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    Subject[PATCH 4.19 103/113] tuntap: synchronize through tfiles array instead of tun->numqueues
    Date
    From: Jason Wang <jasowang@redhat.com>

    [ Upstream commit 9871a9e47a2646fe30ae7fd2e67668a8d30912f6 ]

    When a queue(tfile) is detached through __tun_detach(), we move the
    last enabled tfile to the position where detached one sit but don't
    NULL out last position. We expect to synchronize the datapath through
    tun->numqueues. Unfortunately, this won't work since we're lacking
    sufficient mechanism to order or synchronize the access to
    tun->numqueues.

    To fix this, NULL out the last position during detaching and check
    RCU protected tfile against NULL instead of checking tun->numqueues in
    datapath.

    Cc: YueHaibing <yuehaibing@huawei.com>
    Cc: Cong Wang <xiyou.wangcong@gmail.com>
    Cc: weiyongjun (A) <weiyongjun1@huawei.com>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Fixes: c8d68e6be1c3b ("tuntap: multiqueue support")
    Signed-off-by: Jason Wang <jasowang@redhat.com>
    Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/net/tun.c | 7 ++++++-
    1 file changed, 6 insertions(+), 1 deletion(-)

    --- a/drivers/net/tun.c
    +++ b/drivers/net/tun.c
    @@ -708,6 +708,8 @@ static void __tun_detach(struct tun_file
    tun->tfiles[tun->numqueues - 1]);
    ntfile = rtnl_dereference(tun->tfiles[index]);
    ntfile->queue_index = index;
    + rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
    + NULL);

    --tun->numqueues;
    if (clean) {
    @@ -1090,7 +1092,7 @@ static netdev_tx_t tun_net_xmit(struct s
    tfile = rcu_dereference(tun->tfiles[txq]);

    /* Drop packet if interface is not attached */
    - if (txq >= tun->numqueues)
    + if (!tfile)
    goto drop;

    if (!rcu_dereference(tun->steering_prog))
    @@ -1281,6 +1283,7 @@ static int tun_xdp_xmit(struct net_devic

    rcu_read_lock();

    +resample:
    numqueues = READ_ONCE(tun->numqueues);
    if (!numqueues) {
    rcu_read_unlock();
    @@ -1289,6 +1292,8 @@ static int tun_xdp_xmit(struct net_devic

    tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
    numqueues]);
    + if (unlikely(!tfile))
    + goto resample;

    spin_lock(&tfile->tx_ring.producer_lock);
    for (i = 0; i < n; i++) {

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