On 03/14/2019 08:19 PM, Eric Dumazet wrote:
> Same reasons than the ones explained in commit 4179cb5a4c92
> ("vxlan: test dev->flags & IFF_UP before calling netif_rx()")
>
> netif_rx_ni() or napi_gro_frags() must be called under a strict contract.
>
> At device dismantle phase, core networking clears IFF_UP
> and flush_all_backlogs() is called after rcu grace period
> to make sure no incoming packet might be in a cpu backlog
> and still referencing the device.
>
> A similar protocol is used for gro layer.
>
> Most drivers call netif_rx() from their interrupt handler,
> and since the interrupts are disabled at device dismantle,
> netif_rx() does not have to check dev->flags & IFF_UP
>
> Virtual drivers do not have this guarantee, and must
> therefore make the check themselves.
>
> Fixes: 1bd4978a88ac ("tun: honor IFF_UP in tun_get_user()")
> Signed-off-by: Eric Dumazet <eduma...@google.com>
> Reported-by: syzbot <syzkal...@googlegroups.com>
> ---
> drivers/net/tun.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index
> 1d68921723dc08532b3f5321a52865076ad66336..0d343359f647ff58fee35462358827e61857c837
> 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1763,9 +1763,6 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
> int skb_xdp = 1;
> bool frags = tun_napi_frags_enabled(tfile);
>
> - if (!(tun->dev->flags & IFF_UP))
> - return -EIO;
> -
> if (!(tun->flags & IFF_NO_PI)) {
> if (len < sizeof(pi))
> return -EINVAL;
> @@ -1867,6 +1864,8 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
> err = skb_copy_datagram_from_iter(skb, 0, from, len);
>
> if (err) {
> + err = -EFAULT;
> +drop:
> this_cpu_inc(tun->pcpu_stats->rx_dropped);
> kfree_skb(skb);
> if (frags) {
> @@ -1874,7 +1873,7 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
> mutex_unlock(&tfile->napi_mutex);
> }
>
> - return -EFAULT;
> + return err;
> }
> }
>
> @@ -1958,6 +1957,12 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
> !tfile->detached)
> rxhash = __skb_get_hash_symmetric(skb);
>
> + rcu_read_lock();
> + if (unlikely(!(tun->dev->flags & IFF_UP))) {
> + err = -EIO;
Hmm, it looks like I forgot a rcu_read_unlock() here.
> + goto drop;
> + }
> +
> if (frags) {
> /* Exercise flow dissector code path. */
> u32 headlen = eth_get_headlen(skb->data, skb_headlen(skb));
> @@ -1965,6 +1970,7 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
> if (unlikely(headlen > skb_headlen(skb))) {
> this_cpu_inc(tun->pcpu_stats->rx_dropped);
> napi_free_frags(&tfile->napi);
> + rcu_read_unlock();
> mutex_unlock(&tfile->napi_mutex);
> WARN_ON(1);
> return -ENOMEM;
> @@ -1992,6 +1998,7 @@ static ssize_t tun_get_user(struct tun_struct *tun,
> struct tun_file *tfile,
> } else {
> netif_rx_ni(skb);
> }
> + rcu_read_unlock();
>
> stats = get_cpu_ptr(tun->pcpu_stats);
> u64_stats_update_begin(&stats->syncp);
>
