From: Paolo Abeni <pabeni@redhat.com>

commit 0226436acf2495cde4b93e7400e5a87305c26054 upstream.

Since the blamed commit, closing the first subflow resets the first
subflow socket state to SS_UNCONNECTED.

The current mptcp listen implementation relies only on such
state to prevent touching not-fully-disconnected sockets.

Incoming mptcp fastclose (or paired endpoint removal) unconditionally
closes the first subflow.

All the above allows an incoming fastclose followed by a listen() call
to successfully race with a blocking recvmsg(), potentially causing the
latter to hit a divide by zero bug in cleanup_rbuf/__tcp_select_window().

Address the issue explicitly checking the msk socket state in
mptcp_listen(). An alternative solution would be moving the first
subflow socket state update into mptcp_disconnect(), but in the long
term the first subflow socket should be removed: better avoid relaying
on it for internal consistency check.

Fixes: b29fcfb54cd7 ("mptcp: full disconnect implementation")
Cc: stable@vger.kernel.org
Reported-by: Christoph Paasch <cpaasch@apple.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/414
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
---
Backport notes:
  - Conflicting with a cleanup that has been done after v6.1, see commit
    cfdcfeed6449 ("mptcp: introduce 'sk' to replace 'sock->sk' in
    mptcp_listen()").
  - The conflict was in the context and the new lines didn't need to be
    adapted.
---
 net/mptcp/protocol.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index a6a5c16f2a49..b76cda4187bc 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3752,6 +3752,11 @@ static int mptcp_listen(struct socket *sock, int backlog)
 	pr_debug("msk=%p", msk);
 
 	lock_sock(sock->sk);
+
+	err = -EINVAL;
+	if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)
+		goto unlock;
+
 	ssock = __mptcp_nmpc_socket(msk);
 	if (!ssock) {
 		err = -EINVAL;
-- 
2.40.1

Hi Greg, Sasha,

On 27/07/2023 16:16, Matthieu Baerts wrote:
> From: Paolo Abeni <pabeni@redhat.com>
> 
> commit 0226436acf2495cde4b93e7400e5a87305c26054 upstream.
> 
> Since the blamed commit, closing the first subflow resets the first
> subflow socket state to SS_UNCONNECTED.
> 
> The current mptcp listen implementation relies only on such
> state to prevent touching not-fully-disconnected sockets.
> 
> Incoming mptcp fastclose (or paired endpoint removal) unconditionally
> closes the first subflow.
> 
> All the above allows an incoming fastclose followed by a listen() call
> to successfully race with a blocking recvmsg(), potentially causing the
> latter to hit a divide by zero bug in cleanup_rbuf/__tcp_select_window().
> 
> Address the issue explicitly checking the msk socket state in
> mptcp_listen(). An alternative solution would be moving the first
> subflow socket state update into mptcp_disconnect(), but in the long
> term the first subflow socket should be removed: better avoid relaying
> on it for internal consistency check.
> 
> Fixes: b29fcfb54cd7 ("mptcp: full disconnect implementation")
> Cc: stable@vger.kernel.org
> Reported-by: Christoph Paasch <cpaasch@apple.com>
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/414
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> ---
> Backport notes:
>   - Conflicting with a cleanup that has been done after v6.1, see commit
>     cfdcfeed6449 ("mptcp: introduce 'sk' to replace 'sock->sk' in
>     mptcp_listen()").

Sasha just backported this commit cfdcfeed6449 ("mptcp: introduce 'sk'
to replace 'sock->sk' in mptcp_listen()") so 0226436acf24 ("mptcp: do
not rely on implicit state check in mptcp_listen()") can be applied
without conflict. Thank you for that, good idea, it works for me!

Please drop this patch here then.

Cheers,
Matt
-- 
Tessares | Belgium | Hybrid Access Solutions
www.tessares.net