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    Subject[PATCH 3.16 72/83] net: Fix null de-reference of device refcount
    3.16.78-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>

    commit 10cc514f451a0f239aa34f91bc9dc954a9397840 upstream.

    In event of failure during register_netdevice, free_netdev is
    invoked immediately. free_netdev assumes that all the netdevice
    refcounts have been dropped prior to it being called and as a
    result frees and clears out the refcount pointer.

    However, this is not necessarily true as some of the operations
    in the NETDEV_UNREGISTER notifier handlers queue RCU callbacks for
    invocation after a grace period. The IPv4 callback in_dev_rcu_put
    tries to access the refcount after free_netdev is called which
    leads to a null de-reference-

    44837.761523: <6> Unable to handle kernel paging request at
    virtual address 0000004a88287000
    44837.761651: <2> pc : in_dev_finish_destroy+0x4c/0xc8
    44837.761654: <2> lr : in_dev_finish_destroy+0x2c/0xc8
    44837.762393: <2> Call trace:
    44837.762398: <2> in_dev_finish_destroy+0x4c/0xc8
    44837.762404: <2> in_dev_rcu_put+0x24/0x30
    44837.762412: <2> rcu_nocb_kthread+0x43c/0x468
    44837.762418: <2> kthread+0x118/0x128
    44837.762424: <2> ret_from_fork+0x10/0x1c

    Fix this by waiting for the completion of the call_rcu() in
    case of register_netdevice errors.

    Fixes: 93ee31f14f6f ("[NET]: Fix free_netdev on register_netdev failure.")
    Cc: Sean Tranchetti <stranche@codeaurora.org>
    Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    net/core/dev.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/net/core/dev.c
    +++ b/net/core/dev.c
    @@ -6278,6 +6278,8 @@ int register_netdevice(struct net_device
    ret = notifier_to_errno(ret);
    if (ret) {
    rollback_registered(dev);
    + rcu_barrier();
    +
    dev->reg_state = NETREG_UNREGISTERED;
    }
    /*
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