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

[Andrey Konovalov <andreyknvl@google.com>]

On Thu, Oct 15, 2020 at 4:41 PM Marco Elver <elver@google.com> wrote:
>
> On Wed, 14 Oct 2020 at 22:44, Andrey Konovalov <andreyknvl@google.com> 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?
>
> KASAN itself used to be a debugging tool only. So introducing an "on"
> mode which no longer follows this convention may be confusing.

Yeah, perhaps "on" is not the best name here.

> Instead, maybe the following might be less confusing:
>
> "full" - current "debug", normal KASAN, all debugging help available.
> "opt" - current "on", optimized mode for production.

How about "prod" here?

> "on" - automatic selection => chooses "full" if CONFIG_DEBUG_KERNEL,
> "opt" otherwise.
> "off" - as before.

It actually makes sense to depend on CONFIG_DEBUG_KERNEL, I like this idea.

>
> Also, if there is no other kernel boot parameter named "kasan" yet,
> maybe it could just be "kasan=..." ?

Sounds good to me too.

> > 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.
>
> Perhaps we could make this dependent on CONFIG_DEBUG_KERNEL as above.
> I do not think that having the full/debug KASAN enabled on production
> kernels adds any value because for it to be useful requires somebody
> to actually look at the stacktraces; I think that choice should be
> made explicitly if it's a production kernel. My guess is that we'll
> save explaining performance differences and resulting headaches for
> ourselves and others that way.

Ack.

> > Should we somehow control whether to panic the kernel on a tag fault?
> > Another boot time parameter perhaps?
>
> It already respects panic_on_warn, correct?

Yes, but Android is unlikely to enable panic_on_warn as they have
warnings happening all over. AFAIR Pixel 3/4 kernels actually have a
custom patch that enables kernel panic for KASAN crashes specifically
(even though they don't obviously use KASAN in production), and I
think it's better to provide a similar facility upstream. Maybe call
it panic_on_kasan or something?