From 2c22c06ce426a0d025a3dacd17cd8868f3dbe96b Mon Sep 17 00:00:00 2001 From: Florian Westphal Date: Tue, 4 Feb 2020 18:12:30 +0100 Subject: [PATCH] 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: Christoph Paasch Diagnosed-by: Paolo Abeni Signed-off-by: Florian Westphal Reviewed-by: Mat Martineau Tested-by: Christoph Paasch Signed-off-by: David S. Miller --- net/mptcp/protocol.c | 76 ++++---------------------------------------- 1 file changed, 6 insertions(+), 70 deletions(-) diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c index 3bccee455688..353f2d16b986 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -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) -- 2.30.2