linux-kernel - Re: proc_flush

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Date:   Fri, 22 Dec 2017 01:58:23 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Dave Jones <davej@...emonkey.org.uk>
Cc:     Alexey Dobriyan <adobriyan@...il.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

Dave Jones <davej@...emonkey.org.uk> writes:

> On Thu, Dec 21, 2017 at 07:31:26PM -0600, Eric W. Biederman wrote:
>  > Dave Jones <davej@...emonkey.org.uk> writes:
>  > 
>  > > On Thu, Dec 21, 2017 at 12:38:12PM +0200, Alexey Dobriyan wrote:
>  > >  
>  > >  > > 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 didn't hit this BUG, but I hit the same oops in proc_flush_task.
>  > 
>  > Scratch one idea.
>  > 
>  > If it isn't too much trouble can you try this.
>  > 
>  > I am wondering if somehow the proc_mnt that is NULL is somewhere in the
>  > middle of the stack of pid namespaces.
>  > 
>  > This adds two warnings.  The first just reports which pid namespace in
>  > the stack of pid namespaces is problematic, and the pid number in that
>  > pid namespace.  Which should give a whole lot more to go by.
>  > 
>  > The second warning complains if we manage to create a pid namespace
>  > where the parent pid namespace is not properly set up.  The test to
>  > prevent that looks quite robust, but at this point I don't know where to
>  > look.
>
> Progress ?
>
> [ 1653.030190] ------------[ cut here ]------------
> [ 1653.030852] 1/1: 2 no proc_mnt
> [ 1653.030946] WARNING: CPU: 2 PID: 4420 at kernel/pid.c:213 alloc_pid+0x24f/0x2a0

Yes.  I don't know why Alexey's patch did not fire but this is
confirmation that the first pid allocated was #2 and not #1.

Which explains the pid_mnt not being set, and it is definitely the new
code, changing from the pid bitmap+hash table to an idr.

So it looks like idr_alloc_cyclic in some configuration for the first
allocation returns value #2 instead of value #1.

I don't know that code, and it is quite complicated so I will have to
stare at it some more to even guess why it is doing that.

This is confirmation that reverting those pid changes will fix the
problem.  As they are definitely at fault.

Hmm.  After a little more staring I have a hunch what is going wrong.
It is just possible that there is a failure in alloc_pid during the
first pid allocation and then idr_next gets left at 2.  I need to sleep
before I can think of a patch to test that.

Hmm.  A failure and then restart would also explain why Alexey's patch
did not fire.  An incomplete reset of state.

Eric