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

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Message-ID: <70319f78-2c7c-8141-d751-07f28203db7c@linux.com>
Date:   Thu, 4 Jun 2020 18:23:38 +0300
From:   Alexander Popov <alex.popov@...ux.com>
To:     Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>,
        Elena Reshetova <elena.reshetova@...el.com>
Cc:     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>,
        Will Deacon <will@...nel.org>,
        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 <kernel-hardening@...ts.openwall.com>,
        linux-kbuild@...r.kernel.org,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kernel list <linux-kernel@...r.kernel.org>, gcc@....gnu.org,
        notify@...nel.org
Subject: Re: [PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the
 instrumentation logic

On 04.06.2020 17:01, Jann Horn wrote:
> On Thu, Jun 4, 2020 at 3:51 PM Alexander Popov <alex.popov@...ux.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