Re: [PATCH 2/2] drivers: net: prevent tun_can_build_skb() to exceed xdp size limits

[Andrew Kanner]

On Tue, Jul 25, 2023 at 11:39:46AM +0800, Jason Wang wrote:
> On Tue, Jul 25, 2023 at 6:15 AM Andrew Kanner <andrew.kanner@xxxxxxxxx> wrote:
> >
> > Tested with syzkaller repro with reduced packet size. It was
> > discovered that XDP_PACKET_HEADROOM is not checked in
> > tun_can_build_skb(), although pad may be incremented in
> > tun_build_skb().
> >
> > Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
> > Link: https://syzkaller.appspot.com/text?tag=ReproC&x=12b2593ea80000
> > Signed-off-by: Andrew Kanner <andrew.kanner@xxxxxxxxx>
> > ---
> >  drivers/net/tun.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 18ccbbe9830a..cdf2bd85b383 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1582,7 +1582,13 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
> >  static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> >                               int len, int noblock, bool zerocopy, int *skb_xdp)
> >  {
> > -       if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> > +       int pad = TUN_RX_PAD;
> > +       struct bpf_prog *xdp_prog = rcu_dereference(tun->xdp_prog);
> 
> This misses rcu read lock.
> 
> I wonder if things could be simpler if we move the limit check from
> tun_can_build_skb() to tun_build_skb():
> 
> rcu_read_lock();
> xdp_prog = rcu_dereference(tun->xdp_prog);
>         if (xdp_prog)
>                 pad += XDP_PACKET_HEADROOM;
> buflen += SKB_DATA_ALIGN(len + pad);
> rcu_read_unlock();
> 
> Thanks
> 

Thanks, I missed the part with rcu read lock for some reason.

It's a good idea to move / reduce duplication. Let me think and try to
fix according to your comments, I will resend it as v2.

-- 
Andrew Kanner