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Message-ID: <CAGXu5jJEoTEkOdbVu+AQF8=_NRem3g5vECJYU=n8UNpfHbBHTA@mail.gmail.com>
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: 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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