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Subject: Re: [PATCH 5/5] gcc-plugins/stackleak: Don't instrument
 vgettimeofday.c in arm64 VDSO
To:     Will Deacon <will@kernel.org>, Kees Cook <keescook@chromium.org>
Cc:     Emese Revfy <re.emese@gmail.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Jessica Yu <jeyu@kernel.org>,
        Sven Schnelle <svens@stackframe.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Collingbourne <pcc@google.com>,
        Naohiro Aota <naohiro.aota@wdc.com>,
        Alexander Monakov <amonakov@ispras.ru>,
        Mathias Krause <minipli@googlemail.com>,
        PaX Team <pageexec@freemail.hu>,
        Brad Spengler <spender@grsecurity.net>,
        Laura Abbott <labbott@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        kernel-hardening@lists.openwall.com, linux-kbuild@vger.kernel.org,
        x86@kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, gcc@gcc.gnu.org, notify@kernel.org
References: <20200604134957.505389-1-alex.popov@linux.com>
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From:   Alexander Popov <alex.popov@linux.com>
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Message-ID: <bceea4f8-4b8e-f9ab-f275-572e448e3ec1@linux.com>
Date:   Wed, 10 Jun 2020 18:18:09 +0300
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On 10.06.2020 10:30, Will Deacon wrote:
> 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@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?)
> 
> 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).

Ok, I will do that in the v2 of the patch series.

Best regards,
Alexander