Subject: Re: [PATCH net] mISDN: hfcpci: Fix a use after free in hfcmulti_tx()

From: Dan Carpenter <[email-masked]>
Date: Thu, 22 Oct 2020 17:44:26 +0300

On Thu, Oct 22, 2020 at 04:24:00PM +0200, [email-masked] wrote:
> Hi Dan,
> 
> that looks wrong to me and never was a use after free.
> 
> sp is set either to the address containing the pointer to the actual
> D-channel SKB or to the actual B-channel SKB. This address is not freed
> and will not change in this context. The dev_kfree(*sp) will delete the
> old SKB and the call to  get_next_[bd]frame(), if returning true, will
> place a new SKB into this address, so (*sp) point to this new SKB.
> The len of course need to be the length of the new SKB, not the old one,
> which would be the result of this patch.
> 

Oh, wow.  You're absolutely right.  That's pretty subtle.  Thanks for
catching it.

regards,
dan carpenter

> Best regards
> Karsten
> 
> On 10/22/20 9:07 AM, Dan Carpenter wrote:
> > This frees "*sp" before dereferencing it to get "len = (*sp)->len;".
> > 
> > Fixes: af69fb3a8ffa ("Add mISDN HFC multiport driver")
> > Signed-off-by: Dan Carpenter <[email-masked]>
> > ---
> >  drivers/isdn/hardware/mISDN/hfcmulti.c | 10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/isdn/hardware/mISDN/hfcmulti.c b/drivers/isdn/hardware/mISDN/hfcmulti.c
> > index 7013a3f08429..ce6c160e0df4 100644
> > --- a/drivers/isdn/hardware/mISDN/hfcmulti.c
> > +++ b/drivers/isdn/hardware/mISDN/hfcmulti.c
> > @@ -2152,16 +2152,14 @@ hfcmulti_tx(struct hfc_multi *hc, int ch)
> >  		HFC_wait_nodebug(hc);
> >  	}
> >  
> > +	len = (*sp)->len;
> >  	dev_kfree_skb(*sp);
> >  	/* check for next frame */
> > -	if (bch && get_next_bframe(bch)) {
> > -		len = (*sp)->len;
> > +	if (bch && get_next_bframe(bch))
> >  		goto next_frame;
> > -	}
> > -	if (dch && get_next_dframe(dch)) {
> > -		len = (*sp)->len;
> > +
> > +	if (dch && get_next_dframe(dch))
> >  		goto next_frame;
> > -	}
> >  
> >  	/*
> >  	 * now we have no more data, so in case of transparent,
> >