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