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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