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

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Date:   Mon, 8 May 2017 16:02:30 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Kees Cook <keescook@...omium.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


* 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?

> [...] 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 ...

> [...] 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.

Thanks,

	Ingo