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

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Message-ID: <CAGXu5jJ4iY7QZ9wRu5dmm7RHtLh_V6TQh4huWwLCYPKOr63aiA@mail.gmail.com>
Date: Mon, 8 May 2017 08:24:43 -0700
From: Kees Cook <keescook@...omium.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: 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: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

On Mon, May 8, 2017 at 7:02 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Kees Cook <keescook@...omium.org> wrote:
>
>> > And yes, I realize that there were other such bugs and that such bugs might
>> > occur in the future - but why not push the overhead of the security check to
>> > the kernel build phase? I.e. I'm wondering how well we could do static
>> > analysis during kernel build - would a limited mode of Sparse be good enough
>> > for that? Or we could add a new static checker to tools/, built from first
>> > principles and used primarily for extended syntactical checking.
>>
>> Static analysis is just not going to cover all cases. We've had vulnerabilities
>> where interrupt handlers left KERNEL_DS set, for example. [...]
>
> Got any commit ID of that bug - was it because a function executed by the
> interrupt handler leaked KERNEL_DS?

Ah, it was an exception handler, but the one I was thinking of was this:
https://lwn.net/Articles/419141/

>> [...] If there are performance concerns, let's put this behind a CONFIG. 2-5
>> instructions is not an issue for most people that want this coverage.
>
> That doesn't really _solve_ the performance concerns, it just forces most people
> to enable it by creating a 'security or performance' false dichotomy ...

That's fair, but what I'm trying to say is that many people will want
this, so rejecting it because it's 2 more instructions seems
unreasonable. We have had much more invasive changes added to the
kernel.

>> [...] and it still won't catch everything. Bug-finding is different from making
>> a bug class just unexploitable at all. As we've done before, it's the difference
>> between trying to find format string attacks vs just removing %n from the format
>> parser.
>
> No, it does not make it unexploitable, it could still be exploitable if the
> runtime check is buggy or if there's kernel execution outside of the regular
> system call paths - there's plenty of such hardware functionality on x86 for
> example.

Fine, but this is splitting hairs. This does protect a specific
situation, and it does so very cheaply. The real fix would be to
remove set_fs() entirely. :P

-Kees

-- 
Kees Cook
Pixel Security
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