3.16.61-rc1 review patch. If anyone has any objections, please let me know.
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From: Guillaume Nault g.nault@alphalink.fr
commit 3d609342cc04129ff7568e19316ce3d7451a27e8 upstream.
Commit d02ba2a6110c ("l2tp: fix race in pppol2tp_release with session object destroy") tried to fix a race condition where a PPPoL2TP socket would disappear while the L2TP session was still using it. However, it missed the root issue which is that an L2TP session may accept to be reconnected if its associated socket has entered the release process.
The tentative fix makes the session hold the socket it is connected to. That saves the kernel from crashing, but introduces refcount leakage, preventing the socket from completing the release process. Once stalled, everything the socket depends on can't be released anymore, including the L2TP session and the l2tp_ppp module.
The root issue is that, when releasing a connected PPPoL2TP socket, the session's ->sk pointer (RCU-protected) is reset to NULL and we have to wait for a grace period before destroying the socket. The socket drops the session in its ->sk_destruct callback function, so the session will exist until the last reference on the socket is dropped. Therefore, there is a time frame where pppol2tp_connect() may accept reconnecting a session, as it only checks ->sk to figure out if the session is connected. This time frame is shortened by the fact that pppol2tp_release() calls l2tp_session_delete(), making the session unreachable before resetting ->sk. However, pppol2tp_connect() may grab the session before it gets unhashed by l2tp_session_delete(), but it may test ->sk after the later got reset. The race is not so hard to trigger and syzbot found a pretty reliable reproducer: https://syzkaller.appspot.com/bug?id=418578d2a4389074524e04d641eacb091961b2c...
Before d02ba2a6110c, another race could let pppol2tp_release() overwrite the ->__sk pointer of an L2TP session, thus tricking pppol2tp_put_sk() into calling sock_put() on a socket that is different than the one for which pppol2tp_release() was originally called. To get there, we had to trigger the race described above, therefore having one PPPoL2TP socket being released, while the session it is connected to is reconnecting to a different PPPoL2TP socket. When releasing this new socket fast enough, pppol2tp_release() overwrites the session's ->__sk pointer with the address of the new socket, before the first pppol2tp_put_sk() call gets scheduled. Then the pppol2tp_put_sk() call invoked by the original socket will sock_put() the new socket, potentially dropping its last reference. When the second pppol2tp_put_sk() finally runs, its socket has already been freed.
With d02ba2a6110c, the session takes a reference on both sockets. Furthermore, the session's ->sk pointer is reset in the pppol2tp_session_close() callback function rather than in pppol2tp_release(). Therefore, ->__sk can't be overwritten and pppol2tp_put_sk() is called only once (l2tp_session_delete() will only run pppol2tp_session_close() once, to protect the session against concurrent deletion requests). Now pppol2tp_put_sk() will properly sock_put() the original socket, but the new socket will remain, as l2tp_session_delete() prevented the release process from completing. Here, we don't depend on the ->__sk race to trigger the bug. Getting into the pppol2tp_connect() race is enough to leak the reference, no matter when new socket is released.
So it all boils down to pppol2tp_connect() failing to realise that the session has already been connected. This patch drops the unneeded extra reference counting (mostly reverting d02ba2a6110c) and checks that neither ->sk nor ->__sk is set before allowing a session to be connected.
Fixes: d02ba2a6110c ("l2tp: fix race in pppol2tp_release with session object destroy") Signed-off-by: Guillaume Nault g.nault@alphalink.fr Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Ben Hutchings ben@decadent.org.uk --- net/l2tp/l2tp_ppp.c | 35 +++++++++++++++++------------------ 1 file changed, 17 insertions(+), 18 deletions(-)
--- a/net/l2tp/l2tp_ppp.c +++ b/net/l2tp/l2tp_ppp.c @@ -449,16 +449,6 @@ static void pppol2tp_put_sk(struct rcu_h */ static void pppol2tp_session_close(struct l2tp_session *session) { - struct pppol2tp_session *ps; - - ps = l2tp_session_priv(session); - mutex_lock(&ps->sk_lock); - ps->__sk = rcu_dereference_protected(ps->sk, - lockdep_is_held(&ps->sk_lock)); - RCU_INIT_POINTER(ps->sk, NULL); - if (ps->__sk) - call_rcu(&ps->rcu, pppol2tp_put_sk); - mutex_unlock(&ps->sk_lock); }
/* Really kill the session socket. (Called from sock_put() if @@ -501,15 +491,24 @@ static int pppol2tp_release(struct socke sock_orphan(sk); sock->sk = NULL;
- /* If the socket is associated with a session, - * l2tp_session_delete will call pppol2tp_session_close which - * will drop the session's ref on the socket. - */ session = pppol2tp_sock_to_session(sk); if (session) { + struct pppol2tp_session *ps; + l2tp_session_delete(session); - /* drop the ref obtained by pppol2tp_sock_to_session */ - sock_put(sk); + + ps = l2tp_session_priv(session); + mutex_lock(&ps->sk_lock); + ps->__sk = rcu_dereference_protected(ps->sk, + lockdep_is_held(&ps->sk_lock)); + RCU_INIT_POINTER(ps->sk, NULL); + mutex_unlock(&ps->sk_lock); + call_rcu(&ps->rcu, pppol2tp_put_sk); + + /* Rely on the sock_put() call at the end of the function for + * dropping the reference held by pppol2tp_sock_to_session(). + * The last reference will be dropped by pppol2tp_put_sk(). + */ }
release_sock(sk); @@ -764,7 +763,8 @@ static int pppol2tp_connect(struct socke */ mutex_lock(&ps->sk_lock); if (rcu_dereference_protected(ps->sk, - lockdep_is_held(&ps->sk_lock))) { + lockdep_is_held(&ps->sk_lock)) || + ps->__sk) { mutex_unlock(&ps->sk_lock); error = -EEXIST; goto end; @@ -832,7 +832,6 @@ static int pppol2tp_connect(struct socke
out_no_ppp: /* This is how we get the session context from the socket. */ - sock_hold(sk); sk->sk_user_data = session; rcu_assign_pointer(ps->sk, sk); mutex_unlock(&ps->sk_lock);