Linux: SUID sandbox support

sandbox/linux/suid/sandbox.cc

+// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <asm/unistd.h>
+#include <errno.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <sys/resource.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/time.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#if !defined(CLONE_NEWPID)
+#define CLONE_NEWPID 20000000
+#endif
+
+static const char kSandboxPath[] = "/var/run/chrome-sandbox";
+static const char kChromeBinary[] = "/opt/google/chrome/chrome";
+
+// These are the magic byte values which the sandboxed process uses to request
+// that it be chrooted.
+static const char kMsgChrootMe = 'C';
+static const char kMsgChrootSuccessful = 'O';
+
+static void FatalError(const char *msg, ...) {
+  va_list ap;
+  va_start(ap, msg);
+
+  vfprintf(stderr, msg, ap);
+  fprintf(stderr, ": %s\n", strerror(errno));
+  fflush(stderr);
+  _exit(1);
+}
+
+static int CloneChrootHelperProcess() {
+  int sv[2];
+  if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == -1) {
+    perror("socketpair");
+    return -1;
+  }
+
+  const pid_t pid = syscall(
+      __NR_clone, CLONE_FS | SIGCHLD, 0, 0, 0);
+
+  if (pid == -1) {
+    perror("clone");
+    close(sv[0]);
+    close(sv[1]);
+    return -1;
+  }
+
+  if (pid == 0) {
+    // We share our files structure with an untrusted process. As a security in
+    // depth measure, we make sure that we can't open anything by mistake.
+    // TODO: drop CAP_SYS_RESOURCE
+
+    const struct rlimit nofile = {0, 0};
+    if (setrlimit(RLIMIT_NOFILE, &nofile))
+      FatalError("Setting RLIMIT_NOFILE");
+
+    if (close(sv[1]))
+      FatalError("close");
+
+    char msg;
+    ssize_t bytes;
+    do {
+      bytes = read(sv[0], &msg, 1);
+    } while (bytes == -1 && errno == EINTR);
+
+    if (bytes == 0)
+      exit(0);
+    if (bytes != 1)
+      FatalError("read");
+
+    if (msg != kMsgChrootMe)
+      FatalError("Unknown message from sandboxed process");
+
+    if (chdir(kSandboxPath))
+      FatalError("Cannot chdir into %s", kSandboxPath);
+
+    struct stat st;
+    if (stat("/", &st))
+      FatalError("stat");
+
+    if (st.st_uid || st.st_gid || st.st_mode & S_IWOTH)
+      FatalError("Bad permissions on chroot directory (%s)", kSandboxPath);
+
+    if (chroot(kSandboxPath))
+      FatalError("Cannot chroot into %s", kSandboxPath);
+
+    if (chdir("/"))
+      FatalError("Cannot chdir to / after chroot");
+
+    const char reply = kMsgChrootSuccessful;
+    do {
+      bytes = write(sv[0], &reply, 1);
+    } while (bytes == -1 && errno == EINTR);
+
+    if (bytes != 1)
+      FatalError("Writing reply");
+
+    exit(0);
+  }
+
+  if (close(sv[0])) {
+    close(sv[1]);
+    perror("close");
+    return false;
+  }
+
+  return sv[1];
+}
+
+static bool SpawnChrootHelper() {
+  const int chroot_signal_fd = CloneChrootHelperProcess();
+
+  if (chroot_signal_fd == -1)
+    return false;
+
+  // In the parent process, we install an environment variable containing the
+  // number of the file descriptor.
+  char desc_str[64];
+  snprintf(desc_str, sizeof(desc_str), "%d", chroot_signal_fd);
+
+  if (setenv("SBX_D", desc_str, 1)) {
+    perror("setenv");
+    close(chroot_signal_fd);
+    return false;
+  }
+
+  return true;
+}
+
+static bool MoveToNewPIDNamespace() {
+  const pid_t pid = syscall(
+      __NR_clone, CLONE_NEWPID | SIGCHLD, 0, 0, 0);
+
+  if (pid == -1)
+    return false;
+
+  if (pid)
+    syscall(__NR_exit, 0);
+
+  return true;
+}
+
+static bool DropRoot() {
+  if (prctl(PR_SET_DUMPABLE, 0, 0, 0, 0)) {
+    perror("prctl(PR_SET_DUMPABLE)");
+    return false;
+  }
+
+  if (prctl(PR_GET_DUMPABLE, 0, 0, 0, 0)) {
+    perror("Still dumpable after prctl(PR_SET_DUMPABLE)");
+    return false;
+  }
+
+  gid_t rgid, egid, sgid;
+  if (getresgid(&rgid, &egid, &sgid)) {
+    perror("getresgid");
+    return false;
+  }
+
+  if (setresgid(rgid, rgid, rgid)) {
+    perror("setresgid");
+    return false;
+  }
+
+  uid_t ruid, euid, suid;
+  if (getresuid(&ruid, &euid, &suid)) {
+    perror("getresuid");
+    return false;
+  }
+
+  if (setresuid(ruid, ruid, ruid)) {
+    perror("setresuid");
+    return false;
+  }
+
+  return true;
+}
+
+int main(int argc, char **argv) {
+  if (argc == 1) {
+    fprintf(stderr, "Usage: %s <renderer process> <args...>\n", argv[0]);
+    return 1;
+  }
+
+  if (strcmp(argv[1], kChromeBinary)) {
+    fprintf(stderr, "This wrapper can only run %s!\n", kChromeBinary);
+    return 1;
+  }
+
+  if (!MoveToNewPIDNamespace())
+    return 1;
+  if (!SpawnChrootHelper())
+    return 1;
+  if (!DropRoot())
+    return 1;
+
+  execv(argv[1], &argv[1]);
+  FatalError("execv failed");
+
+  return 1;
+}