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

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Message-ID: <202006091149.6C78419@keescook>
Date:   Tue, 9 Jun 2020 12:09:27 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Alexander Popov <alex.popov@...ux.com>,
        Emese Revfy <re.emese@...il.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Jessica Yu <jeyu@...nel.org>,
        Sven Schnelle <svens@...ckframe.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Naohiro Aota <naohiro.aota@....com>,
        Alexander Monakov <amonakov@...ras.ru>,
        Mathias Krause <minipli@...glemail.com>,
        PaX Team <pageexec@...email.hu>,
        Brad Spengler <spender@...ecurity.net>,
        Laura Abbott <labbott@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        kernel-hardening@...ts.openwall.com, linux-kbuild@...r.kernel.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, gcc@....gnu.org, notify@...nel.org
Subject: Re: [PATCH 5/5] gcc-plugins/stackleak: Don't instrument
 vgettimeofday.c in arm64 VDSO

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@...ux.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