From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <1494625675.29205.21.camel@redhat.com>
From: Rik van Riel <riel@redhat.com>
Date: Fri, 12 May 2017 17:47:55 -0400
In-Reply-To: <20170512214144.GT390@ZenIV.linux.org.uk>
References: 
	<CA+55aFwvQfs_X+paQF6Luc0Rq+W3J2fKuHRou7=ANcquDdXdDA@mail.gmail.com>
	 <20170512072802.5a686f23@mschwideX1>
	 <CAGXu5j+EatK=DYONRkgovwLgytAnbG8jnAZaMSLckZFNVj3gig@mail.gmail.com>
	 <20170512075458.09a3a1ce@mschwideX1>
	 <CAGXu5jL9vUrn4kpjO+qa4cHmWBypeqP17OGbrMs=5Nz0YpQMZw@mail.gmail.com>
	 <CA+55aFzbiBqsYb7vwO=+L4Vp_GOgPu+DBOrq4fBnyzq5DbBehg@mail.gmail.com>
	 <CAGXu5j+xmyJ6RhtPw9rUgs7k3sZ1KKWffvyGGG--oPfu9W42ng@mail.gmail.com>
	 <20170512202106.GO22219@n2100.armlinux.org.uk>
	 <20170512210645.GS390@ZenIV.linux.org.uk>
	 <CAGXu5jJu=VTqp2tzkPB4RAVxdGC+_SSQwrUwdzWpu24AA-zEcg@mail.gmail.com>
	 <20170512214144.GT390@ZenIV.linux.org.uk>
Content-Type: text/plain; charset="utf-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 8bit
Subject: Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode
List-Archive: <https://lore.kernel.org/kernel-hardening/>
List-Post: <mailto:kernel-hardening@lists.openwall.com>
To: Al Viro <viro@ZenIV.linux.org.uk>, Kees Cook <keescook@chromium.org>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>, 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>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Arnd Bergmann <arnd@arndb.de>, Brian Gerst <brgerst@gmail.com>, Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, Linux API <linux-api@vger.kernel.org>, Oleg Nesterov <oleg@redhat.com>, Daniel Micay <danielmicay@gmail.com>, James Morse <james.morse@arm.com>, "Eric W .
 Biederman" <ebiederm@xmission.com>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Paolo Bonzini <pbonzini@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Thomas Garnier <thgarnie@google.com>, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
List-ID: <linux-s390.vger.kernel.org>

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.