[PATCH] Add packet filtering based on process\'s security context.

Tetsuo Handa
2007-11-20T04:50:49Z

This patch allows LSM modules filter incoming connections/datagrams
based on the process's security context who is attempting to pick up.

There are already hooks to filter incoming connections/datagrams
based on the socket's security context, but these hooks are not
applicable when one wants to do TCP Wrapper-like filtering
(e.g. App1 is permitted to accept TCP connections from 192.168.0.0/16).

There is a side effect which unlikely occurs.

If a socket is shared by multiple processes with different policy,
the process who should be able to accept this connection
will not be able to accept this connection
because socket_post_accept() aborts this connection.
But if socket_post_accept() doesn't abort this connection,
the process who must not be able to accept this connection
will repeat accept() forever, which is a worse side effect.

Similarly, if a socket is shared by multiple processes with different policy,
the process who should be able to pick up this datagram
will not be able to pick up this datagram
because socket_post_recv_datagram() discards this datagram.
But if socket_post_recv_datagram() doesn't discard this datagram,
the process who must not be able to pick up this datagram
will repeat recvmsg() forever, which is a worse side effect.

Signed-off-by: Kentaro Takeda <takedakn@nttdata.co.jp>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

include/linux/security.h | 34 +++++++++++++++++++++++++++++-----
net/core/datagram.c | 26 ++++++++++++++++++++++++--
net/socket.c | 7 +++++--
security/dummy.c | 13 ++++++++++---
security/security.c | 10 ++++++++--
5 files changed, 76 insertions(+), 14 deletions(-)

--- linux-2.6.24-rc2-mm1.orig/include/linux/security.h
+++ linux-2.6.24-rc2-mm1/include/linux/security.h
@@ -778,8 +778,12 @@ struct request_sock;
* @socket_post_accept:
* This hook allows a security module to copy security
* information into the newly created socket's inode.
+ * This hook also allows a security module to filter connections
+ * from unwanted peers based on the process accepting this connection.
+ * The connection will be aborted if this hook returns nonzero.
* @sock contains the listening socket structure.
* @newsock contains the newly created server socket for connection.
+ * Return 0 if permission is granted.
* @socket_sendmsg:
* Check permission before transmitting a message to another socket.
* @sock contains the socket structure.
@@ -793,6 +797,15 @@ struct request_sock;
* @size contains the size of message structure.
* @flags contains the operational flags.
* Return 0 if permission is granted.
+ * @socket_post_recv_datagram:
+ * Check permission after receiving a datagram.
+ * This hook allows a security module to filter packets
+ * from unwanted peers based on the process receiving this datagram.
+ * The packet will be discarded if this hook returns nonzero.
+ * @sk contains the socket.
+ * @skb contains the socket buffer (may be NULL).
+ * @flags contains the operational flags.
+ * Return 0 if permission is granted.
* @socket_getsockname:
* Check permission before the local address (name) of the socket object
* @sock is retrieved.
@@ -1384,12 +1397,13 @@ struct security_operations {
struct sockaddr * address, int addrlen);
int (*socket_listen) (struct socket * sock, int backlog);
int (*socket_accept) (struct socket * sock, struct socket * newsock);
- void (*socket_post_accept) (struct socket * sock,
- struct socket * newsock);
+ int (*socket_post_accept) (struct socket *sock, struct socket *newsock);
int (*socket_sendmsg) (struct socket * sock,
struct msghdr * msg, int size);
int (*socket_recvmsg) (struct socket * sock,
struct msghdr * msg, int size, int flags);
+ int (*socket_post_recv_datagram) (struct sock *sk, struct sk_buff *skb,
+ unsigned int flags);
int (*socket_getsockname) (struct socket * sock);
int (*socket_getpeername) (struct socket * sock);
int (*socket_getsockopt) (struct socket * sock, int level, int optname);
@@ -2294,10 +2308,12 @@ int security_socket_bind(struct socket *
int security_socket_connect(struct socket *sock, struct sockaddr *address, int addrlen);
int security_socket_listen(struct socket *sock, int backlog);
int security_socket_accept(struct socket *sock, struct socket *newsock);
-void security_socket_post_accept(struct socket *sock, struct socket *newsock);
+int security_socket_post_accept(struct socket *sock, struct socket *newsock);
int security_socket_sendmsg(struct socket *sock, struct msghdr *msg, int size);
int security_socket_recvmsg(struct socket *sock, struct msghdr *msg,
int size, int flags);
+int security_socket_post_recv_datagram(struct sock *sk, struct sk_buff *skb,
+ unsigned int flags);
int security_socket_getsockname(struct socket *sock);
int security_socket_getpeername(struct socket *sock);
int security_socket_getsockopt(struct socket *sock, int level, int optname);
@@ -2373,9 +2389,10 @@ static inline int security_socket_accept
return 0;
}

-static inline void security_socket_post_accept(struct socket * sock,
- struct socket * newsock)
+static inline int security_socket_post_accept(struct socket *sock,
+ struct socket *newsock)
{
+ return 0;
}

static inline int security_socket_sendmsg(struct socket * sock,
@@ -2391,6 +2408,13 @@ static inline int security_socket_recvms
return 0;
}

