Re: [PATCH v4 2/2] UML: add support for KASAN under x86_64

[David Gow]

On Thu, Jun 30, 2022 at 9:29 PM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
>
> On Thu, Jun 30, 2022 at 2:54 PM Vincent Whitchurch
> <vincent.whitchurch@xxxxxxxx> wrote:
> >
> > On Thu, Jun 30, 2022 at 11:41:04AM +0200, Dmitry Vyukov wrote:
> > > On Thu, 30 Jun 2022 at 10:08, David Gow <davidgow@xxxxxxxxxx> wrote:
> > > > diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
> > > > index 1c2d4b29a3d4..a089217e2f0e 100644
> > > > --- a/arch/um/kernel/Makefile
> > > > +++ b/arch/um/kernel/Makefile
> > > > @@ -27,6 +27,9 @@ obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
> > > >  obj-$(CONFIG_STACKTRACE) += stacktrace.o
> > > >  obj-$(CONFIG_GENERIC_PCI_IOMAP) += ioport.o
> > > >
> > > > +KASAN_SANITIZE_stacktrace.o := n
> > > > +KASAN_SANITIZE_sysrq.o := n
> > >
> > > Why are these needed?
> > > It's helpful to leave some comments for any of *_SANITIZE:=n.
> > > Otherwise later it's unclear if it's due to some latent bugs, some
> > > inherent incompatibility, something that can be fixed, etc.
> >
> > I believe I saw the stacktrace code itself triggering KASAN splats and
> > causing recursion when sanitization was not disabled on it.  I noticed
> > that other architectures disabled sanitization of their stacktrace code,
> > eg. ARM in commit 4d576cab16f57e1f87978f ("ARM: 9028/1: disable KASAN in
> > call stack capturing routines"), so I did not investigate it further.
> >
> > (Note that despite the name, sysrq.c is also just stacktrace code.)
>
> Stack trace collection code might trigger KASAN splats when walking
> stack frames, but this can be resolved by using unchecked accesses.
> The main reason to disable instrumentation here is for performance
> reasons, see the upcoming patch for arm64 [1] for some details.
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?id=802b91118d11

Ah -- that does it! Using READ_ONCE_NOCHECK() in dump_trace() gets rid
of the nasty recursive KASAN failures we were getting in the tests.

I'll send out v5 with those files instrumented again.

Thanks!
-- David