Date: Fri, 12 May 2017 14:17:19 -0700 From: Kees Cook <keescook@...omium.org> To: Al Viro <viro@...iv.linux.org.uk> Cc: Russell King - ARM Linux <linux@...linux.org.uk>, Linus Torvalds <torvalds@...ux-foundation.org>, Mark Rutland <mark.rutland@....com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Greg KH <greg@...ah.com>, Heiko Carstens <heiko.carstens@...ibm.com>, LKML <linux-kernel@...r.kernel.org>, David Howells <dhowells@...hat.com>, Dave Hansen <dave.hansen@...el.com>, "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, linux-s390 <linux-s390@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>, Will Deacon <will.deacon@....com>, Christian Borntraeger <borntraeger@...ibm.com>, René Nyffenegger <mail@...enyffenegger.ch>, Catalin Marinas <catalin.marinas@....com>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Rik van Riel <riel@...hat.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Arnd Bergmann <arnd@...db.de>, Brian Gerst <brgerst@...il.com>, Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Linux API <linux-api@...r.kernel.org>, Oleg Nesterov <oleg@...hat.com>, Daniel Micay <danielmicay@...il.com>, James Morse <james.morse@....com>, "Eric W . Biederman" <ebiederm@...ssion.com>, Martin Schwidefsky <schwidefsky@...ibm.com>, Paolo Bonzini <pbonzini@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Thomas Garnier <thgarnie@...gle.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com> Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode On Fri, May 12, 2017 at 2:06 PM, Al Viro <viro@...iv.linux.org.uk> wrote: > On Fri, May 12, 2017 at 09:21:06PM +0100, Russell King - ARM Linux wrote: >> On Fri, May 12, 2017 at 12:30:02PM -0700, Kees Cook wrote: >> > I'm clearly not explaining things well enough. I shouldn't say >> > "corruption", I should say "malicious manipulation". The methodology >> > of attacks against the stack are quite different from the other kinds >> > of attacks like use-after-free, heap overflow, etc. Being able to >> > exhaust the kernel stack (either due to deep recursion or unbounded >> > alloca()) >> >> I really hope we don't have alloca() use in the kernel. Do you have >> evidence to support that assertion? >> >> IMHO alloca() (or similar) should not be present in any kernel code >> because we have a limited stack - we have kmalloc() etc for that kind >> of thing. > > No alloca(), but there are VLAs. Said that, the whole "what if they > can bugger thread_info and/or task_struct and go after set_fs() state" > is idiocy, of course - in that case the box is fucked, no matter what. Two things are at risk from stack exhaustion: thread_info (mainly addr_limit) when on the stack (fixed by THREAD_INFO_IN_TASK), and overflow into adjacent allocations (fixed by VMAP_STACK). The latter is fundamentally a heap overflow. -Kees -- Kees Cook Pixel Security
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