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Date:   Fri, 12 May 2017 14:04:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Brian Gerst <brgerst@...il.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Oleg Nesterov <oleg@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Greg KH <greg@...ah.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Rik van Riel <riel@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>,
        Borislav Petkov <bp@...en8.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Micay <danielmicay@...il.com>,
        James Morse <james.morse@....com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

On Fri, May 12, 2017 at 2:00 PM, Kees Cook <keescook@...omium.org> wrote:
> On Fri, May 12, 2017 at 1:45 PM, Russell King - ARM Linux
> <linux@...linux.org.uk> wrote:
>> On Fri, May 12, 2017 at 10:30:44PM +0200, Peter Zijlstra wrote:
>>> On Fri, May 12, 2017 at 09:21:06PM +0100, Russell King - ARM Linux wrote:
>>> > On Fri, May 12, 2017 at 12:30:02PM -0700, Kees Cook wrote:
>>> > > I'm clearly not explaining things well enough. I shouldn't say
>>> > > "corruption", I should say "malicious manipulation". The methodology
>>> > > of attacks against the stack are quite different from the other kinds
>>> > > of attacks like use-after-free, heap overflow, etc. Being able to
>>> > > exhaust the kernel stack (either due to deep recursion or unbounded
>>> > > alloca())
>>> >
>>> > I really hope we don't have alloca() use in the kernel.  Do you have
>>> > evidence to support that assertion?
>>> >
>>> > IMHO alloca() (or similar) should not be present in any kernel code
>>> > because we have a limited stack - we have kmalloc() etc for that kind
>>> > of thing.
>>>
>>> On stack variable length arrays get implemented by the compiler doing
>>> alloca(), and we sadly have a few of those around.
>>
>> I hope their size is appropriately limited, but something tells me it
>> would be foolish to assume that.
>>
>>> But yes, fully agreed on the desirability of alloca() and things.
>>
>> Hmm, I wonder if -fno-builtin-alloca would prevent those... it looks
>> like it certainly would prevent an explicit alloca() call.
>
> Building with -Werror=vla is exciting. :)
>
> A lot of it is in crypto (which are relatively static sizes, just
> using function callbacks), but there is plenty more.

I meant to also paste an example (which is harmless, I haven't looked
extensively at other examples):

        unsigned long alignmask = crypto_tfm_alg_alignmask(tfm);
        unsigned int size = crypto_tfm_alg_blocksize(tfm);
        u8 buffer[size + alignmask];

Looking at all the places (and having tried to remove a few of these
in pstore), I think it might be quite frustrating to eliminate them
all and then declare VLAs dead. I'm not against trying, though. :)

-Kees

-- 
Kees Cook
Pixel Security

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