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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 <cpaasch@apple.com>
Diagnosed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Tested-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Florian Westphal 2020-02-04 18:12:30 +01:00 committed by David S. Miller
parent 8b7a07c7d4
commit 2c22c06ce4
1 changed files with 6 additions and 70 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

76
net/mptcp/protocol.c
View File

@ -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)