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

[Dmitry Vyukov]

On Tue, Oct 20, 2020 at 12:51 AM Kostya Serebryany <kcc@xxxxxxxxxx> wrote:
>
> Hi,
> I would like to hear opinions from others in CC on these choices:
> * Production use of In-kernel MTE should be based on stripped-down
> KASAN, or implemented independently?

Andrey, what are the fundamental consequences of basing MTE on KASAN?
I would assume that there are none as we can change KASAN code and
special case some code paths as necessary.

> * Should we aim at a single boot-time flag (with several values) or
> for several independent flags (OFF/SYNC/ASYNC, Stack traces on/off)

We won't be able to answer this question for several years until we
have actual hardware/users...
It's definitely safer to aim at multiple options. I would reuse the fs
opt parsing code as we seem to have lots of potential things to
configure so that we can do:
kasan_options=quarantine=off,fault=panic,trap=async

I am also always confused by the term "debug" when configuring the
kernel. In some cases it's for debugging of the subsystem (for
developers of KASAN), in some cases it adds additional checks to catch
misuses of the subsystem. in some - it just adds more debugging output
on console. And in this case it's actually neither of these. But I am
not sure what's a better name ("full"?). Even if we split options into
multiple, we still can have some kind of presents that just flip all
other options into reasonable values.



> Andrey, please give us some idea of the CPU and RAM overheads other
> than those coming from MTE
> * stack trace collection and storage
> * adding redzones to every allocation - not strictly needed for MTE,
> but convenient to store the stack trace IDs.
>
> Andrey: with production MTE we should not be using quarantine, which
> means storing the stack trace IDs
> in the deallocated memory doesn't provide good report quality.
> We may need to consider another approach, e.g. the one used in HWASAN
> (separate ring buffer, per thread or per core)
>
> --kcc
>
>
> On Fri, Oct 16, 2020 at 8:52 AM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> >
> > On Fri, Oct 16, 2020 at 3:31 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> > >
> > > On Fri, 16 Oct 2020 at 15:17, 'Andrey Konovalov' via kasan-dev
> > > <kasan-dev@xxxxxxxxxxxxxxxx> wrote:
> > > [...]
> > > > > > 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?
> > >
> > > SGTM.
> > >
> > > [...]
> > > >
> > > > > > 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?
> > >
> > > Best would be if kasan= can take another option, e.g.
> > > "kasan=prod,panic". I think you can change the strcmp() to a
> > > str_has_prefix() for the checks for full/prod/on/off, and then check
> > > if what comes after it is ",panic".
> > >
> > > Thanks,
> > > -- Marco
> >
> > CC Kostya and Serban.