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Date: Fri, 12 May 2017 17:47:55 -0400
From: Rik van Riel <riel@...hat.com>
To: Al Viro <viro@...IV.linux.org.uk>, Kees Cook <keescook@...omium.org>
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>, 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, 2017-05-12 at 22:41 +0100, Al Viro wrote:
> On Fri, May 12, 2017 at 02:17:19PM -0700, Kees Cook wrote:
> 
> > Two things are at risk from stack exhaustion: thread_info (mainly
> > addr_limit) when on the stack (fixed by THREAD_INFO_IN_TASK), and
> 
> Really?  Let's take a look at arm, for example:
> 
> struct thread_info {
>         unsigned long           flags;          /* low level flags */
>         int                     preempt_count;  /* 0 => preemptable,
> <0 => bug */
>         mm_segment_t            addr_limit;     /* address limit */
>         struct task_struct      *task;          /* main task
> structure */
> 
> and current() is defined as current_thread_info()->task.
> 
> Seriously, look at these beasts.  Overwriting ->addr_limit is nowhere
> near
> the top threat.  If attacker can overwrite thread_info, you have
> lost.

That is why THREAD_INFO_IN_TASK exists. It moves
the struct thread_info to a location away from the
stack, which means a stack overflow will not overwrite
the thread_info.

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