From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode
Date: Fri, 12 May 2017 22:06:46 +0100
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In-Reply-To: <20170512202106.GO22219@n2100.armlinux.org.uk>
Sender: Al Viro <viro@ftp.linux.org.uk>
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Kees Cook <keescook@chromium.org>, Linus Torvalds <torvalds@linux-foundation.org>, Mark Rutland <mark.rutland@arm.com>, Kernel Hardening <kernel-hardening@lists.openwall.com>, Greg KH <greg@kroah.com>, Heiko Carstens <heiko.carstens@de.ibm.com>, LKML <linux-kernel@vger.kernel.org>, David Howells <dhowells@redhat.com>, Dave Hansen <dave.hansen@intel.com>, "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>, Pavel Tikhomirov <ptikhomirov@virtuozzo.com>, linux-s390 <linux-s390@vger.kernel.org>, the arch/x86 maintainers <x86@kernel.org>, Will Deacon <will.deacon@arm.com>, Christian Borntraeger <borntraeger@de.ibm.com>, =?iso-8859-1?Q?Ren=E9?= Nyffenegger <mail@renenyffenegger.ch>, Catalin Marinas <catalin.marinas@arm.com>, "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>, Rik van Riel <riel@redhat.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Arnd
List-Id: linux-api@vger.kernel.org

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.