Re: [PATCH mm v3 00/34] kasan: switch tag-based modes to stack ring from per-object metadata

[Andrey Konovalov]

On Mon, Sep 5, 2022 at 11:05 PM <andrey.konovalov@xxxxxxxxx> wrote:
>
> From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>
> 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 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 proposed 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.
>
> At this point, it is unknown whether the performance impact from this
> contention would be significant compared to the slowdown introduced by
> collecting stack traces due to the planned changes to the latter part,
> see the section below.
>
> For now, the proposed implementation is deemed to be good enough, but this
> might need to be revisited once the stack collection becomes faster.
>
> 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!

Hi Andrew,

Could you consider picking up this series into mm?

Most of the patches have a Reviewed-by tag from Marco, and I've
addressed the last few comments he had in v3.

Thanks!