Send property_service AVC messages to the kernel audit system

The property service uses an SELinux userspace check to determine if a
process is allowed to set a property. If the security check fails, a
userspace SELinux denial is generated. Currently, these denials are only
sent to dmesg.

Instead of sending these denials to dmesg, send it to the kernel audit
system. This will cause these userspace denials to be treated similarly
to kernel generated denials (eg, logd will pick them up and process
them). This will ensure that denials generated by the property service
will show up in logcat / dmesg / event log.

After this patch, running "setprop asdf asdf" from the unprivileged adb
shell user will result in the following audit message:

  type=1107 audit(39582851.013:48): pid=1 uid=0 auid=4294967295
  ses=4294967295 subj=u:r:init:s0 msg='avc: denied { set } for
  property=asdf pid=5537 uid=2000 gid=2000 scontext=u:r:shell:s0
  tcontext=u:object_r:default_prop:s0 tclass=property_service'

Test: manual
Bug: 27878170
Change-Id: I0b8994888653501f2f315eaa63d9e2ba32d851ef

2 files changed, 32 insertions, 4 deletions

diff --git a/init/Android.mk b/init/Android.mk
index ecdf5db70..111fe895f 100644
--- a/init/Android.mk
+++ b/init/Android.mk
@@ -54,7 +54,7 @@ LOCAL_SRC_FILES:= \
     service.cpp \
     util.cpp \
 
-LOCAL_STATIC_LIBRARIES := libbase libselinux liblog libprocessgroup
+LOCAL_STATIC_LIBRARIES := libbase libselinux liblog libprocessgroup libnl
 LOCAL_WHOLE_STATIC_LIBRARIES := libcap
 LOCAL_MODULE := libinit
 LOCAL_SANITIZE := integer
@@ -103,7 +103,8 @@ LOCAL_STATIC_LIBRARIES := \
     libdl \
     libsparse_static \
     libz \
-    libprocessgroup
+    libprocessgroup \
+    libnl \
 
 # Create symlinks.
 LOCAL_POST_INSTALL_CMD := $(hide) mkdir -p $(TARGET_ROOT_OUT)/sbin; \
diff --git a/init/log.cpp b/init/log.cpp
index 861834011..6b32526df 100644
--- a/init/log.cpp
+++ b/init/log.cpp
@@ -19,6 +19,8 @@
 #include <fcntl.h>
 #include <string.h>
 
+#include <linux/audit.h>
+#include <netlink/netlink.h>
 #include <selinux/selinux.h>
 
 void InitKernelLogging(char* argv[]) {
@@ -38,6 +40,24 @@ void InitKernelLogging(char* argv[]) {
     android::base::InitLogging(argv, &android::base::KernelLogger);
 }
 
+static void selinux_avc_log(char* buf, size_t buf_len) {
+    size_t str_len = strnlen(buf, buf_len);
+
+    // trim newline at end of string
+    buf[str_len - 1] = '\0';
+
+    struct nl_sock* sk = nl_socket_alloc();
+    if (sk == NULL) {
+        return;
+    }
+    nl_connect(sk, NETLINK_AUDIT);
+    int result;
+    do {
+        result = nl_send_simple(sk, AUDIT_USER_AVC, 0, buf, str_len);
+    } while (result == -NLE_INTR);
+    nl_socket_free(sk);
+}
+
 int selinux_klog_callback(int type, const char *fmt, ...) {
     android::base::LogSeverity severity = android::base::ERROR;
     if (type == SELINUX_WARNING) {
@@ -48,8 +68,15 @@ int selinux_klog_callback(int type, const char *fmt, ...) {
     char buf[1024];
     va_list ap;
     va_start(ap, fmt);
-    vsnprintf(buf, sizeof(buf), fmt, ap);
+    int res = vsnprintf(buf, sizeof(buf), fmt, ap);
     va_end(ap);
-    android::base::KernelLogger(android::base::MAIN, severity, "selinux", nullptr, 0, buf);
+    if (res <= 0) {
+        return 0;
+    }
+    if (type == SELINUX_AVC) {
+        selinux_avc_log(buf, sizeof(buf));
+    } else {
+        android::base::KernelLogger(android::base::MAIN, severity, "selinux", nullptr, 0, buf);
+    }
     return 0;
 }