Re: [PATCH RFC 0/8] kasan: hardware tag-based mode for production use on arm64

[Andrey Konovalov]

On Wed, Oct 14, 2020 at 10:44 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
>
> This patchset is not complete (see particular TODOs in the last patch),
> and I haven't performed any benchmarking yet, but I would like to start the
> discussion now and hear people's opinions regarding the questions mentioned
> below.
>
> === Overview
>
> This patchset adopts the existing hardware tag-based KASAN mode [1] for
> use in production as a memory corruption mitigation. Hardware tag-based
> KASAN relies on arm64 Memory Tagging Extension (MTE) [2] to perform memory
> and pointer tagging. Please see [3] and [4] for detailed analysis of how
> MTE helps to fight memory safety problems.
>
> The current plan is reuse CONFIG_KASAN_HW_TAGS for production, but add a
> boot time switch, that allows to choose between a debugging mode, that
> includes all KASAN features as they are, and a production mode, that only
> includes the essentials like tag checking.
>
> It is essential that switching between these modes doesn't require
> rebuilding the kernel with different configs, as this is required by the
> Android GKI initiative [5].
>
> The last patch of this series adds a new boot time parameter called
> kasan_mode, which can have the following values:
>
> - "kasan_mode=on" - only production features
> - "kasan_mode=debug" - all debug features
> - "kasan_mode=off" - no checks at all (not implemented yet)
>
> Currently outlined differences between "on" and "debug":
>
> - "on" doesn't keep track of alloc/free stacks, and therefore doesn't
>   require the additional memory to store those
> - "on" uses asyncronous tag checking (not implemented yet)
>
> === Questions
>
> The intention with this kind of a high level switch is to hide the
> implementation details. Arguably, we could add multiple switches that allow
> to separately control each KASAN or MTE feature, but I'm not sure there's
> much value in that.
>
> Does this make sense? Any preference regarding the name of the parameter
> and its values?
>
> What should be the default when the parameter is not specified? I would
> argue that it should be "debug" (for hardware that supports MTE, otherwise
> "off"), as it's the implied default for all other KASAN modes.
>
> Should we somehow control whether to panic the kernel on a tag fault?
> Another boot time parameter perhaps?
>
> Any ideas as to how properly estimate the slowdown? As there's no
> MTE-enabled hardware yet, the only way to test these patches is use an
> emulator (like QEMU). The delay that is added by the emulator (for setting
> and checking the tags) is different from the hardware delay, and this skews
> the results.
>
> A question to KASAN maintainers: what would be the best way to support the
> "off" mode? I see two potential approaches: add a check into each kasan
> callback (easier to implement, but we still call kasan callbacks, even
> though they immediately return), or add inline header wrappers that do the
> same.

CC Kostya and Serban.

>
> === Notes
>
> This patchset is available here:
>
> https://github.com/xairy/linux/tree/up-prod-mte-rfc1
>
> and on Gerrit here:
>
> https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/3460
>
> This patchset is based on v5 of "kasan: add hardware tag-based mode for
> arm64" patchset [1].
>
> For testing in QEMU hardware tag-based KASAN requires:
>
> 1. QEMU built from master [6] (use "-machine virt,mte=on -cpu max" arguments
>    to run).
> 2. GCC version 10.
>
> [1] https://lore.kernel.org/linux-arm-kernel/cover.1602535397.git.andreyknvl@xxxxxxxxxx/
> [2] https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/enhancing-memory-safety
> [3] https://arxiv.org/pdf/1802.09517.pdf
> [4] https://github.com/microsoft/MSRC-Security-Research/blob/master/papers/2020/Security%20analysis%20of%20memory%20tagging.pdf
> [5] https://source.android.com/devices/architecture/kernel/generic-kernel-image
> [6] https://github.com/qemu/qemu
>
> Andrey Konovalov (8):
>   kasan: simplify quarantine_put call
>   kasan: rename get_alloc/free_info
>   kasan: introduce set_alloc_info
>   kasan: unpoison stack only with CONFIG_KASAN_STACK
>   kasan: mark kasan_init_tags as __init
>   kasan, arm64: move initialization message
>   arm64: kasan: Add system_supports_tags helper
>   kasan: add and integrate kasan_mode boot param
>
>  arch/arm64/include/asm/memory.h  |  1 +
>  arch/arm64/kernel/sleep.S        |  2 +-
>  arch/arm64/mm/kasan_init.c       |  3 ++
>  arch/x86/kernel/acpi/wakeup_64.S |  2 +-
>  include/linux/kasan.h            | 14 ++---
>  mm/kasan/common.c                | 90 ++++++++++++++++++--------------
>  mm/kasan/generic.c               | 18 ++++---
>  mm/kasan/hw_tags.c               | 63 ++++++++++++++++++++--
>  mm/kasan/kasan.h                 | 25 ++++++---
>  mm/kasan/quarantine.c            |  5 +-
>  mm/kasan/report.c                | 22 +++++---
>  mm/kasan/report_sw_tags.c        |  2 +-
>  mm/kasan/sw_tags.c               | 14 +++--
>  13 files changed, 182 insertions(+), 79 deletions(-)
>
> --
> 2.28.0.1011.ga647a8990f-goog
>