Re: proc_flush_task oops

[Dave Jones <davej@codemonkey.org.uk>]

On Thu, Dec 21, 2017 at 12:38:12PM +0200, Alexey Dobriyan wrote:
 > On 12/21/17, Eric W. Biederman <ebiederm@xmission.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