selinux: handle TCP SYN-ACK packets correctly in selinux_ip_postroute()

In selinux_ip_postroute() we perform access checks based on the
packet's security label.  For locally generated traffic we get the
packet's security label from the associated socket; this works in all
cases except for TCP SYN-ACK packets.  In the case of SYN-ACK packet's
the correct security label is stored in the connection's request_sock,
not the server's socket.  Unfortunately, at the point in time when
selinux_ip_postroute() is called we can't query the request_sock
directly, we need to recreate the label using the same logic that
originally labeled the associated request_sock.

See the inline comments for more explanation.

Reported-by: Janak Desai <Janak.Desai@gtri.gatech.edu>
Tested-by: Janak Desai <Janak.Desai@gtri.gatech.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Paul Moore <pmoore@redhat.com>

1 files changed, 53 insertions, 15 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 877bab748c87..cc076a9b0344 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3847,6 +3847,30 @@ static int selinux_skb_peerlbl_sid(struct sk_buff *skb, u16 family, u32 *sid)
         return 0;
 }
 
+/**
+ * selinux_conn_sid - Determine the child socket label for a connection
+ * @sk_sid: the parent socket's SID
+ * @skb_sid: the packet's SID
+ * @conn_sid: the resulting connection SID
+ *
+ * If @skb_sid is valid then the user:role:type information from @sk_sid is
+ * combined with the MLS information from @skb_sid in order to create
+ * @conn_sid.  If @skb_sid is not valid then then @conn_sid is simply a copy
+ * of @sk_sid.  Returns zero on success, negative values on failure.
+ *
+ */
+static int selinux_conn_sid(u32 sk_sid, u32 skb_sid, u32 *conn_sid)
+{
+        int err = 0;
+
+        if (skb_sid != SECSID_NULL)
+                err = security_sid_mls_copy(sk_sid, skb_sid, conn_sid);
+        else
+                *conn_sid = sk_sid;
+
+        return err;
+}
+
 /* socket security operations */
 
 static int socket_sockcreate_sid(const struct task_security_struct *tsec,
@@ -4453,7 +4477,7 @@ static int selinux_inet_conn_request(struct sock *sk, struct sk_buff *skb,
         struct sk_security_struct *sksec = sk->sk_security;
         int err;
         u16 family = sk->sk_family;
-        u32 newsid;
+        u32 connsid;
         u32 peersid;
 
         /* handle mapped IPv4 packets arriving via IPv6 sockets */
@@ -4463,16 +4487,11 @@ static int selinux_inet_conn_request(struct sock *sk, struct sk_buff *skb,
         err = selinux_skb_peerlbl_sid(skb, family, &peersid);
         if (err)
                 return err;
-        if (peersid == SECSID_NULL) {
-                req->secid = sksec->sid;
-                req->peer_secid = SECSID_NULL;
-        } else {
-                err = security_sid_mls_copy(sksec->sid, peersid, &newsid);
-                if (err)
-                        return err;
-                req->secid = newsid;
-                req->peer_secid = peersid;
-        }
+        err = selinux_conn_sid(sksec->sid, peersid, &connsid);
+        if (err)
+                return err;
+        req->secid = connsid;
+        req->peer_secid = peersid;
 
         return selinux_netlbl_inet_conn_request(req, family);
 }
@@ -4846,12 +4865,12 @@ static unsigned int selinux_ip_postroute(struct sk_buff *skb, int ifindex,
         if (!secmark_active && !peerlbl_active)
                 return NF_ACCEPT;
 
-        /* if the packet is being forwarded then get the peer label from the
-         * packet itself; otherwise check to see if it is from a local
-         * application or the kernel, if from an application get the peer label
-         * from the sending socket, otherwise use the kernel's sid */
         sk = skb->sk;
         if (sk == NULL) {
+                /* Without an associated socket the packet is either coming
+                 * from the kernel or it is being forwarded; check the packet
+                 * to determine which and if the packet is being forwarded
+                 * query the packet directly to determine the security label. */
                 if (skb->skb_iif) {
                         secmark_perm = PACKET__FORWARD_OUT;
                         if (selinux_skb_peerlbl_sid(skb, family, &peer_sid))
@@ -4860,7 +4879,26 @@ static unsigned int selinux_ip_postroute(struct sk_buff *skb, int ifindex,
                         secmark_perm = PACKET__SEND;
                         peer_sid = SECINITSID_KERNEL;
                 }
+        } else if (sk->sk_state == TCP_LISTEN) {
+                /* Locally generated packet but the associated socket is in the
+                 * listening state which means this is a SYN-ACK packet.  In
+                 * this particular case the correct security label is assigned
+                 * to the connection/request_sock but unfortunately we can't
+                 * query the request_sock as it isn't queued on the parent
+                 * socket until after the SYN-ACK packet is sent; the only
+                 * viable choice is to regenerate the label like we do in
+                 * selinux_inet_conn_request().  See also selinux_ip_output()
+                 * for similar problems. */
+                u32 skb_sid;
+                struct sk_security_struct *sksec = sk->sk_security;
+                if (selinux_skb_peerlbl_sid(skb, family, &skb_sid))
+                        return NF_DROP;
+                if (selinux_conn_sid(sksec->sid, skb_sid, &peer_sid))
+                        return NF_DROP;
+                secmark_perm = PACKET__SEND;
         } else {
+                /* Locally generated packet, fetch the security label from the
+                 * associated socket. */
                 struct sk_security_struct *sksec = sk->sk_security;
                 peer_sid = sksec->sid;
                 secmark_perm = PACKET__SEND;