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

[Jason Wang]

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

> +
> +       if (xdp_prog)
> +               pad += XDP_PACKET_HEADROOM;
> +
> +       if (SKB_DATA_ALIGN(len + pad) +
>             SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE) {
>                 *skb_xdp = 0;
>                 return false;
> --
> 2.39.3
>