From e9a525829a354c92983a35455ccab16d1b0d3892 Mon Sep 17 00:00:00 2001
From: Narayan Kamath <narayan@google.com>
Date: Mon, 7 Nov 2016 16:22:48 +0000
Subject: [PATCH] Zygote : Block SIGCHLD during fork.

We close the android logging related sockets prior as late as possible
before every fork to avoid having to whitelist them. If one of the
zygote's children dies after this point (but prior to the fork), we can
end up reopening the logging sockets from the SIGCHLD signal handler.

To prevent this from happening, block SIGCHLD during this critical
section.

Bug: 32693692
Test: Manual

Change-Id: Ia4763a0bfd821a25b68aba3939cb0a6cbf248de0
---
 core/jni/com_android_internal_os_Zygote.cpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/core/jni/com_android_internal_os_Zygote.cpp b/core/jni/com_android_internal_os_Zygote.cpp
index af117d18bf458..bba3d76492260 100644
--- a/core/jni/com_android_internal_os_Zygote.cpp
+++ b/core/jni/com_android_internal_os_Zygote.cpp
@@ -449,6 +449,20 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
                                      jstring instructionSet, jstring dataDir) {
   SetSigChldHandler();
 
+  sigset_t sigchld;
+  sigemptyset(&sigchld);
+  sigaddset(&sigchld, SIGCHLD);
+
+  // Temporarily block SIGCHLD during forks. The SIGCHLD handler might
+  // log, which would result in the logging FDs we close being reopened.
+  // This would cause failures because the FDs are not whitelisted.
+  //
+  // Note that the zygote process is single threaded at this point.
+  if (sigprocmask(SIG_BLOCK, &sigchld, nullptr) == -1) {
+    ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+    RuntimeAbort(env, __LINE__, "Call to sigprocmask(SIG_BLOCK, { SIGCHLD }) failed.");
+  }
+
   // Close any logging related FDs before we start evaluating the list of
   // file descriptors.
   __android_log_close();
@@ -482,6 +496,11 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
       RuntimeAbort(env, __LINE__, "Unable to reopen whitelisted descriptors.");
     }
 
+    if (sigprocmask(SIG_UNBLOCK, &sigchld, nullptr) == -1) {
+      ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+      RuntimeAbort(env, __LINE__, "Call to sigprocmask(SIG_UNBLOCK, { SIGCHLD }) failed.");
+    }
+
     // Keep capabilities across UID change, unless we're staying root.
     if (uid != 0) {
       EnableKeepCapabilities(env);