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Subject: Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode
From: Rik van Riel <riel@redhat.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>,
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Date: Fri, 12 May 2017 17:47:55 -0400
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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.