Re: [PATCH 5/5] gcc-plugins/stackleak: Don't instrument vgettimeofday.c in arm64 VDSO

Kees Cook
2020-06-09T19:09:31Z

On Thu, Jun 04, 2020 at 02:58:06PM +0100, Will Deacon wrote:
> On Thu, Jun 04, 2020 at 04:49:57PM +0300, Alexander Popov wrote:
> > Don't try instrumenting functions in arch/arm64/kernel/vdso/vgettimeofday.c.
> > Otherwise that can cause issues if the cleanup pass of stackleak gcc plugin
> > is disabled.
> >
> > Signed-off-by: Alexander Popov <alex.popov@linux.com>
> > ---
> > arch/arm64/kernel/vdso/Makefile | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
> > index 3862cad2410c..9b84cafbd2da 100644
> > --- a/arch/arm64/kernel/vdso/Makefile
> > +++ b/arch/arm64/kernel/vdso/Makefile
> > @@ -32,7 +32,8 @@ UBSAN_SANITIZE := n
> > OBJECT_FILES_NON_STANDARD := y
> > KCOV_INSTRUMENT := n
> >
> > -CFLAGS_vgettimeofday.o = -O2 -mcmodel=tiny -fasynchronous-unwind-tables
> > +CFLAGS_vgettimeofday.o = -O2 -mcmodel=tiny -fasynchronous-unwind-tables \
> > + $(DISABLE_STACKLEAK_PLUGIN)
>
> I can pick this one up via arm64, thanks. Are there any other plugins we
> should be wary of? It looks like x86 filters out $(GCC_PLUGINS_CFLAGS)
> when building the vDSO.

I didn't realize/remember that arm64 retained the kernel build flags for
vDSO builds. (I'm used to x86 throwing all its flags away for its vDSO.)

How does 32-bit ARM do its vDSO?

My quick run-through on plugins:

arm_ssp_per_task_plugin.c
32-bit ARM only (but likely needs disabling for 32-bit ARM vDSO?)

cyc_complexity_plugin.c
compile-time reporting only

latent_entropy_plugin.c
this shouldn't get triggered for the vDSO (no __latent_entropy
nor __init attributes in vDSO), but perhaps explicitly disabling
it would be a sensible thing to do, just for robustness?

randomize_layout_plugin.c
this shouldn't get triggered (again, lacking attributes), but
should likely be disabled too.

sancov_plugin.c
This should be tracking the KCOV directly (see
scripts/Makefile.kcov), which is already disabled here.

structleak_plugin.c
This should be fine in the vDSO, but there's not security
boundary here, so it wouldn't be important to KEEP it enabled.

--
Kees Cook