Re: [PATCH] ISDN: fix double free bug in isdn_net

[Jesper Juhl]

On 16/08/06, Karsten Keil <kkeil@xxxxxxx> wrote:
> On Wed, Aug 16, 2006 at 10:22:46PM +0200, Jesper Juhl wrote:
> > On 15/08/06, Karsten Keil <kkeil@xxxxxxx> wrote:
> > >On Tue, Aug 15, 2006 at 02:15:03AM -0700, David Miller wrote:
> > >> From: "Jesper Juhl" <jesper.juhl@xxxxxxxxx>
> > >> Date: Tue, 15 Aug 2006 11:08:35 +0200
> > >>
> > >> > Hmm, perhaps I made a mistake and missed a path. Maybe it would be
> > >> > better to fix if by making isdn_writebuf_skb_stub() always set the skb
> > >> > to NULL when it does free it. That would add a few more assignments
> > >> > but should ensure the right result always.
> > >> > What do you say?
> > >>
> > >> Do we know if the ->writebuf_skb() method ever frees the skb?  If it
> > >> never does, then yes your suggestion would be one way to handle this.
> > >
> > >
> > >It does if it consumes the skb (then it returns skb->len).
> > >But the skb have not to be freed imediately in this case, it maybe
> > >queued or used until all bytes are written to the physical device.
> > >
> > >If it returns any other value the skb is not freed.
> > >
> > >This logic came from using skb for transparent data too.
> > >Here it was possible, that the hw driver only take some bytes from the
> > >skb (so it returns < skb->len), then the isdn layer should requeue
> > >the skb so no transparent data get lost.
> > >
> > >But this mechanism was never used in drivers, only 3 states:
> > >
> > >The driver accept the packet then it is responsible for the skb
> > >and return skb->len or the driver do not accept it (e.g. buffer full,
> > >conntection is going down), then it return 0 and does not free the
> > >skb.
> > >
> > >If some internal error in the HW driver occur, it should return a
> > >negative value and it also do not free the skb.
> > >
> >
> > Ok, if I understand you correctly, then there's no actual problem here.
> > right?
> >
>
> I not aware of any problems in this code (besides it should be cleaned up
> and rewritten).
>
> Was here any trigger for your patch ?

Well, the trigger for the initial patch was Coverity bug #1060 .
I looked at it and thought it looked valid, so I tried to cook up a
patch for was I believed looked like an actual problem.


Here's what Coverity had to say :

CID: 1060
Checker: USE_AFTER_FREE (help)
File: base/src/linux-2.6/drivers/isdn/i4l/isdn_net.c

...
1006 	void isdn_net_writebuf_skb(isdn_net_local *lp, struct sk_buff *skb)
1007 	{
1008 		int ret;
1009 		int len = skb->len;     /* save len */
1010 	
1011 		/* before obtaining the lock the caller should have checked that
1012 		   the lp isn't busy */
1013 		if (isdn_net_lp_busy(lp)) {
1014 			printk("isdn BUG at %s:%d!\n", __FILE__, __LINE__);
1015 			goto error;
1016 		}
1017 	
1018 		if (!(lp->flags & ISDN_NET_CONNECTED)) {
1019 			printk("isdn BUG at %s:%d!\n", __FILE__, __LINE__);
1020 			goto error;
1021 		}

Event freed_arg: Pointer "skb" freed by function
"isdn_writebuf_skb_stub" [model]
Also see events: [double_free]

1022 		ret = isdn_writebuf_skb_stub(lp->isdn_device, lp->isdn_channel, 1, skb);

At conditional (1): "ret != len" taking true path

1023 		if (ret != len) {
1024 			/* we should never get here */
1025 			printk(KERN_WARNING "%s: HL driver queue full\n", lp->name);
1026 			goto error;
1027 		}
1028 		
1029 		lp->transcount += len;
1030 		isdn_net_inc_frame_cnt(lp);
1031 		return;
1032 	
1033 	 error:

Event double_free: Double free of pointer "skb" in call to "kfree_skb" [model]
Also see events: [freed_arg]

1034 		dev_kfree_skb(skb);
1035 		lp->stats.tx_errors++;
1036 	
1037 	}
...


--
Jesper Juhl <jesper.juhl@xxxxxxxxx>
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