Re: [Xen-devel] race condition in xen-gntdev

[Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Jun 29, 2015 at 10:39:26AM -0400, Konrad Rzeszutek Wilk wrote:
> On Fri, Jun 26, 2015 at 03:28:24AM +0200, Marek Marczykowski-GÃrecki wrote:
> > On Mon, Jun 22, 2015 at 03:14:16PM -0400, Daniel De Graaf wrote:
> > > The reason that gntdev_release didn't have a lock is because there are not
> > > supposed to be any references to the areas pointed to by priv->maps when 
> > > it
> > > is called.  However, since the MMU notifier has not yet been unregistered,
> > > it is apparently possible to race here; the comment on 
> > > mmu_notifier_unregister
> > > seems to confirm this as a possibility (as do the backtraces).
> > > 
> > > I think adding the lock will be sufficient.
> > 
> > Ok, so here is the patch:
> 
> Awesome!
> 
> Since you are the one who has been seeing this particular fault - any chance
> you could give it some soak time? If I recall your emails correctly it takes
> about a week or so before you saw the crash?

Sure. I've already installed patched kernel, will report back results
later.

> 
> > 
> > -----------8<------------
> > 
> > From b876e14888bdafa112c3265e6420543fa74aa709 Mon Sep 17 00:00:00 2001
> > From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=
> >  <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > Date: Fri, 26 Jun 2015 02:16:49 +0200
> > Subject: [PATCH] xen/grant: fix race condition in gntdev_release
> > 
> > While gntdev_release is called, MMU notifier is still registered and
> > can traverse priv->maps list even if no pages are mapped (which is the
> > case - gntdev_release is called after all). But gntdev_release will
> > clear that list, so make sure that only one of those things happens at
> > the same time.
> > 
> > Signed-off-by: Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/xen/gntdev.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> > index 8927485..4bd23bb 100644
> > --- a/drivers/xen/gntdev.c
> > +++ b/drivers/xen/gntdev.c
> > @@ -568,12 +568,14 @@ static int gntdev_release(struct inode *inode, struct 
> > file *flip)
> >  
> >     pr_debug("priv %p\n", priv);
> >  
> > +   mutex_lock(&priv->lock);
> >     while (!list_empty(&priv->maps)) {
> >             map = list_entry(priv->maps.next, struct grant_map, next);
> >             list_del(&map->next);
> >             gntdev_put_map(NULL /* already removed */, map);
> >     }
> >     WARN_ON(!list_empty(&priv->freeable_maps));
> > +   mutex_unlock(&priv->lock);
> >  
> >     if (use_ptemod)
> >             mmu_notifier_unregister(&priv->mn, priv->mm);
> > -- 
> > 1.9.3
> > 
> > 
> > -- 
> > Best Regards,
> > Marek Marczykowski-GÃrecki
> > Invisible Things Lab
> > A: Because it messes up the order in which people normally read text.
> > Q: Why is top-posting such a bad thing?
> 
> 

-- 
Best Regards,
Marek Marczykowski-GÃrecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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