Re: [PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the instrumentation logic

From: Alexander Popov <alex.popov@linux.com>
To: Jann Horn <jannh@google.com>, Kees Cook <keescook@chromium.org>,
	Elena Reshetova <elena.reshetova@intel.com>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>,
	Naohiro Aota <naohiro.aota@wdc.com>,
	Sven Schnelle <svens@stackframe.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	the arch/x86 maintainers <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>,
	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 <linux-arm-kernel@lists.infradead.org>,
	notify@kernel.org, Florian Weimer <fweimer@redhat.com>,
	gcc@gcc.gnu.org, Brad Spengler <spender@grsecurity.net>,
	kernel list <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 1/5] gcc-plugins/stackleak: Exclude alloca() from the instrumentation logic
Date: Thu, 4 Jun 2020 18:23:38 +0300	[thread overview]
Message-ID: <70319f78-2c7c-8141-d751-07f28203db7c@linux.com> (raw)
In-Reply-To: <CAG48ez05JOvqzYGr3PvyQGwFURspFWvNvf-b8Y613PX0biug8w@mail.gmail.com>

On 04.06.2020 17:01, Jann Horn wrote:
> On Thu, Jun 4, 2020 at 3:51 PM Alexander Popov <alex.popov@linux.com> wrote:
>> Some time ago Variable Length Arrays (VLA) were removed from the kernel.
>> The kernel is built with '-Wvla'. Let's exclude alloca() from the
>> instrumentation logic and make it simpler. The build-time assertion
>> against alloca() is added instead.
> [...]
>> +                       /* Variable Length Arrays are forbidden in the kernel */
>> +                       gcc_assert(!is_alloca(stmt));
> 
> There is a patch series from Elena and Kees on the kernel-hardening
> list that deliberately uses __builtin_alloca() in the syscall entry
> path to randomize the stack pointer per-syscall - see
> <https://lore.kernel.org/kernel-hardening/20200406231606.37619-4-keescook@chromium.org/>.

Thanks, Jann.

At first glance, leaving alloca() handling in stackleak instrumentation logic
would allow to integrate stackleak and this version of random_kstack_offset.

Kees, Elena, did you try random_kstack_offset with upstream stackleak?

It looks to me that without stackleak erasing random_kstack_offset can be
weaker. I mean, if next syscall has a bigger stack randomization gap, the data
on thread stack from the previous syscall is not overwritten and can be used. Am
I right?

Another aspect: CONFIG_STACKLEAK_METRICS can be used for guessing kernel stack
offset, which is bad. It should be disabled if random_kstack_offset is on.

Best regards,
Alexander

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