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Date: Tue, 9 May 2017 08:34:04 +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:
> 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/
Ok, so that's CVE-2010-4258, where an oops with KERNEL_DS set was used to escalate
privileges, due to the kernel's oops handler not cleaning up the KERNEL_DS. The
exploit used another bug, a crash in a network protocol handler, to execute the
oops handler with KERNEL_DS set.
The explanation of the exploit itself points out that it's a very interesting bug
and I agree, it's not a general kernel bug but a bug in a very narrow code path
(oops handling) that caused this, and I don't see how that example can be turned
into a general example: it was a bug in oops handling to let the process continue
execution (and perform the CLEARTID operation) *and* leak the address limit at
KERNEL_DS.
By similar argument a bug in the runtime checking of the address limit may allow
exploits. Consider the oops path cleanup a similarly sensitive code path as the
address limit check.
To handle this category of exploits it would be enough to add a runtime check to
the _oops handling code itself_ (to make sure we've set addr_limit back to USER_DS
even if we crash in a KERNEL_DS code area), not to every system call!
That check would avoid that particular historic pattern, if combined with static
analysis that ensured that KERNEL_DS is always set/restored correctly. (Which btw.
I believe some of the regular static scans of the kernel are already doing today.)
Furthermore, to go back to your original argument:
> Static analysis is just not going to cover all cases.
it's not even true that a runtime check will 'cover all cases': for example a
similar bug to CVE-2010-4258 could still be exploited:
- Note that the actual put_user() was not prevented via the runtime check - the
runtime check would run *after* the buggy put_user() was done. The runtime
check warns or panics after the fact, which might (or might not) be enough to
prevent the exploit.
- Also note that a slightly different form of the bug would still be exploitable,
even with the runtime check: for example if the task-shutdown code can be made
to unconditionally set KERNEL_DS, but after the put_user(), then the runtime
check would not 'cover all cases'.
So the argument for doing this runtime check after every system call is very
dubious.
Thanks,
Ingo
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