• Catherine Zhang's avatar
    [SECURITY]: TCP/UDP getpeersec · 2c7946a7
    Catherine Zhang authored
    This patch implements an application of the LSM-IPSec networking
    controls whereby an application can determine the label of the
    security association its TCP or UDP sockets are currently connected to
    via getsockopt and the auxiliary data mechanism of recvmsg.
    
    Patch purpose:
    
    This patch enables a security-aware application to retrieve the
    security context of an IPSec security association a particular TCP or
    UDP socket is using.  The application can then use this security
    context to determine the security context for processing on behalf of
    the peer at the other end of this connection.  In the case of UDP, the
    security context is for each individual packet.  An example
    application is the inetd daemon, which could be modified to start
    daemons running at security contexts dependent on the remote client.
    
    Patch design approach:
    
    - Design for TCP
    The patch enables the SELinux LSM to set the peer security context for
    a socket based on the security context of the IPSec security
    association.  The application may retrieve this context using
    getsockopt.  When called, the kernel determines if the socket is a
    connected (TCP_ESTABLISHED) TCP socket and, if so, uses the dst_entry
    cache on the socket to retrieve the security associations.  If a
    security association has a security context, the context string is
    returned, as for UNIX domain sockets.
    
    - Design for UDP
    Unlike TCP, UDP is connectionless.  This requires a somewhat different
    API to retrieve the peer security context.  With TCP, the peer
    security context stays the same throughout the connection, thus it can
    be retrieved at any time between when the connection is established
    and when it is torn down.  With UDP, each read/write can have
    different peer and thus the security context might change every time.
    As a result the security context retrieval must be done TOGETHER with
    the packet retrieval.
    
    The solution is to build upon the existing Unix domain socket API for
    retrieving user credentials.  Linux offers the API for obtaining user
    credentials via ancillary messages (i.e., out of band/control messages
    that are bundled together with a normal message).
    
    Patch implementation details:
    
    - Implementation for TCP
    The security context can be retrieved by applications using getsockopt
    with the existing SO_PEERSEC flag.  As an example (ignoring error
    checking):
    
    getsockopt(sockfd, SOL_SOCKET, SO_PEERSEC, optbuf, &optlen);
    printf("Socket peer context is: %s\n", optbuf);
    
    The SELinux function, selinux_socket_getpeersec, is extended to check
    for labeled security associations for connected (TCP_ESTABLISHED ==
    sk->sk_state) TCP sockets only.  If so, the socket has a dst_cache of
    struct dst_entry values that may refer to security associations.  If
    these have security associations with security contexts, the security
    context is returned.
    
    getsockopt returns a buffer that contains a security context string or
    the buffer is unmodified.
    
    - Implementation for UDP
    To retrieve the security context, the application first indicates to
    the kernel such desire by setting the IP_PASSSEC option via
    getsockopt.  Then the application retrieves the security context using
    the auxiliary data mechanism.
    
    An example server application for UDP should look like this:
    
    toggle = 1;
    toggle_len = sizeof(toggle);
    
    setsockopt(sockfd, SOL_IP, IP_PASSSEC, &toggle, &toggle_len);
    recvmsg(sockfd, &msg_hdr, 0);
    if (msg_hdr.msg_controllen > sizeof(struct cmsghdr)) {
        cmsg_hdr = CMSG_FIRSTHDR(&msg_hdr);
        if (cmsg_hdr->cmsg_len <= CMSG_LEN(sizeof(scontext)) &&
            cmsg_hdr->cmsg_level == SOL_IP &&
            cmsg_hdr->cmsg_type == SCM_SECURITY) {
            memcpy(&scontext, CMSG_DATA(cmsg_hdr), sizeof(scontext));
        }
    }
    
    ip_setsockopt is enhanced with a new socket option IP_PASSSEC to allow
    a server socket to receive security context of the peer.  A new
    ancillary message type SCM_SECURITY.
    
    When the packet is received we get the security context from the
    sec_path pointer which is contained in the sk_buff, and copy it to the
    ancillary message space.  An additional LSM hook,
    selinux_socket_getpeersec_udp, is defined to retrieve the security
    context from the SELinux space.  The existing function,
    selinux_socket_getpeersec does not suit our purpose, because the
    security context is copied directly to user space, rather than to
    kernel space.
    
    Testing:
    
    We have tested the patch by setting up TCP and UDP connections between
    applications on two machines using the IPSec policies that result in
    labeled security associations being built.  For TCP, we can then
    extract the peer security context using getsockopt on either end.  For
    UDP, the receiving end can retrieve the security context using the
    auxiliary data mechanism of recvmsg.
    Signed-off-by: default avatarCatherine Zhang <cxzhang@watson.ibm.com>
    Acked-by: default avatarJames Morris <jmorris@namei.org>
    Acked-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    2c7946a7
sock.c 41.8 KB