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Date: Mon, 8 May 2017 08:26:23 -0700
From: Kees Cook <keescook@...omium.org>
To: Daniel Micay <danielmicay@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>, Thomas Garnier <thgarnie@...gle.com>, 
	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: Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

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.

-Kees

-- 
Kees Cook
Pixel Security

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