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Date: Wed, 01 Jun 2022 11:27:42 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Duoming Zhou <duoming@....edu.cn>, linux-hams@...r.kernel.org
Cc: jreuter@...na.de, ralf@...ux-mips.org, davem@...emloft.net,
edumazet@...gle.com, kuba@...nel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, thomas@...erried.de
Subject: Re: [PATCH net v5] ax25: Fix ax25 session cleanup problems
On Mon, 2022-05-30 at 23:21 +0800, Duoming Zhou wrote:
> There are session cleanup problems in ax25_release() and
> ax25_disconnect(). If we setup a session and then disconnect,
> the disconnected session is still in "LISTENING" state that
> is shown below.
>
> Active AX.25 sockets
> Dest Source Device State Vr/Vs Send-Q Recv-Q
> DL9SAU-4 DL9SAU-3 ??? LISTENING 000/000 0 0
> DL9SAU-3 DL9SAU-4 ??? LISTENING 000/000 0 0
>
> The first reason is caused by del_timer_sync() in ax25_release().
> The timers of ax25 are used for correct session cleanup. If we use
> ax25_release() to close ax25 sessions and ax25_dev is not null,
> the del_timer_sync() functions in ax25_release() will execute.
> As a result, the sessions could not be cleaned up correctly,
> because the timers have stopped.
>
> In order to solve this problem, this patch adds a device_up flag
> in ax25_dev in order to judge whether the device is up. If there
> are sessions to be cleaned up, the del_timer_sync() in
> ax25_release() will not execute. What's more, we add ax25_cb_del()
> in ax25_kill_by_device(), because the timers have been stopped
> and there are no functions that could delete ax25_cb if we do not
> call ax25_release(). Finally, we reorder the position of
> ax25_list_lock in ax25_cb_del() in order to synchronize among
> different functions that call ax25_cb_del().
>
> The second reason is caused by improper check in ax25_disconnect().
> The incoming ax25 sessions which ax25->sk is null will close
> heartbeat timer, because the check "if(!ax25->sk || ..)" is
> satisfied. As a result, the session could not be cleaned up properly.
>
> In order to solve this problem, this patch changes the improper
> check to "if(ax25->sk && ..)" in ax25_disconnect().
>
> What`s more, the ax25_disconnect() may be called twice, which is
> not necessary. For example, ax25_kill_by_device() calls
> ax25_disconnect() and sets ax25->state to AX25_STATE_0, but
> ax25_release() calls ax25_disconnect() again.
>
> In order to solve this problem, this patch add a check in
> ax25_release(). If the flag of ax25->sk equals to SOCK_DEAD,
> the ax25_disconnect() in ax25_release() should not be executed.
>
> Fixes: 82e31755e55f ("ax25: Fix UAF bugs in ax25 timers")
> Fixes: 8a367e74c012 ("ax25: Fix segfault after sock connection timeout")
> Reported-and-tested-by: Thomas Osterried <thomas@...erried.de>
> Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> ---
> Changes since v1:
> - Add ax25_cb_del() in ax25_kill_by_device().
> - Mitigate race conditions through lock.
> - Fix session cleanup problem in ax25_disconnect().
> - Fix ax25_disconnect() may be called twice problem.
> - Change check in ax25_disconnect() to "if(ax25->sk && ..)".
