linux-kernel - Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

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Message-ID: <CAJcbSZGhE0ecpqS4jmezqwGXf_o4cakTVkE+m67kJJvhao1ENA@mail.gmail.com>
Date:   Mon, 8 May 2017 12:51:34 -0700
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Daniel Micay <danielmicay@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Howells <dhowells@...hat.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

On Mon, May 8, 2017 at 8:26 AM, Kees Cook <keescook@...omium.org> wrote:
> On Mon, May 8, 2017 at 8:22 AM, Daniel Micay <danielmicay@...il.com> wrote:
>> On Mon, 2017-05-08 at 09:52 +0200, Ingo Molnar wrote:
>>>
>>> ... it's just not usable in that form for a regular maintenance flow.
>>>
>>> So what would be more useful is to add a specific Sparse check that
>>> only checks
>>> KERNEL_DS, to add it as a regular (.config driven) build option and
>>> make sure the
>>> kernel build has zero warnings.
>>>
>>> From that point on we can declare that this kind of bug won't occur
>>> anymore, if
>>> the Sparse implementation of the check is correct.
>>>
>>> But there's a (big) problem with that development model: Sparse is not
>>> part of the
>>> kernel tree and adding a feature to it while making the kernel depend
>>> on that
>>> brand new feature is a logistical nightmare. The overhead is quite
>>> similar to
>>> adding new features to a compiler - it happens at a glacial pace and
>>> is only done
>>> for major features really, at considerable expense. I don't think this
>>> is an
>>> adequate model for 'extended syntax checking' of the kernel,
>>> especially when it
>>> comes to correctness that has such obvious security impact.
>>>
>>> Thanks,
>>>
>>>       Ingo
>>
>> There's the option of using GCC plugins now that the infrastructure was
>> upstreamed from grsecurity. It can be used as part of the regular build
>> process and as long as the analysis is pretty simple it shouldn't hurt
>> compile time much.
>
> Well, and that the situation may arise due to memory corruption, not
> from poorly-matched set_fs() calls, which static analysis won't help
> solve. We need to catch this bad kernel state because it is a very bad
> state to run in.

Of course, I agree with Kees points on this and previous emails.

A static analysis solution is hard to scale across functions and build
time can suffer. I don't think the coverage will be good enough to
consider this change and static analysis as similar.

>
> -Kees
>
> --
> Kees Cook
> Pixel Security



-- 
Thomas