Allow chrome_sandbox to act as a helper program and find the socket with a gi...

sandbox/linux/suid/linux_util.c

+// 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.
+
+// The following is duplicated from base/linux_utils.cc.
+// We shouldn't link against C++ code in a setuid binary.
+
+#include "linux_util.h"
+
+#include <dirent.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+// expected prefix of the target of the /proc/self/fd/%d link for a socket
+static const char kSocketLinkPrefix[] = "socket:[";
+
+// Parse a symlink in /proc/pid/fd/$x and return the inode number of the
+// socket.
+//   inode_out: (output) set to the inode number on success
+//   path: e.g. /proc/1234/fd/5 (must be a UNIX domain socket descriptor)
+static bool ProcPathGetInode(ino_t* inode_out, const char* path) {
+  char buf[256];
+  const ssize_t n = readlink(path, buf, sizeof(buf) - 1);
+  if (n == -1)
+    return false;
+  buf[n] = 0;
+
+  if (memcmp(kSocketLinkPrefix, buf, sizeof(kSocketLinkPrefix) - 1))
+    return false;
+
+  char *endptr;
+  const unsigned long long int inode_ul =
+      strtoull(buf + sizeof(kSocketLinkPrefix) - 1, &endptr, 10);
+  if (*endptr != ']')
+    return false;
+
+  if (inode_ul == ULLONG_MAX)
+    return false;
+
+  *inode_out = inode_ul;
+  return true;
+}
+
+bool FindProcessHoldingSocket(pid_t* pid_out, ino_t socket_inode) {
+  bool already_found = false;
+
+  DIR* proc = opendir("/proc");
+  if (!proc)
+    return false;
+
+  const uid_t uid = getuid();
+  struct dirent* dent;
+  while ((dent = readdir(proc))) {
+    char *endptr;
+    const unsigned long int pid_ul = strtoul(dent->d_name, &endptr, 10);
+    if (pid_ul == ULONG_MAX || *endptr)
+      continue;
+
+    // We have this setuid code here because the zygote and its children have
+    // /proc/$pid/fd owned by root. While scanning through /proc, we add this
+    // extra check so users cannot accidentally gain information about other
+    // users' processes. To determine process ownership, we use the property
+    // that if user foo owns process N, then /proc/N is owned by foo.
+    {
+      char buf[256];
+      struct stat statbuf;
+      snprintf(buf, sizeof(buf), "/proc/%lu", pid_ul);
+      if (stat(buf, &statbuf) < 0)
+        continue;
+      if (uid != statbuf.st_uid)
+        continue;
+    }
+
+    char buf[256];
+    snprintf(buf, sizeof(buf), "/proc/%lu/fd", pid_ul);
+    DIR* fd = opendir(buf);
+    if (!fd)
+      continue;
+
+    while ((dent = readdir(fd))) {
+      if (snprintf(buf, sizeof(buf), "/proc/%lu/fd/%s", pid_ul,
+                   dent->d_name) >= sizeof(buf) - 1) {
+        continue;
+      }
+
+      ino_t fd_inode;
+      if (ProcPathGetInode(&fd_inode, buf)) {
+        if (fd_inode == socket_inode) {
+          if (already_found) {
+            closedir(fd);
+            closedir(proc);
+            return false;
+          }
+
+          already_found = true;
+          *pid_out = pid_ul;
+          break;
+        }
+      }
+    }
+    closedir(fd);
+  }
+  closedir(proc);
+
+  return already_found;
+}