Validate sender credentials on netlink msg receive

Verify that netlink messages are actually from the kernel,
and not from a userspace program.

Change-Id: I709c0efe9ba0258f6d79ebcde531d7f7bbe780b2

libsysutils/src/NetlinkListener.cpp

@@ -17,6 +17,7 @@
 
 #include <sys/types.h>
 #include <sys/socket.h>
+#include <linux/netlink.h>
 #include <string.h>
 
 #define LOG_TAG "NetlinkListener"
@@ -32,10 +33,32 @@ NetlinkListener::NetlinkListener(int socket) :
 bool NetlinkListener::onDataAvailable(SocketClient *cli)
 {
     int socket = cli->getSocket();
-    int count;
+    ssize_t count;
+    char cred_msg[CMSG_SPACE(sizeof(struct ucred))];
+    struct sockaddr_nl snl;
+    struct iovec iov = {mBuffer, sizeof(mBuffer)};
+    struct msghdr hdr = {&snl, sizeof(snl), &iov, 1, cred_msg, sizeof(cred_msg), 0};
 
-    if ((count = recv(socket, mBuffer, sizeof(mBuffer), 0)) < 0) {
-        SLOGE("recv failed (%s)", strerror(errno));
+    if ((count = recvmsg(socket, &hdr, 0)) < 0) {
+        SLOGE("recvmsg failed (%s)", strerror(errno));
+        return false;
+    }
+
+    if ((snl.nl_groups != 1) || (snl.nl_pid != 0)) {
+        SLOGE("ignoring non-kernel netlink multicast message");
+        return false;
+    }
+
+    struct cmsghdr * cmsg = CMSG_FIRSTHDR(&hdr);
+
+    if (cmsg == NULL || cmsg->cmsg_type != SCM_CREDENTIALS) {
+        SLOGE("ignoring message with no sender credentials");
+        return false;
+    }
+
+    struct ucred * cred = (struct ucred *)CMSG_DATA(cmsg);
+    if (cred->uid != 0) {
+        SLOGE("ignoring message from non-root UID %d", cred->uid);
         return false;
     }