Subject: Re: [PATCH] e1000: enhance frame fragment detection

From: Neil Horman <[email-masked]>
Date: Tue, 5 Jan 2010 17:04:13 -0500

On Tue, Jan 05, 2010 at 01:44:25PM -0800, Brandeburg, Jesse wrote:
> Neil, I couple of comments below, I was just looking at the implementation 
> of this for e1000e.
> 
> On Mon, 28 Dec 2009, Neil Horman wrote:
> 
> > Hey all-
> > 	A security discussion was recently given:
> > http://events.ccc.de/congress/2009/Fahrplan//events/3596.en.html
> > And a patch that I submitted awhile back was brought up.  Apparently some of
> > their testing revealed that they were able to force a buffer fragment in e1000
> > in which the trailing fragment was greater than 4 bytes.  As a result the
> > fragment check I introduced failed to detect the fragement and a partial invalid
> > frame was passed up into the network stack.  I've written this patch to correct
> > it.  I'm in the process of testing it now, but it makes good logical sense to
> > me.  Effectively it maintains a per-adapter state variable which detects a
> > non-EOP frame, and discards it and subsequent non-EOP frames leading up to _and_
> > _including_ the next positive-EOP frame (as it is by definition the last
> > fragment).  This should prevent any and all partial frames from entering the
> > network stack from e1000
> > 
> > Regards
> > Neil
> > 
> > 
> >  e1000.h      |    3 ++-
> >  e1000_main.c |   14 ++++++++++++--
> >  2 files changed, 14 insertions(+), 3 deletions(-)
> > 
> > 
> > diff --git a/drivers/net/e1000/e1000.h b/drivers/net/e1000/e1000.h
> > index 2a567df..3d421ab 100644
> > --- a/drivers/net/e1000/e1000.h
> > +++ b/drivers/net/e1000/e1000.h
> > @@ -331,7 +331,8 @@ struct e1000_adapter {
> >  enum e1000_state_t {
> >  	__E1000_TESTING,
> >  	__E1000_RESETTING,
> > -	__E1000_DOWN
> > +	__E1000_DOWN,
> > +	__E1000_DISCARDING
> >  };
> >  
> >  extern char e1000_driver_name[];
> > diff --git a/drivers/net/e1000/e1000_main.c b/drivers/net/e1000/e1000_main.c
> > index 7e855f9..0731779 100644
> > --- a/drivers/net/e1000/e1000_main.c
> > +++ b/drivers/net/e1000/e1000_main.c
> > @@ -3850,16 +3850,26 @@ static bool e1000_clean_rx_irq(struct e1000_adapter *adapter,
> >  
> >  		length = le16_to_cpu(rx_desc->length);
> >  		/* !EOP means multiple descriptors were used to store a single
> > -		 * packet, also make sure the frame isn't just CRC only */
> > -		if (unlikely(!(status & E1000_RXD_STAT_EOP) || (length <= 4))) {
> > +		 * packet, if thats the case we need to toss it.  In fact, we
> > +		 * to toss every packet with the EOP bit clear and the next
> > +		 * frame that _does_ have the EOP bit set, as it is by
> > +		 * definition only a frame fragment
> > +		 */
> > +		if (unlikely(!(status & E1000_RXD_STAT_EOP)))
> > +			set_bit(__E1000_DISCARDING, &adapter->flags);
> 
> test_bit and set_bit and clear_bit are atomic operations, isn't that quite 
> a bit of overhead for something that is already being done in a guaranteed 
> single context?
> 
> > +
> > +		if (test_bit(__E1000_DISCARDING, &adapter->flags)) {
> >  			/* All receives must fit into a single buffer */
> >  			E1000_DBG("%s: Receive packet consumed multiple"
> >  				  " buffers\n", netdev->name);
> >  			/* recycle */
> >  			buffer_info->skb = skb;
> > +			if (status & E1000_RXD_STAT_EOP)
> > +				clear_bit(__E1000_DISCARDING, &adapter->flags);
> 
> couldn't these simply be read/modify/write assignments (aka |=)
> 
> That would significantly avoid the extra cycles needed to implement three 
> atomic ops.
> 
They certainly could be non-atomic assignments, but the other flags in the
adapter falgs are atomic and I dont think its safe to mix and match the
accesses, lest we get a waw race somewhere.

If you really think we need to save the save the cycles the best thing to
probably do is define a new flags field separate from adapter->flags that can be
accessed with non-atomics.

Let me know if you would prefer that, and I'll happily re-spin the patch.
Neil