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Wed, 10 Jun 2020 08:24:30 -0700 (PDT) Received: from [192.168.42.234] ([213.87.161.124]) by smtp.gmail.com with ESMTPSA id 11sm9227lju.118.2020.06.10.08.24.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Jun 2020 08:24:27 -0700 (PDT) Reply-To: alex.popov@linux.com Subject: Re: [PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the instrumentation logic To: Kees Cook Cc: Jann Horn , Elena Reshetova , Emese Revfy , Miguel Ojeda , Masahiro Yamada , Michal Marek , Andrew Morton , Masahiro Yamada , Thiago Jung Bauermann , Luis Chamberlain , Jessica Yu , Sven Schnelle , Iurii Zaikin , Catalin Marinas , Will Deacon , Vincenzo Frascino , Thomas Gleixner , Peter Collingbourne , Naohiro Aota , Alexander Monakov , Mathias Krause , PaX Team , Brad Spengler , Laura Abbott , Florian Weimer , Kernel Hardening , linux-kbuild@vger.kernel.org, the arch/x86 maintainers , Linux ARM , kernel list , gcc@gcc.gnu.org, notify@kernel.org References: <20200604134957.505389-1-alex.popov@linux.com> <20200604134957.505389-2-alex.popov@linux.com> <70319f78-2c7c-8141-d751-07f28203db7c@linux.com> <202006091133.412F0E89@keescook> From: Alexander Popov Autocrypt: addr=alex.popov@linux.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <202006091133.412F0E89@keescook> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09.06.2020 21:39, Kees Cook wrote: > On Thu, Jun 04, 2020 at 06:23:38PM +0300, Alexander Popov wrote: >> On 04.06.2020 17:01, Jann Horn wrote: >>> On Thu, Jun 4, 2020 at 3:51 PM Alexander Popov 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 >>> . >> >> Thanks, Jann. >> >> At first glance, leaving alloca() handling in stackleak instrumentation logic >> would allow to integrate stackleak and this version of random_kstack_offset. > > Right, it seems there would be a need for this coverage to remain, > otherwise the depth of stack erasure might be incorrect. > > It doesn't seem like the other patches strictly depend on alloca() > support being removed, though? Ok, I will leave alloca() support, reorganize the patch series and send v2. >> Kees, Elena, did you try random_kstack_offset with upstream stackleak? > > I didn't try that combination yet, no. It seemed there would likely > still be further discussion about the offset series first (though the > thread has been silent -- I'll rebase and resend it after rc2). Ok, please add me to CC list. Best regards, Alexander >> 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? > > That's correct. I think the combination is needed, but I don't think > they need to be strictly tied together. > >> 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. > > Agreed.