netdev - Re: [PATCH for stable 3.10] xen-netback: drop SKB from internal queue if frontend is disconnected

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Date:	Fri, 11 Jul 2014 14:30:51 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	<xen-devel@...ts.xen.org>, <netdev@...r.kernel.org>,
	Philipp Hahn <hahn@...vention.de>
Subject: Re: [PATCH for stable 3.10] xen-netback: drop SKB from internal
 queue if frontend is disconnected

On Fri, 2014-07-11 at 14:28 +0100, Wei Liu wrote:
> On Fri, Jul 11, 2014 at 02:15:29PM +0100, Ian Campbell wrote:
> > On Fri, 2014-07-11 at 14:08 +0100, Wei Liu wrote:
> > > In 88a810def7 ("xen-netback: fix refcnt unbalance for 3.10"), we moved
> > > the ref counting code from xenvif_disconnect to xenvif_free.
> > 
> > Since this was a 3.10 specific backport that explains why this is also
> > such?
> > 
> 
> Yes.

OK.

> > > It can occur that frontend is disconnected while there's still SKB
> > > stuck in netback's rx_queue in rare case. When netback thread wakes up,
> > > it will try to write to an already unmapped ring, resulting in kernel
> > > oops.
> > > 
> > > Moving the ref counting back to xenvif_disconnect isn't an option as it
> > > reintroduces an old bug. Further more, writing into a dead frontend's
> > > ring and memory is just wrong. Dropping those SKBs seems to be a good
> > > strategy.
> > > 
> > > This patch fixes that corner case: introduce a flag to indicate whether
> > > frontend ring is mapped. If the ring is unmapped, just drop those SKBs.
> > > 
> > > This bug only manifests in 3.10 kernel. Kernel >=3.12 doesn't have it.
> > 
> > What about 3.11?
> > 
> 
> 3.11 is long dead while 3.10 is LTS kernel. I don't think GregKH accepts
> patch for 3.11 anymore.

Whether Greg would take a patch is orthogonal to whether the bug exists
in 3.11 though. Someone might want to fix their own kernel locally for
example...

Ian.

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