Fold uevent message origin checking from init into libcutils.

Change-Id: I7cbc0cb840d1b4962e6c360fdad9a9c0ce20e87b

1 files changed, 70 insertions, 0 deletions

diff --git a/libcutils/uevent.c b/libcutils/uevent.c
new file mode 100644
index 000000000..3533c00f6
--- /dev/null
+++ b/libcutils/uevent.c
@@ -0,0 +1,70 @@
+/*
+ * Copyright (C) 2011 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <cutils/uevent.h>
+
+#include <errno.h>
+#include <strings.h>
+
+#include <linux/netlink.h>
+
+/**
+ * Like recv(), but checks that messages actually originate from the kernel.
+ */
+ssize_t uevent_checked_recv(int socket, void *buffer, size_t length) {
+    struct iovec iov = { buffer, length };
+    struct sockaddr_nl addr;
+    char control[CMSG_SPACE(sizeof(struct ucred))];
+    struct msghdr hdr = {
+        &addr,
+        sizeof(addr),
+        &iov,
+        1,
+        control,
+        sizeof(control),
+        0,
+    };
+
+    ssize_t n = recvmsg(socket, &hdr, 0);
+    if (n <= 0) {
+        return n;
+    }
+
+    if (addr.nl_groups == 0 || addr.nl_pid != 0) {
+        /* ignoring non-kernel or unicast netlink message */
+        goto out;
+    }
+
+    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr);
+    if (cmsg == NULL || cmsg->cmsg_type != SCM_CREDENTIALS) {
+        /* ignoring netlink message with no sender credentials */
+        goto out;
+    }
+
+    struct ucred *cred = (struct ucred *)CMSG_DATA(cmsg);
+    if (cred->uid != 0) {
+        /* ignoring netlink message from non-root user */
+        goto out;
+    }
+
+    return n;
+
+out:
+    /* clear residual potentially malicious data */
+    bzero(buffer, length);
+    errno = EIO;
+    return -1;
+}