Redirect fopen("/dev/urandom") so that NSS can properly seed it's RNG.

content/browser/zygote_main_linux.cc

Index: content/browser/zygote_main_linux.cc
diff --git a/content/browser/zygote_main_linux.cc b/content/browser/zygote_main_linux.cc
index 020d7771ca6ed9cfe8351dcba1098d10b8a47f84..2c5e5fec57396921fc0ed45132580996cbd7df62 100644
--- a/content/browser/zygote_main_linux.cc
+++ b/content/browser/zygote_main_linux.cc
@@ -7,6 +7,7 @@
 #include <dlfcn.h>
 #include <fcntl.h>
 #include <pthread.h>
+#include <stdio.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ -25,6 +26,7 @@
 #include "base/pickle.h"
 #include "base/process_util.h"
 #include "base/rand_util.h"
+#include "base/rand_util_c.h"
 #include "base/sys_info.h"
 #include "build/build_config.h"
 #include "crypto/nss_util.h"
@@ -64,6 +66,8 @@ static const int kMagicSandboxIPCDescriptor = 5;
 static const int kZygoteIdDescriptor = 7;
 static bool g_suid_sandbox_active = false;
 
+static const char kUrandomDevPath[] = "/dev/urandom";
+
 #if defined(SECCOMP_SANDBOX)
 static int g_proc_fd = -1;
 #endif
@@ -632,16 +636,26 @@ static bool g_am_zygote_or_renderer = false;
 typedef struct tm* (*LocaltimeFunction)(const time_t* timep);
 typedef struct tm* (*LocaltimeRFunction)(const time_t* timep,
                                          struct tm* result);
+typedef FILE* (*FopenFunction)(const char* path, const char* mode);
+typedef int (*XstatFunction)(int version, const char *path, struct stat *buf);
+typedef int (*Xstat64Function)(int version, const char *path,
+                               struct stat64 *buf);
 
 static pthread_once_t g_libc_localtime_funcs_guard = PTHREAD_ONCE_INIT;
 static LocaltimeFunction g_libc_localtime;
 static LocaltimeRFunction g_libc_localtime_r;
 
+static pthread_once_t g_libc_file_io_funcs_guard = PTHREAD_ONCE_INIT;
+static FopenFunction g_libc_fopen;
+static FopenFunction g_libc_fopen64;
+static XstatFunction g_libc_xstat;
+static Xstat64Function g_libc_xstat64;
+
 static void InitLibcLocaltimeFunctions() {
   g_libc_localtime = reinterpret_cast<LocaltimeFunction>(
-                         dlsym(RTLD_NEXT, "localtime"));
+      dlsym(RTLD_NEXT, "localtime"));
   g_libc_localtime_r = reinterpret_cast<LocaltimeRFunction>(
-                         dlsym(RTLD_NEXT, "localtime_r"));
+      dlsym(RTLD_NEXT, "localtime_r"));
 
   if (!g_libc_localtime || !g_libc_localtime_r) {
     // http://code.google.com/p/chromium/issues/detail?id=16800
@@ -686,6 +700,125 @@ struct tm* localtime_r(const time_t* timep, struct tm* result) {
   }
 }
 
+// TODO(sergeyu): Currently this code doesn't work properly under ASAN
+// - it crashes content_unittests. Make sure it works properly and
+// enable it here. http://crbug.com/123263
+#if !defined(ADDRESS_SANITIZER)
+
+static void InitLibcFileIOFunctions() {
+  g_libc_fopen = reinterpret_cast<FopenFunction>(
+      dlsym(RTLD_NEXT, "fopen"));
+  g_libc_fopen64 = reinterpret_cast<FopenFunction>(
+      dlsym(RTLD_NEXT, "fopen64"));
+
+  if (!g_libc_fopen) {
+    LOG(FATAL) << "Failed to get fopen() from libc.";
+  } else if (!g_libc_fopen64) {
+#if !defined(OS_OPENBSD) && !defined(OS_FREEBSD)
+    LOG(WARNING) << "Failed to get fopen64() from libc. Using fopen() instead.";
+#endif  // !defined(OS_OPENBSD) && !defined(OS_FREEBSD)
+    g_libc_fopen64 = g_libc_fopen;
+  }
+
+  // TODO(sergeyu): This works only on systems with glibc. Fix it to
+  // work properly on other systems if necessary.
+  g_libc_xstat = reinterpret_cast<XstatFunction>(
+      dlsym(RTLD_NEXT, "__xstat"));
+  g_libc_xstat64 = reinterpret_cast<Xstat64Function>(
+      dlsym(RTLD_NEXT, "__xstat64"));
+
+  if (!g_libc_xstat) {
+    LOG(FATAL) << "Failed to get __xstat() from libc.";
+  }
+  if (!g_libc_xstat64) {
+    LOG(WARNING) << "Failed to get __xstat64() from libc.";
+  }
+}
+
+// fopen() and fopen64() are intercepted here so that NSS can open
+// /dev/urandom to seed its random number generator. NSS is used by
+// remoting in the sendbox.

