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Date: Mon, 8 May 2017 12:51:34 -0700
From: Thomas Garnier <>
To: Kees Cook <>
Cc: Daniel Micay <>, Ingo Molnar <>, 
	Martin Schwidefsky <>, Heiko Carstens <>, 
	Dave Hansen <>, Arnd Bergmann <>, 
	Thomas Gleixner <>, David Howells <>, 
	René Nyffenegger <>, 
	Andrew Morton <>, 
	"Paul E . McKenney" <>, "Eric W . Biederman" <>, 
	Oleg Nesterov <>, Pavel Tikhomirov <>, 
	Ingo Molnar <>, "H . Peter Anvin" <>, Andy Lutomirski <>, 
	Paolo Bonzini <>, Rik van Riel <>, 
	Josh Poimboeuf <>, Borislav Petkov <>, Brian Gerst <>, 
	"Kirill A . Shutemov" <>, 
	Christian Borntraeger <>, Russell King <>, 
	Will Deacon <>, Catalin Marinas <>, 
	Mark Rutland <>, James Morse <>, 
	linux-s390 <>, LKML <>, 
	Linux API <>, "the arch/x86 maintainers" <>, 
	"" <>, 
	Kernel Hardening <>, 
	Linus Torvalds <>, Peter Zijlstra <>
Subject: Re: 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 <> wrote:
> On Mon, May 8, 2017 at 8:22 AM, Daniel Micay <> 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


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