>
> include/net/ax25.h | 1 +
> net/ax25/af_ax25.c | 27 +++++++++++++++++----------
> net/ax25/ax25_dev.c | 1 +
> net/ax25/ax25_subr.c | 2 +-
> 4 files changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/include/net/ax25.h b/include/net/ax25.h
> index 0f9790c455b..a427a05672e 100644
> --- a/include/net/ax25.h
> +++ b/include/net/ax25.h
> @@ -228,6 +228,7 @@ typedef struct ax25_dev {
> ax25_dama_info dama;
> #endif
> refcount_t refcount;
> + bool device_up;
> } ax25_dev;
>
> typedef struct ax25_cb {
> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> index 363d47f9453..289f355e185 100644
> --- a/net/ax25/af_ax25.c
> +++ b/net/ax25/af_ax25.c
> @@ -62,12 +62,12 @@ static void ax25_free_sock(struct sock *sk)
> */
> static void ax25_cb_del(ax25_cb *ax25)
> {
> + spin_lock_bh(&ax25_list_lock);
> if (!hlist_unhashed(&ax25->ax25_node)) {
> - spin_lock_bh(&ax25_list_lock);
> hlist_del_init(&ax25->ax25_node);
> - spin_unlock_bh(&ax25_list_lock);
> ax25_cb_put(ax25);
> }
> + spin_unlock_bh(&ax25_list_lock);
> }
>
> /*
> @@ -81,6 +81,7 @@ static void ax25_kill_by_device(struct net_device *dev)
>
> if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
> return;
> + ax25_dev->device_up = false;
>
> spin_lock_bh(&ax25_list_lock);
> again:
> @@ -91,6 +92,7 @@ static void ax25_kill_by_device(struct net_device *dev)
> spin_unlock_bh(&ax25_list_lock);
> ax25_disconnect(s, ENETUNREACH);
> s->ax25_dev = NULL;
> + ax25_cb_del(s);
> spin_lock_bh(&ax25_list_lock);
> goto again;
> }
> @@ -103,6 +105,7 @@ static void ax25_kill_by_device(struct net_device *dev)
> dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
> ax25_dev_put(ax25_dev);
> }
> + ax25_cb_del(s);
> release_sock(sk);
> spin_lock_bh(&ax25_list_lock);
> sock_put(sk);
> @@ -995,9 +998,11 @@ static int ax25_release(struct socket *sock)
> if (sk->sk_type == SOCK_SEQPACKET) {
> switch (ax25->state) {
> case AX25_STATE_0:
> - release_sock(sk);
> - ax25_disconnect(ax25, 0);
> - lock_sock(sk);
> + if (!sock_flag(ax25->sk, SOCK_DEAD)) {
> + release_sock(sk);
> + ax25_disconnect(ax25, 0);
> + lock_sock(sk);
> + }
> ax25_destroy_socket(ax25);
> break;
>
> @@ -1053,11 +1058,13 @@ static int ax25_release(struct socket *sock)
> ax25_destroy_socket(ax25);
> }
> if (ax25_dev) {
> - del_timer_sync(&ax25->timer);
> - del_timer_sync(&ax25->t1timer);
> - del_timer_sync(&ax25->t2timer);
> - del_timer_sync(&ax25->t3timer);
> - del_timer_sync(&ax25->idletimer);
> + if (!ax25_dev->device_up) {
> + del_timer_sync(&ax25->timer);
> + del_timer_sync(&ax25->t1timer);
> + del_timer_sync(&ax25->t2timer);
> + del_timer_sync(&ax25->t3timer);
> + del_timer_sync(&ax25->idletimer);
> + }
> dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
> ax25_dev_put(ax25_dev);
> }
> diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
> index d2a244e1c26..5451be15e07 100644
> --- a/net/ax25/ax25_dev.c
> +++ b/net/ax25/ax25_dev.c
> @@ -62,6 +62,7 @@ void ax25_dev_device_up(struct net_device *dev)
> ax25_dev->dev = dev;
> dev_hold_track(dev, &ax25_dev->dev_tracker, GFP_ATOMIC);
> ax25_dev->forward = NULL;
> + ax25_dev->device_up = true;
>
> ax25_dev->values[AX25_VALUES_IPDEFMODE] = AX25_DEF_IPDEFMODE;
> ax25_dev->values[AX25_VALUES_AXDEFMODE] = AX25_DEF_AXDEFMODE;
> diff --git a/net/ax25/ax25_subr.c b/net/ax25/ax25_subr.c
> index 3a476e4f6cd..9ff98f46dc6 100644
> --- a/net/ax25/ax25_subr.c
> +++ b/net/ax25/ax25_subr.c
> @@ -268,7 +268,7 @@ void ax25_disconnect(ax25_cb *ax25, int reason)
> del_timer_sync(&ax25->t3timer);
> del_timer_sync(&ax25->idletimer);
> } else {
> - if (!ax25->sk || !sock_flag(ax25->sk, SOCK_DESTROY))
> + if (ax25->sk && !sock_flag(ax25->sk, SOCK_DESTROY))
> ax25_stop_heartbeat(ax25);
> ax25_stop_t1timer(ax25);
> ax25_stop_t2timer(ax25);
Side note outside the scope of this patch: I think the ax25
implementation is prone to other races, as it looks like a bit of code
lives under the assumption that once acquired bh_lock_sock() it can
modify the socket status arbitrary. It can't: it will still race with a
different process context acquiring the plain socket lock.
As said, outside the scope of this patch.
Paolo
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