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Date: Tue, 9 May 2017 09:30:02 -0700
From: Kees Cook <keescook@...omium.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Daniel Micay <danielmicay@...il.com>, 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>,
Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address
limit before returning to user-mode
On Mon, May 8, 2017 at 11:56 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Kees Cook <keescook@...omium.org> wrote:
>
>> > 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.
[attempting some thread-merging]
> 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.
Right, I didn't mean to suggest that vulnerability would be fixed by
this solution. I was trying to show how there can be some pretty
complex interaction with exceptions/interrupts/etc that would make
pure static analysis still miss things.
> If memory corruption corrupted the task state into having addr_limit set to
> KERNEL_DS then there's already a fair chance that it's game over: it could also
> have set *uid to 0, or changed a sensitive PF_ flag, or a number of other
> things...
>
> Furthermore, think about it: there's literally an infinite amount of corrupted
> task states that could be a security problem and that could be checked after every
> system call. Do we want to check every one of them?
Right, but this "slippery slope" argument isn't the best way to reject
security changes. Let me take a step back and describe the threat, and
where we should likely spend time:
The primary threat with addr_limit getting changed is that a
narrowly-scoped attack (traditionally stack exhaustion or
adjacent-stack large-index writes) could be leveraged into opening the
entire kernel to writes (by allowing all syscalls with a
copy_to_user() call to suddenly be able to write to kernel memory).
So, really, the flaw is having addr_limit at all. Removing set_fs()
should, I think, allow this to become a const (or at least should get
us a lot closer).
The main path to corrupting addr_limit has been via stack corruption.
On architectures with CONFIG_THREAD_INFO_IN_TASK, this risk is greatly
reduced already, but it's not universally available yet. (And as long
as we're talking about stack attacks, CONFIG_VMAP_STACK makes
cross-stack overflows go away, and cross-stack indexing harder, but
that's not really about addr_limit since currently nothing with
VMAP_STACK doesn't already have THREAD_INFO_IN_TASK.)
So, left with a still exploitable target in memory that allows such an
expansion of attack method, I still think it's worth keeping this
patch series, but if we can drop set_fs() I could probably be
convinced the benefit of the series doesn't exceed the cost on
THREAD_INFO_IN_TASK-architectures (x86, arm64, s390). But that means
at least currently keeping it on arm, for example. If we can make
addr_limit const, well, we don't need the series at all.
-Kees
--
Kees Cook
Pixel Security
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