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

From: Kees Cook <keescook@chromium•org>
To: Will Deacon <will@kernel•org>
Cc: kernel-hardening@lists•openwall.com,
	Catalin Marinas <catalin.marinas@arm•com>,
	Masahiro Yamada <yamada.masahiro@socionext•com>,
	Vincenzo Frascino <vincenzo.frascino@arm•com>,
	Sven Schnelle <svens@stackframe•org>,
	Naohiro Aota <naohiro.aota@wdc•com>,
	Masahiro Yamada <masahiroy@kernel•org>,
	x86@kernel•org, Emese Revfy <re.emese@gmail•com>,
	Iurii Zaikin <yzaikin@google•com>,
	PaX Team <pageexec@freemail•hu>,
	Laura Abbott <labbott@redhat•com>,
	Mathias Krause <minipli@googlemail•com>,
	Alexander Popov <alex.popov@linux•com>,
	linux-kbuild@vger•kernel.org,
	Alexander Monakov <amonakov@ispras•ru>,
	Michal Marek <michal.lkml@markovi•net>,
	Thomas Gleixner <tglx@linutronix•de>,
	Peter Collingbourne <pcc@google•com>,
	linux-arm-kernel@lists•infradead.org, notify@kernel•org,
	Florian Weimer <fweimer@redhat•com>,
	gcc@gcc•gnu.org, Brad Spengler <spender@grsecurity•net>,
	linux-kernel@vger•kernel.org,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail•com>,
	Luis Chamberlain <mcgrof@kernel•org>,
	Jessica Yu <jeyu@kernel•org>,
	Andrew Morton <akpm@linux-foundation•org>,
	Thiago Jung Bauermann <bauerman@linux•ibm.com>
Subject: Re: [PATCH 5/5] gcc-plugins/stackleak: Don't instrument vgettimeofday.c in arm64 VDSO
Date: Tue, 9 Jun 2020 12:09:27 -0700	[thread overview]
Message-ID: <202006091149.6C78419@keescook> (raw)
In-Reply-To: <20200604135806.GA3170@willie-the-truck>

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

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