times, the report contains the latest alloc/free
    stack traces, not necessarily the ones relevant to the buggy allocation.)
    - Allows to better identify and mark use-after-free bugs, effectively
    making the CONFIG_KASAN_TAGS_IDENTIFY functionality always-on.
    - Has fixed memory overhead.

    The disadvantage:

    - If the affected object was allocated/freed long before the bug happened
    and the stack trace events were purged from the stack ring, the report
    will have no stack traces.

    Discussion
    ==========

    The current implementation of the stack ring uses a single ring buffer for
    the whole kernel. This might lead to contention due to atomic accesses to
    the ring buffer index on multicore systems.

    It is unclear to me whether the performance impact from this contention
    is significant compared to the slowdown introduced by collecting stack
    traces.

    While these patches are being reviewed, I will do some tests on the arm64
    hardware that I have. However, I do not have a large multicore arm64
    system to do proper measurements.

    A considered alternative is to keep a separate ring buffer for each CPU
    and then iterate over all of them when printing a bug report. This approach
    requires somehow figuring out which of the stack rings has the freshest
    stack traces for an object if multiple stack rings have them.

    Further plans
    =============

    This series is a part of an effort to make KASAN stack trace collection
    suitable for production. This requires stack trace collection to be fast
    and memory-bounded.

    The planned steps are:

    1. Speed up stack trace collection (potentially, by using SCS;
    patches on-hold until steps #2 and #3 are completed).
    2. Keep stack trace handles in the stack ring (this series).
    3. Add a memory-bounded mode to stack depot or provide an alternative
    memory-bounded stack storage.
    4. Potentially, implement stack trace collection sampling to minimize
    the performance impact.

    Thanks!

    Andrey Konovalov (32):
    kasan: check KASAN_NO_FREE_META in __kasan_metadata_size
    kasan: rename kasan_set_*_info to kasan_save_*_info
    kasan: move is_kmalloc check out of save_alloc_info
    kasan: split save_alloc_info implementations
    kasan: drop CONFIG_KASAN_TAGS_IDENTIFY
    kasan: introduce kasan_print_aux_stacks
    kasan: introduce kasan_get_alloc_track
    kasan: introduce kasan_init_object_meta
    kasan: clear metadata functions for tag-based modes
    kasan: move kasan_get_*_meta to generic.c
    kasan: introduce kasan_requires_meta
    kasan: introduce kasan_init_cache_meta
    kasan: drop CONFIG_KASAN_GENERIC check from kasan_init_cache_meta
    kasan: only define kasan_metadata_size for Generic mode
    kasan: only define kasan_never_merge for Generic mode
    kasan: only define metadata offsets for Generic mode
    kasan: only define metadata structs for Generic mode
    kasan: only define kasan_cache_create for Generic mode
    kasan: pass tagged pointers to kasan_save_alloc/free_info
    kasan: move kasan_get_alloc/free_track definitions
    kasan: simplify invalid-free reporting
    kasan: cosmetic changes in report.c
    kasan: use kasan_addr_to_slab in print_address_description
    kasan: move kasan_addr_to_slab to common.c
    kasan: make kasan_addr_to_page static
    kasan: simplify print_report
    kasan: introduce complete_report_info
    kasan: fill in cache and object in complete_report_info
    kasan: rework function arguments in report.c
    kasan: introduce kasan_complete_mode_report_info
    kasan: implement stack ring for tag-based modes
    kasan: better identify bug types for tag-based modes

    include/linux/kasan.h | 55 +++++-------
    include/linux/slab.h | 2 +-
    lib/Kconfig.kasan | 8 --
    mm/kasan/common.c | 173 ++++----------------------------------
    mm/kasan/generic.c | 154 ++++++++++++++++++++++++++++++---
    mm/kasan/kasan.h | 138 ++++++++++++++++++++----------
    mm/kasan/report.c | 130 +++++++++++++---------------
    mm/kasan/report_generic.c | 45 +++++++++-
    mm/kasan/report_tags.c | 114 ++++++++++++++++++-------
    mm/kasan/tags.c | 61 +++++++-------
    10 files changed, 491 insertions(+), 389 deletions(-)

    --
    2.25.1

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