Commit 2c22c06c authored by Florian Westphal's avatar Florian Westphal Committed by David S. Miller
Browse files

mptcp: fix use-after-free on tcp fallback 



When an mptcp socket connects to a tcp peer or when a middlebox interferes
with tcp options, mptcp needs to fall back to plain tcp.
Problem is that mptcp is trying to be too clever in this case:

It attempts to close the mptcp meta sk and transparently replace it with
the (only) subflow tcp sk.

Unfortunately, this is racy -- the socket is already exposed to userspace.
Any parallel calls to send/recv/setsockopt etc. can cause use-after-free:

BUG: KASAN: use-after-free in atomic_try_cmpxchg include/asm-generic/atomic-instrumented.h:693 [inline]
CPU: 1 PID: 2083 Comm: syz-executor.1 Not tainted 5.5.0 #2
 atomic_try_cmpxchg include/asm-generic/atomic-instrumented.h:693 [inline]
 queued_spin_lock include/asm-generic/qspinlock.h:78 [inline]
 do_raw_spin_lock include/linux/spinlock.h:181 [inline]
 __raw_spin_lock_bh include/linux/spinlock_api_smp.h:136 [inline]
 _raw_spin_lock_bh+0x71/0xd0 kernel/locking/spinlock.c:175
 spin_lock_bh include/linux/spinlock.h:343 [inline]
 __lock_sock+0x105/0x190 net/core/sock.c:2414
 lock_sock_nested+0x10f/0x140 net/core/sock.c:2938
 lock_sock include/net/sock.h:1516 [inline]
 mptcp_setsockopt+0x2f/0x1f0 net/mptcp/protocol.c:800
 __sys_setsockopt+0x152/0x240 net/socket.c:2130
 __do_sys_setsockopt net/socket.c:2146 [inline]
 __se_sys_setsockopt net/socket.c:2143 [inline]
 __x64_sys_setsockopt+0xba/0x150 net/socket.c:2143
 do_syscall_64+0xb7/0x3d0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

While the use-after-free can be resolved, there is another problem:
sock->ops and sock->sk assignments are not atomic, i.e. we may get calls
into mptcp functions with sock->sk already pointing at the subflow socket,
or calls into tcp functions with a mptcp meta sk.

Remove the fallback code and call the relevant functions for the (only)
subflow in case the mptcp socket is connected to tcp peer.

Reported-by: default avatarChristoph Paasch <cpaasch@apple.com>
Diagnosed-by: default avatarPaolo Abeni <pabeni@redhat.com>
Signed-off-by: default avatarFlorian Westphal <fw@strlen.de>
Reviewed-by: default avatarMat Martineau <mathew.j.martineau@linux.intel.com>
Tested-by: default avatarChristoph Paasch <cpaasch@apple.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 8b7a07c7

net/mptcp/protocol.c

+6 −70
Original line number Diff line number Diff line
@@ -24,58 +24,6 @@ 


#define MPTCP_SAME_STATE TCP_MAX_STATES



static void __mptcp_close(struct sock *sk, long timeout);



static const struct proto_ops *tcp_proto_ops(struct sock *sk)

{

#if IS_ENABLED(CONFIG_MPTCP_IPV6)

	if (sk->sk_family == AF_INET6)

		return &inet6_stream_ops;

#endif

	return &inet_stream_ops;

}



/* MP_CAPABLE handshake failed, convert msk to plain tcp, replacing

 * socket->sk and stream ops and destroying msk

 * return the msk socket, as we can't access msk anymore after this function

 * completes

 * Called with msk lock held, releases such lock before returning

 */

static struct socket *__mptcp_fallback_to_tcp(struct mptcp_sock *msk,

					      struct sock *ssk)

{

	struct mptcp_subflow_context *subflow;

	struct socket *sock;

	struct sock *sk;



	sk = (struct sock *)msk;

	sock = sk->sk_socket;

	subflow = mptcp_subflow_ctx(ssk);



	/* detach the msk socket */

	list_del_init(&subflow->node);

	sock_orphan(sk);

	sock->sk = NULL;



	/* socket is now TCP */

	lock_sock(ssk);

	sock_graft(ssk, sock);

	if (subflow->conn) {

		/* We can't release the ULP data on a live socket,

		 * restore the tcp callback

		 */

		mptcp_subflow_tcp_fallback(ssk, subflow);

		sock_put(subflow->conn);

		subflow->conn = NULL;

	}

	release_sock(ssk);

	sock->ops = tcp_proto_ops(ssk);



	/* destroy the left-over msk sock */

	__mptcp_close(sk, 0);

	return sock;

}



/* If msk has an initial subflow socket, and the MP_CAPABLE handshake has not

 * completed yet or has failed, return the subflow socket.

 * Otherwise return NULL.
@@ -93,10 +41,6 @@ static bool __mptcp_needs_tcp_fallback(const struct mptcp_sock *msk) 
	return msk->first && !sk_is_mptcp(msk->first);

}



/* if the mp_capable handshake has failed, it fallbacks msk to plain TCP,

 * releases the socket lock and returns a reference to the now TCP socket.

 * Otherwise returns NULL

 */

static struct socket *__mptcp_tcp_fallback(struct mptcp_sock *msk)

{

	sock_owned_by_me((const struct sock *)msk);
@@ -105,15 +49,11 @@ static struct socket *__mptcp_tcp_fallback(struct mptcp_sock *msk) 
		return NULL;



	if (msk->subflow) {

		/* the first subflow is an active connection, discart the

		 * paired socket

		 */

		msk->subflow->sk = NULL;

		sock_release(msk->subflow);

		msk->subflow = NULL;

		release_sock((struct sock *)msk);

		return msk->subflow;

	}



	return __mptcp_fallback_to_tcp(msk, msk->first);

	return NULL;

}



static bool __mptcp_can_create_subflow(const struct mptcp_sock *msk)
@@ -640,12 +580,14 @@ static void mptcp_subflow_shutdown(struct sock *ssk, int how) 
}



/* Called with msk lock held, releases such lock before returning */

static void __mptcp_close(struct sock *sk, long timeout)

static void mptcp_close(struct sock *sk, long timeout)

{

	struct mptcp_subflow_context *subflow, *tmp;

	struct mptcp_sock *msk = mptcp_sk(sk);

	LIST_HEAD(conn_list);



	lock_sock(sk);



	mptcp_token_destroy(msk->token);

	inet_sk_state_store(sk, TCP_CLOSE);
@@ -662,12 +604,6 @@ static void __mptcp_close(struct sock *sk, long timeout) 
	sk_common_release(sk);

}



static void mptcp_close(struct sock *sk, long timeout)

{

	lock_sock(sk);

	__mptcp_close(sk, timeout);

}



static void mptcp_copy_inaddrs(struct sock *msk, const struct sock *ssk)

{

#if IS_ENABLED(CONFIG_MPTCP_IPV6)

CRA Git | Maintained and supported by SUSTech CRA and CCSE