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Date:   Fri, 12 May 2017 22:41:44 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     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>,
        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: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode

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.

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