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    Subject[PATCH 4.16 41/81] l2tp: hold reference on tunnels in netlink dumps
    Date
    4.16-stable review patch.  If anyone has any objections, please let me know.

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

    From: Guillaume Nault <g.nault@alphalink.fr>


    [ Upstream commit 5846c131c39b6d0add36ec19dc8650700690f930 ]

    l2tp_tunnel_find_nth() is unsafe: no reference is held on the returned
    tunnel, therefore it can be freed whenever the caller uses it.
    This patch defines l2tp_tunnel_get_nth() which works similarly, but
    also takes a reference on the returned tunnel. The caller then has to
    drop it after it stops using the tunnel.

    Convert netlink dumps to make them safe against concurrent tunnel
    deletion.

    Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
    Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/l2tp/l2tp_core.c | 20 ++++++++++++++++++++
    net/l2tp/l2tp_core.h | 2 ++
    net/l2tp/l2tp_netlink.c | 11 ++++++++---
    3 files changed, 30 insertions(+), 3 deletions(-)

    --- a/net/l2tp/l2tp_core.c
    +++ b/net/l2tp/l2tp_core.c
    @@ -183,6 +183,26 @@ struct l2tp_tunnel *l2tp_tunnel_get(cons
    }
    EXPORT_SYMBOL_GPL(l2tp_tunnel_get);

    +struct l2tp_tunnel *l2tp_tunnel_get_nth(const struct net *net, int nth)
    +{
    + const struct l2tp_net *pn = l2tp_pernet(net);
    + struct l2tp_tunnel *tunnel;
    + int count = 0;
    +
    + rcu_read_lock_bh();
    + list_for_each_entry_rcu(tunnel, &pn->l2tp_tunnel_list, list) {
    + if (++count > nth) {
    + l2tp_tunnel_inc_refcount(tunnel);
    + rcu_read_unlock_bh();
    + return tunnel;
    + }
    + }
    + rcu_read_unlock_bh();
    +
    + return NULL;
    +}
    +EXPORT_SYMBOL_GPL(l2tp_tunnel_get_nth);
    +
    /* Lookup a session. A new reference is held on the returned session. */
    struct l2tp_session *l2tp_session_get(const struct net *net,
    struct l2tp_tunnel *tunnel,
    --- a/net/l2tp/l2tp_core.h
    +++ b/net/l2tp/l2tp_core.h
    @@ -212,6 +212,8 @@ static inline void *l2tp_session_priv(st
    }

    struct l2tp_tunnel *l2tp_tunnel_get(const struct net *net, u32 tunnel_id);
    +struct l2tp_tunnel *l2tp_tunnel_get_nth(const struct net *net, int nth);
    +
    void l2tp_tunnel_free(struct l2tp_tunnel *tunnel);

    struct l2tp_session *l2tp_session_get(const struct net *net,
    --- a/net/l2tp/l2tp_netlink.c
    +++ b/net/l2tp/l2tp_netlink.c
    @@ -487,14 +487,17 @@ static int l2tp_nl_cmd_tunnel_dump(struc
    struct net *net = sock_net(skb->sk);

    for (;;) {
    - tunnel = l2tp_tunnel_find_nth(net, ti);
    + tunnel = l2tp_tunnel_get_nth(net, ti);
    if (tunnel == NULL)
    goto out;

    if (l2tp_nl_tunnel_send(skb, NETLINK_CB(cb->skb).portid,
    cb->nlh->nlmsg_seq, NLM_F_MULTI,
    - tunnel, L2TP_CMD_TUNNEL_GET) < 0)
    + tunnel, L2TP_CMD_TUNNEL_GET) < 0) {
    + l2tp_tunnel_dec_refcount(tunnel);
    goto out;
    + }
    + l2tp_tunnel_dec_refcount(tunnel);

    ti++;
    }
    @@ -848,7 +851,7 @@ static int l2tp_nl_cmd_session_dump(stru

    for (;;) {
    if (tunnel == NULL) {
    - tunnel = l2tp_tunnel_find_nth(net, ti);
    + tunnel = l2tp_tunnel_get_nth(net, ti);
    if (tunnel == NULL)
    goto out;
    }
    @@ -856,6 +859,7 @@ static int l2tp_nl_cmd_session_dump(stru
    session = l2tp_session_get_nth(tunnel, si);
    if (session == NULL) {
    ti++;
    + l2tp_tunnel_dec_refcount(tunnel);
    tunnel = NULL;
    si = 0;
    continue;
    @@ -865,6 +869,7 @@ static int l2tp_nl_cmd_session_dump(stru
    cb->nlh->nlmsg_seq, NLM_F_MULTI,
    session, L2TP_CMD_SESSION_GET) < 0) {
    l2tp_session_dec_refcount(session);
    + l2tp_tunnel_dec_refcount(tunnel);
    break;
    }
    l2tp_session_dec_refcount(session);

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