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From:   Kees Cook <keescook@chromium.org>
Date:   Mon, 16 Apr 2018 11:29:13 -0700
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Subject: Re: [PATCH v11 2/6] x86/entry: Add STACKLEAK erasing the kernel stack
 at the end of syscalls
To:     Alexander Popov <alex.popov@linux.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Kernel Hardening <kernel-hardening@lists.openwall.com>,
        PaX Team <pageexec@freemail.hu>,
        Brad Spengler <spender@grsecurity.net>,
        Andy Lutomirski <luto@kernel.org>,
        Tycho Andersen <tycho@tycho.ws>,
        Mark Rutland <mark.rutland@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Borislav Petkov <bp@alien8.de>,
        Richard Sandiford <richard.sandiford@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "Dmitry V . Levin" <ldv@altlinux.org>,
        Emese Revfy <re.emese@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Garnier <thgarnie@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Josef Bacik <jbacik@fb.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
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        Ding Tianhong <dingtianhong@huawei.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
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        Florian Weimer <fweimer@redhat.com>,
        Boris Lukashev <blukashev@sempervictus.com>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
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On Fri, Apr 6, 2018 at 7:22 AM, Alexander Popov <alex.popov@linux.com> wrote:
> This commit introduces the architecture-specific code filling the used
> part of the kernel stack with a poison value before returning to the
> userspace. Full STACKLEAK feature also contains the gcc plugin which
> comes in a separate commit.

Thanks for sending this again! And thanks for the updated reasoning
for why this remains a valuable addition:

https://lkml.kernel.org/r/1523024546-6150-1-git-send-email-alex.popov@linux.com

I, too, remain convinced this is a good protection to have, even as we
slowly remove VLAs and try to improve the compiler's initialization of
stack variables.

Dave, Ingo, Linus: how does this look? With the assembly rewritten
into C, the entry changes are very small:

>  arch/x86/entry/entry_32.S        |  7 ++++++
>  arch/x86/entry/entry_64.S        |  3 +++
>  arch/x86/entry/entry_64_compat.S |  5 ++++
>  arch/x86/entry/erase.c           | 54 ++++++++++++++++++++++++++++++++++++++++

I'd really like to get people's Ack/Review. :)

Laura, can this C version work for arm64 as well?

Thanks,

-Kees

-- 
Kees Cook
Pixel Security