+static inline int security_socket_post_recv_datagram(struct sock *sk,
+ struct sk_buff *skb,
+ unsigned int flags)
+{
+ return 0;
+}
+
static inline int security_socket_getsockname(struct socket * sock)
{
return 0;
--- linux-2.6.24-rc2-mm1.orig/net/socket.c
+++ linux-2.6.24-rc2-mm1/net/socket.c
@@ -1442,13 +1442,16 @@ asmlinkage long sys_accept(int fd, struc
goto out_fd;
}

+ /* Filter connections from unwanted peers. */
+ err = security_socket_post_accept(sock, newsock);
+ if (err)
+ goto out_fd;
+
/* File flags are not inherited via accept() unlike another OSes. */

fd_install(newfd, newfile);
err = newfd;

- security_socket_post_accept(sock, newsock);
-
out_put:
fput_light(sock->file, fput_needed);
out:
--- linux-2.6.24-rc2-mm1.orig/security/dummy.c
+++ linux-2.6.24-rc2-mm1/security/dummy.c
@@ -753,10 +753,10 @@ static int dummy_socket_accept (struct s
return 0;
}

-static void dummy_socket_post_accept (struct socket *sock,
- struct socket *newsock)
+static int dummy_socket_post_accept(struct socket *sock,
+ struct socket *newsock)
{
- return;
+ return 0;
}

static int dummy_socket_sendmsg (struct socket *sock, struct msghdr *msg,
@@ -771,6 +771,12 @@ static int dummy_socket_recvmsg (struct
return 0;
}

+static int dummy_socket_post_recv_datagram(struct sock *sk, struct sk_buff *skb,
+ unsigned int flags)
+{
+ return 0;
+}
+
static int dummy_socket_getsockname (struct socket *sock)
{
return 0;
@@ -1104,6 +1110,7 @@ void security_fixup_ops (struct security
set_to_dummy_if_null(ops, socket_post_accept);
set_to_dummy_if_null(ops, socket_sendmsg);
set_to_dummy_if_null(ops, socket_recvmsg);
+ set_to_dummy_if_null(ops, socket_post_recv_datagram);
set_to_dummy_if_null(ops, socket_getsockname);
set_to_dummy_if_null(ops, socket_getpeername);
set_to_dummy_if_null(ops, socket_setsockopt);
--- linux-2.6.24-rc2-mm1.orig/net/core/datagram.c
+++ linux-2.6.24-rc2-mm1/net/core/datagram.c
@@ -55,6 +55,7 @@
#include <net/checksum.h>
#include <net/sock.h>
#include <net/tcp_states.h>
+#include <linux/security.h>

/*
* Is a socket 'connection oriented' ?
@@ -148,6 +149,7 @@ struct sk_buff *skb_recv_datagram(struct
{
struct sk_buff *skb;
long timeo;
+ unsigned long cpu_flags;
/*
* Caller is allowed not to check sk->sk_err before skb_recv_datagram()
*/
@@ -166,8 +168,6 @@ struct sk_buff *skb_recv_datagram(struct
* However, this function was corrent in any case. 8)
*/
if (flags & MSG_PEEK) {
- unsigned long cpu_flags;
-
spin_lock_irqsave(&sk->sk_receive_queue.lock,
cpu_flags);
skb = skb_peek(&sk->sk_receive_queue);
@@ -178,6 +178,11 @@ struct sk_buff *skb_recv_datagram(struct
} else
skb = skb_dequeue(&sk->sk_receive_queue);

+ /* Filter packets from unwanted peers. */
+ error = security_socket_post_recv_datagram(sk, skb, flags);
+ if (error)
+ goto force_dequeue;
+
if (skb)
return skb;

@@ -190,6 +195,23 @@ struct sk_buff *skb_recv_datagram(struct

return NULL;

+force_dequeue:
+ /* Drop this packet because LSM says "Don't pass it to the caller". */
+ if (!(flags & MSG_PEEK))
+ goto no_peek;
+ /*
+ * If this packet is MSG_PEEK'ed, dequeue it forcibly
+ * so that this packet won't prevent the caller from picking up
+ * next packet.
+ */
+ spin_lock_irqsave(&sk->sk_receive_queue.lock, cpu_flags);
+ if (skb == skb_peek(&sk->sk_receive_queue)) {
+ __skb_unlink(skb, &sk->sk_receive_queue);
+ atomic_dec(&skb->users);
+ }
+ spin_unlock_irqrestore(&sk->sk_receive_queue.lock, cpu_flags);
+no_peek:
+ kfree_skb(skb);
no_packet:
*err = error;
return NULL;
--- linux-2.6.24-rc2-mm1.orig/security/security.c
+++ linux-2.6.24-rc2-mm1/security/security.c
@@ -869,9 +869,9 @@ int security_socket_accept(struct socket
return security_ops->socket_accept(sock, newsock);
}

-void security_socket_post_accept(struct socket *sock, struct socket *newsock)
+int security_socket_post_accept(struct socket *sock, struct socket *newsock)
{
- security_ops->socket_post_accept(sock, newsock);
+ return security_ops->socket_post_accept(sock, newsock);
}

int security_socket_sendmsg(struct socket *sock, struct msghdr *msg, int size)
@@ -885,6 +885,12 @@ int security_socket_recvmsg(struct socke
return security_ops->socket_recvmsg(sock, msg, size, flags);
}

+int security_socket_post_recv_datagram(struct sock *sk, struct sk_buff *skb,
+ unsigned int flags)
+{
+ return security_ops->socket_post_recv_datagram(sk, skb, flags);
+}
+
int security_socket_getsockname(struct socket *sock)
{
return security_ops->socket_getsockname(sock);