linux-kernel - Re: [PATCH] pidns: remove recursion from free_pid

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Date:	Tue, 9 Oct 2012 12:08:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Greg KH <greg@...ah.com>
Cc:	Andrew Vagin <avagin@...nvz.org>, linux-kernel@...r.kernel.org,
	criu@...nvz.org, Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] pidns: remove recursion from free_pid_ns (v3)

On Tue, 9 Oct 2012 12:03:00 -0700
Greg KH <greg@...ah.com> wrote:

> On Tue, Oct 09, 2012 at 11:48:21AM -0700, Andrew Morton wrote:
> > On Sat,  6 Oct 2012 23:56:33 +0400
> > Andrew Vagin <avagin@...nvz.org> wrote:
> > 
> > > Here is a stack trace of recursion:
> > > free_pid_ns(parent)
> > >   put_pid_ns(parent)
> > >     kref_put(&ns->kref, free_pid_ns);
> > >       free_pid_ns
> > > 
> > > This patch turns recursion into loops.
> > > 
> > > pidns can be nested many times, so in case of recursion
> > > a simple user space program can provoke a kernel panic
> > > due to exceed of a kernel stack.
> > 
> > So we should backport this into earlier kernels.
> > 
> > > --- a/include/linux/kref.h
> > > +++ b/include/linux/kref.h
> > > @@ -95,6 +95,18 @@ static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref)
> > >  	return kref_sub(kref, 1, release);
> > >  }
> > >  
> > > +/**
> > > + * kref_put - decrement refcount for object.
> > > + * @kref: object.
> > > + *
> > > + * Decrement the refcount.
> > > + * Return 1 if refcount is zero.
> > > + */
> > > +static inline int __kref_put(struct kref *kref)
> > > +{
> > > +	return atomic_dec_and_test(&kref->refcount);
> > > +}
> > 
> > Greg might be interested in this.
> > 
> > It's a pretty specialised thing and perhaps it needs some stern words
> > in the description explaining when and why it should and shouldn't be
> > used.
> > 
> > I wonder if people might (ab)use this to avoid the "doesn't
> > have a release function" warning.
> 
> Yes they would, please don't do this at all.
> 
> In fact, why is it needed?  It doesn't solve anything (if it does,
> something in the way the kref is being used is wrong.)
> 

It's right there in the changelog.  The patch fixes deep
kref_put->release->kref_put recursion by turning the operation for
pidns into a loop.
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