Add DropPrivileges()

util/posix/drop_privileges.cc

Index: util/posix/drop_privileges.cc
diff --git a/util/posix/drop_privileges.cc b/util/posix/drop_privileges.cc
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <unistd.h>
+
+#include "base/logging.h"
+#include "build/build_config.h"
+
+namespace crashpad {
+
+void DropPrivileges() {
+  gid_t gid = getgid();
+  uid_t uid = getuid();
+
+#if defined(OS_MACOSX)
+  // Based on the POSIX.1-2008 2013 edition documentation for setreuid() and
+  // setregid(), setreuid() and setregid() alone should be sufficient to drop
+  // privileges. The standard specifies that the saved ID should be set to the
+  // effective ID whenever the real ID is not -1, and whenever the effective ID
+  // is set not equal to the real ID. This code never specifies -1, so the
+  // setreuid() and setregid() alone should work according to the standard.
+  //
+  // In practice, on Mac OS X, setuid() and setgid() (or seteuid() and
+  // setegid()) must be called first, otherwise, setreuid() and setregid() do
+  // not alter the saved IDs, leaving open the possibility for future privilege
+  // escalation. This bug is filed as radar 18987552.
+  gid_t egid = getegid();
+  PCHECK(setgid(gid) == 0) << "setgid";
+  PCHECK(setregid(gid, gid) == 0) << "setregid";
+
+  uid_t euid = geteuid();
+  PCHECK(setuid(uid) == 0) << "setuid";
+  PCHECK(setreuid(uid, uid) == 0) << "setreuid";
+
+  if (uid != 0) {
+    // Because the setXid()+setreXid() interface to change IDs is fragile,
+    // ensure that privileges cannot be regained. This can only be done if the
+    // real user ID (and now the effective user ID as well) is not root, because
+    // root always has permission to change identity.
+    if (euid != uid) {
+      CHECK_EQ(seteuid(euid), -1);
+    }
+    if (egid != gid) {
+      CHECK_EQ(setegid(egid), -1);
+    }
+  }
+#elif defined(OS_LINUX)
+  PCHECK(setresgid(gid, gid, gid) == 0) << "setresgid";
+  PCHECK(setresuid(uid, uid, uid) == 0) << "setresuid";
+
+  // Don’t check to see if privileges can be regained on Linux, because on
+  // Linux, it’s not as simple as ensuring that this can’t be done if non-root.
+  // Instead, the ability to change user and group IDs are controlled by the
+  // CAP_SETUID and CAP_SETGID capabilities, which may be granted to non-root

[Robert Sesek, 2014/11/14 23:36:03]

Arguably, this function should drop those capabilities.

+  // processes. Since the setresXid() interface is well-defined, it shouldn’t be
+  // necessary to perform any additional checking anyway.
+#else
+#error Port this function to your system.
+#endif
+}
+
+}  // namespace crashpad