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    Subject[PATCH 3.12 30/84] ppp: defer netns reference release for ppp channel
    Date
    From: WANG Cong <xiyou.wangcong@gmail.com>

    3.12-stable review patch. If anyone has any objections, please let me know.

    ===============

    commit 205e1e255c479f3fd77446415706463b282f94e4 upstream.

    Matt reported that we have a NULL pointer dereference
    in ppp_pernet() from ppp_connect_channel(),
    i.e. pch->chan_net is NULL.

    This is due to that a parallel ppp_unregister_channel()
    could happen while we are in ppp_connect_channel(), during
    which pch->chan_net set to NULL. Since we need a reference
    to net per channel, it makes sense to sync the refcnt
    with the life time of the channel, therefore we should
    release this reference when we destroy it.

    Fixes: 1f461dcdd296 ("ppp: take reference on channels netns")
    Reported-by: Matt Bennett <Matt.Bennett@alliedtelesis.co.nz>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: linux-ppp@vger.kernel.org
    Cc: Guillaume Nault <g.nault@alphalink.fr>
    Cc: Cyrill Gorcunov <gorcunov@openvz.org>
    Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
    Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    drivers/net/ppp/ppp_generic.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
    index 14a8d2958698..ab79c0f13d0a 100644
    --- a/drivers/net/ppp/ppp_generic.c
    +++ b/drivers/net/ppp/ppp_generic.c
    @@ -2317,8 +2317,6 @@ ppp_unregister_channel(struct ppp_channel *chan)
    spin_lock_bh(&pn->all_channels_lock);
    list_del(&pch->list);
    spin_unlock_bh(&pn->all_channels_lock);
    - put_net(pch->chan_net);
    - pch->chan_net = NULL;

    pch->file.dead = 1;
    wake_up_interruptible(&pch->file.rwait);
    @@ -2925,6 +2923,9 @@ ppp_disconnect_channel(struct channel *pch)
    */
    static void ppp_destroy_channel(struct channel *pch)
    {
    + put_net(pch->chan_net);
    + pch->chan_net = NULL;
    +
    atomic_dec(&channel_count);

    if (!pch->file.dead) {
    --
    2.10.1
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