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

[Will Deacon]

On Tue, Jun 09, 2020 at 12:09:27PM -0700, Kees Cook wrote:
> 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@xxxxxxxxx>
> > > ---
> > >  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?)

On arm64, the 32-bit toolchain is picked up via CC_COMPAT -- does that still
get the plugins?

> 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.

Thanks for going through these. In general though, it seems like an
opt-in strategy would make more sense, as it doesn't make an awful lot
of sense to me for the plugins to be used to build the vDSO.

So I would prefer that this patch filters out $(GCC_PLUGINS_CFLAGS).

Will