linux-kernel - Re: [PATCH] exit: move exit_task_namespaces() after exit_task

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Date:   Thu, 14 Dec 2017 21:08:17 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] exit: move exit_task_namespaces() after exit_task_work()

On Thu, Dec 14, 2017 at 12:17:57PM -0800, Cong Wang wrote:
> syzbot reported we have a use-after-free when mqueue_evict_inode()
> is called on __cleanup_mnt() path, where the ipc ns is already
> freed by the previous exit_task_namespaces(). We can just move
> it after after exit_task_work() to avoid this use-after-free.

What's to prevent somebody else holding a reference to the same
inode past the exit(2)?  IOW, I don't believe that this is fixing
anything - in the best case, your patch papers over a specific
reproducer.