linux-kernel - Re: [PATCH] ipc: introduce ipc_valid_object() helper to sort out IPC

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Date:	Tue, 17 Dec 2013 14:18:02 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Greg Thelen <gthelen@...gle.com>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: Re: [PATCH] ipc: introduce ipc_valid_object() helper to sort out
 IPC_RMID races

On Tue, 2013-12-17 at 19:46 -0200, Rafael Aquini wrote:
> On Tue, Dec 17, 2013 at 01:27:49PM -0800, Davidlohr Bueso wrote:
> > Ccing Manfred.
> > 
> > On Tue, 2013-12-17 at 17:03 -0200, Rafael Aquini wrote:
> > > After the locking semantics for the SysV IPC API got improved, a couple of
> > > IPC_RMID race windows were opened because we ended up dropping the
> > > 'kern_ipc_perm.deleted' check performed way down in ipc_lock().
> > > The spotted races got sorted out by re-introducing the old test within
> > > the racy critical sections.
> > > 
> > > This patch introduces ipc_valid_object() to consolidate the way we cope with
> > > IPC_RMID races by using the same abstraction across the API implementation.
> > 
> > This is certainly a good function to have. Some comments below.
> > 
[...]
> > > 
> > >  		shm_file = shp->shm_file;
> > > -
> > > -		/* check if shm_destroy() is tearing down shp */
> > > -		if (shm_file == NULL) {
> > > -			err = -EIDRM;
> > > -			goto out_unlock0;
> > > -		}
> > 
> > Ok, this seems safe, we can always rely on .deleted for validity since
> > shm_destroy() ends up calling shm_rmid() which sets .deleted. So this
> > change is really moving what we're checking against just a few
> > instructions later.
> >
> 
> Yep, I did change it cause it seems that there's no reason to delay the return
> condition if we raced with shm_destroy(), anyways.
>  

Right, but I was referring to moving what we consider as valid.

static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp)
{
	struct file *shm_file;

	shm_file = shp->shm_file;
	shp->shm_file = NULL;   <--- we currently use this.
	ns->shm_tot -= (shp->shm_segsz + PAGE_SIZE - 1) >> PAGE_SHIFT;
	shm_rmid(ns, shp); <--- with your patch we now use this.
	shm_unlock(shp);
	...
}

... and it makes since since shm was the only one not using .deleted for
RMID racing checks.

Thanks,
Davidlohr

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