[wtc, 2012/04/17 22:02:47]

Typo: sendbox => sandbox

+
+// fopen() call may be redirected to fopen64() in stdio.h using
+// __REDIRECT(), which sets asm name for fopen() to "fopen64". This
+// means that we cannot override fopen() directly here. Instead the
+// the code below defines fopen_override() function with asm name

[wtc, 2012/04/17 22:02:47]

Typo: remove "the"

I suggest you just move "the" to the front of "fopen_override() function".

+// "fopen", so that all references to fopen() will resolve to this
+// function.
+__attribute__ ((__visibility__("default")))
+FILE* fopen_override(const char* path, const char* mode)  __asm__ ("fopen");
+
+__attribute__ ((__visibility__("default")))
+FILE* fopen_override(const char* path, const char* mode) {
+  if (g_am_zygote_or_renderer && strcmp(path, kUrandomDevPath) == 0) {
+    int fd = HANDLE_EINTR(dup(GetUrandomFD()));
+    if (fd < 0) {
+      PLOG(ERROR) << "dup() failed.";
+      return NULL;
+    }
+    return fdopen(fd, mode);
+  } else {
+    CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                             InitLibcFileIOFunctions));
+    return g_libc_fopen(path, mode);
+  }
+}
+
+__attribute__ ((__visibility__("default")))
+FILE* fopen64(const char* path, const char* mode) {
+  if (g_am_zygote_or_renderer && strcmp(path, kUrandomDevPath) == 0) {
+    int fd = HANDLE_EINTR(dup(GetUrandomFD()));
+    if (fd < 0) {
+      PLOG(ERROR) << "dup() failed.";
+      return NULL;
+    }
+    return fdopen(fd, mode);
+  } else {
+    CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                             InitLibcFileIOFunctions));
+    return g_libc_fopen64(path, mode);
+  }
+}
+
+// stat() is subject to the same problem as fopen(), so we have to use
+// the same trick to override it.

[wtc, 2012/04/17 22:02:47]

You should explain why you need to intercept stat().

Which functions we need to intercept depends on what functions
NSS calls on "/dev/urandom".  So this CL may be broken
by a future change to the NSS code in its unix_rand.c
file.

I agree with agl that we should also patch NSS to fail (or
even call abort()) if it cannot use /dev/urandom on
platforms that are known to have /dev/urandom:
http://bonsai.mozilla.org/cvsblame.cgi?file=mozilla/security/nss/lib/freebl/u...

http://bonsai.mozilla.org/cvsblame.cgi?file=mozilla/security/nss/lib/freebl/u...

+__attribute__ ((__visibility__("default")))
+int xstat_override(int version,
+                   const char *path,
+                   struct stat *buf)  __asm__ ("__xstat");
+
+__attribute__ ((__visibility__("default")))
+int xstat_override(int version, const char *path, struct stat *buf) {
+  if (g_am_zygote_or_renderer && strcmp(path, kUrandomDevPath) == 0) {
+    int result = __fxstat(version, GetUrandomFD(), buf);
+    return result;
+  } else {
+    CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                             InitLibcFileIOFunctions));
+    return g_libc_xstat(version, path, buf);
+  }
+}
+
+__attribute__ ((__visibility__("default")))
+int xstat64_override(int version,
+                     const char *path,
+                     struct stat64 *buf)  __asm__ ("__xstat64");
+
+__attribute__ ((__visibility__("default")))
+int xstat64_override(int version, const char *path, struct stat64 *buf) {
+  if (g_am_zygote_or_renderer && strcmp(path, kUrandomDevPath) == 0) {
+    int result = __fxstat64(version, GetUrandomFD(), buf);
+    return result;
+  } else {
+    CHECK_EQ(0, pthread_once(&g_libc_file_io_funcs_guard,
+                             InitLibcFileIOFunctions));
+    CHECK(g_libc_xstat64);
+    return g_libc_xstat64(version, path, buf);
+  }
+}
+
+#endif  // !ADDRESS_SANITIZER
+
 #endif  // !CHROMIUM_SELINUX
 
 // This function triggers the static and lazy construction of objects that need