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Date:   Thu, 21 Dec 2017 09:25:35 -0500
From:   Dave Jones <davej@...emonkey.org.uk>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com, Gargi Sharma <gs051095@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: proc_flush_task oops

On Thu, Dec 21, 2017 at 12:38:12PM +0200, Alexey Dobriyan wrote:
 > On 12/21/17, Eric W. Biederman <ebiederm@...ssion.com> wrote:
 > > I have stared at this code, and written some test programs and I can't
 > > see what is going on.  alloc_pid by design and in implementation (as far
 > > as I can see) is always single threaded when allocating the first pid
 > > in a pid namespace.  idr_init always initialized idr_next to 0.
 > >
 > > So how we can get past:
 > >
 > > 	if (unlikely(is_child_reaper(pid))) {
 > > 		if (pid_ns_prepare_proc(ns)) {
 > > 			disable_pid_allocation(ns);
 > > 			goto out_free;
 > > 		}
 > > 	}
 > >
 > > with proc_mnt still set to NULL is a mystery to me.
 > >
 > > Is there any chance the idr code doesn't always return the lowest valid
 > > free number?  So init gets assigned something other than 1?
 > 
 > Well, this theory is easy to test (attached).

I'll give this a shot and report back when I get to the office.

 > There is a "valid" way to break the code via kernel.ns_last_pid:
 > unshare+write+fork but the reproducer doesn't seem to use it (or it does?)

that sysctl is root only, so that isn't at play here.

	Dav

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