Re: [PATCH] linux 2.9.10-rc1: Fix oops in unix_dgram_sendmsg when using SELinux and SOCK_SEQPACKET

[Chris Wright]

* James Morris (jmorris@xxxxxxxxxx) wrote:
> What's happening is that mixing stream and dgram ops for SEQPACKET is
> having some unfortunate side effects.

Agreed.

> One of these is that there is a race between client sendmsg() and server
> accept().  The server child socket is attached via sock_graft() after the 
> client has entered unix_dgram_sendmsg() and called 
> 
> 	security_unix_may_send(sk->sk_socket, other->sk_socket);
> 
> other->sk_socket will thus be null, causing the oops in SELinux and any 
> other LSM which tries to dereference the pointer.

Yup.  And it's not much of a race, the window is wide open.  One
malicious app simply has to do:

bind()
listen()
connect()
send() <-- Oops

> The fix is a combination of some of Ross's ideas:
> 
> 1) SOCK_SEQPACKET is connection oriented, and there no need to call 
> security_unix_may_send() for each packet.  security_unix_stream_connect() 
> is sufficient.

Why not make a unix_seq_sendmsg, which is a very small wrapper?
e.g.
static int unix_seq_sendmsg(struct kiocb *kiocb, struct socket *sock,
			    struct msghdr *msg, size_t len)
{
	struct sock *sk = sock->sk;

	if (sk->sk_type == SOCK_SEQPACKET && sk->sk_state != TCP_ESTABLISHED)
		return -ENOTCONN;
	if (msg->msg_name || msg->msg_namelen)
		return -EINVAL;
	return unix_dgram_sendmsg(kiocb, sock, msg, len);
}


Also, I missed how MSG_EOR is honored.

> 2) Ensure that unix_dgram_sendmsg() fails for SOCK_SEQPACKET sockets which
> are not connected, otherwise someone could bypass LSM by sending on an
> unconnected socket.

Agreed, not connected, it should fail IMHO.

> Note that this only solves the problem for the LSM hook.

Does the above stop the other issue?  My laptop died, so I'm not able to
test ATM.

thanks,
-chris
-- 
Linux Security Modules     http://lsm.immunix.org     http://lsm.bkbits.net
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