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References: <20200604134957.505389-1-alex.popov@linux.com> <20200604134957.505389-2-alex.popov@linux.com>
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From:   Jann Horn <jannh@google.com>
Date:   Thu, 4 Jun 2020 16:01:30 +0200
Message-ID: <CAG48ez05JOvqzYGr3PvyQGwFURspFWvNvf-b8Y613PX0biug8w@mail.gmail.com>
Subject: Re: [PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the
 instrumentation logic
To:     Alexander Popov <alex.popov@linux.com>,
        Kees Cook <keescook@chromium.org>,
        Elena Reshetova <elena.reshetova@intel.com>
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>,
        Will Deacon <will@kernel.org>,
        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>,
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        Kernel Hardening <kernel-hardening@lists.openwall.com>,
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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/>.