linux-kernel - Re: [PATCH v3] target: close target_put_sess_cmd() vs. core_tmr_abort

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Date:	Mon, 18 Mar 2013 19:39:11 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Jörn Engel <joern@...fs.org>,
	linux-kernel@...r.kernel.org,
	target-devel <target-devel@...r.kernel.org>
Subject: Re: [PATCH v3] target: close target_put_sess_cmd() vs.
 core_tmr_abort_task() race

On Mon, Mar 18, 2013 at 07:11:12PM -0700, Nicholas A. Bellinger wrote:
> On Mon, 2013-03-18 at 19:34 -0400, Jörn Engel wrote:
> > And because I'm lame and stupid, here's v3.
> > 
> > It is possible for one thread to to take se_sess->sess_cmd_lock in
> > core_tmr_abort_task() before taking a reference count on
> > se_cmd->cmd_kref, while another thread in target_put_sess_cmd() drops
> > se_cmd->cmd_kref before taking se_sess->sess_cmd_lock.
> > 
> > This introduces kref_put_and_lock() and uses it in
> > target_put_sess_cmd() to close the race window.
> > 
> > Signed-off-by: Joern Engel <joern@...fs.org>
> > ---
> >  drivers/target/target_core_transport.c |   17 +++++++++++------
> >  include/linux/kref.h                   |   26 ++++++++++++++++++++++++++
> >  2 files changed, 37 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
> > index 04ec9cb..b98c158 100644
> > --- a/drivers/target/target_core_transport.c
> > +++ b/drivers/target/target_core_transport.c
> > @@ -2207,21 +2207,19 @@ static void target_release_cmd_kref(struct kref *kref)
> >  {
> >  	struct se_cmd *se_cmd = container_of(kref, struct se_cmd, cmd_kref);
> >  	struct se_session *se_sess = se_cmd->se_sess;
> > -	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&se_sess->sess_cmd_lock, flags);
> >  	if (list_empty(&se_cmd->se_cmd_list)) {
> > -		spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
> > +		spin_unlock(&se_sess->sess_cmd_lock);
> >  		se_cmd->se_tfo->release_cmd(se_cmd);
> >  		return;
> >  	}
> >  	if (se_sess->sess_tearing_down && se_cmd->cmd_wait_set) {
> > -		spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
> > +		spin_unlock(&se_sess->sess_cmd_lock);
> >  		complete(&se_cmd->cmd_wait_comp);
> >  		return;
> >  	}
> >  	list_del(&se_cmd->se_cmd_list);
> > -	spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);
> > +	spin_unlock(&se_sess->sess_cmd_lock);
> >  
> >  	se_cmd->se_tfo->release_cmd(se_cmd);
> 
> The fact that this changes ->release_cmd() to be called with irqs still
> disabled is going to cause a problem for any fabrics that take a _bh
> lock from this code-path.
> 
> iscsi-target is the only fabric AFAICT that this could potentially be
> problematic for, but as I'm currently in the process of converting it to
> use ->cmd_kref for v3.10 code some extra refactoring to avoid these
> locks from within ->release_cmd() should not be a big deal..
> 
> So that said, applied to target-pending/for-next.

Really?  Don't you need my ack as you are adding a new kref api call
here.

greg k-h
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