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// Copyright (c) 2013 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 "sandbox/linux/services/credentials.h"
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/capability.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include "base/basictypes.h"
#include "base/bind.h"
#include "base/logging.h"
#include "base/strings/string_number_conversions.h"
#include "base/template_util.h"
#include "base/threading/thread.h"
namespace {
struct CapFreeDeleter {
inline void operator()(cap_t cap) const {
int ret = cap_free(cap);
CHECK_EQ(0, ret);
// Wrapper to manage libcap2's cap_t type.
typedef scoped_ptr<typeof(*((cap_t)0)), CapFreeDeleter> ScopedCap;
struct CapTextFreeDeleter {
inline void operator()(char* cap_text) const {
int ret = cap_free(cap_text);
CHECK_EQ(0, ret);
// Wrapper to manage the result from libcap2's cap_from_text().
typedef scoped_ptr<char, CapTextFreeDeleter> ScopedCapText;
struct FILECloser {
inline void operator()(FILE* f) const {
PCHECK(0 == fclose(f));
// Don't use ScopedFILE in base/file_util.h since it doesn't check fclose().
// TODO(jln): fix base/.
typedef scoped_ptr<FILE, FILECloser> ScopedFILE;
struct DIRCloser {
void operator()(DIR* d) const {
PCHECK(0 == closedir(d));
typedef scoped_ptr<DIR, DIRCloser> ScopedDIR;
COMPILE_ASSERT((base::is_same<uid_t, gid_t>::value), UidAndGidAreSameType);
// generic_id_t can be used for either uid_t or gid_t.
typedef uid_t generic_id_t;
// Write a uid or gid mapping from |id| to |id| in |map_file|.
bool WriteToIdMapFile(const char* map_file, generic_id_t id) {
ScopedFILE f(fopen(map_file, "w"));
const uid_t inside_id = id;
const uid_t outside_id = id;
int num = fprintf(f.get(), "%d %d 1\n", inside_id, outside_id);
if (num < 0) return false;
// Manually call fflush() to catch permission failures.
int ret = fflush(f.get());
if (ret) {
VLOG(1) << "Could not write to id map file";
return false;
return true;
// Checks that the set of RES-uids and the set of RES-gids have
// one element each and return that element in |resuid| and |resgid|
// respectively. It's ok to pass NULL as one or both of the ids.
bool GetRESIds(uid_t* resuid, gid_t* resgid) {
uid_t ruid, euid, suid;
gid_t rgid, egid, sgid;
PCHECK(getresuid(&ruid, &euid, &suid) == 0);
PCHECK(getresgid(&rgid, &egid, &sgid) == 0);
const bool uids_are_equal = (ruid == euid) && (ruid == suid);
const bool gids_are_equal = (rgid == egid) && (rgid == sgid);
if (!uids_are_equal || !gids_are_equal) return false;
if (resuid) *resuid = euid;
if (resgid) *resgid = egid;
return true;
// chroot() and chdir() to /proc/<tid>/fdinfo.
void ChrootToThreadFdInfo(base::PlatformThreadId tid, bool* result) {
*result = false;
COMPILE_ASSERT((base::is_same<base::PlatformThreadId, int>::value),
const std::string current_thread_fdinfo = "/proc/" +
base::IntToString(tid) + "/fdinfo/";
// Make extra sure that /proc/<tid>/fdinfo is unique to the thread.
CHECK(0 == unshare(CLONE_FILES));
int chroot_ret = chroot(current_thread_fdinfo.c_str());
if (chroot_ret) {
PLOG(ERROR) << "Could not chroot";
// CWD is essentially an implicit file descriptor, so be careful to not leave
// it behind.
PCHECK(0 == chdir("/"));
*result = true;
// chroot() to an empty dir that is "safe". To be safe, it must not contain
// any subdirectory (chroot-ing there would allow a chroot escape) and it must
// be impossible to create an empty directory there.
// We achieve this by doing the following:
// 1. We create a new thread, which will create a new /proc/<tid>/ directory
// 2. We chroot to /proc/<tid>/fdinfo/
// This is already "safe", since fdinfo/ does not contain another directory and
// one cannot create another directory there.
// 3. The thread dies
// After (3) happens, the directory is not available anymore in /proc.
bool ChrootToSafeEmptyDir() {
base::Thread chrooter("sandbox_chrooter");
if (!chrooter.Start()) return false;
bool is_chrooted = false;
base::Bind(&ChrootToThreadFdInfo, chrooter.thread_id(), &is_chrooted));
// Make sure our task has run before committing the return value.
return is_chrooted;
} // namespace.
namespace sandbox {
Credentials::Credentials() {
Credentials::~Credentials() {
bool Credentials::HasOpenDirectory(int proc_fd) {
int proc_self_fd = -1;
if (proc_fd >= 0) {
proc_self_fd = openat(proc_fd, "self/fd", O_DIRECTORY | O_RDONLY);
} else {
proc_self_fd = openat(AT_FDCWD, "/proc/self/fd", O_DIRECTORY | O_RDONLY);
if (proc_self_fd < 0) {
// If this process has been chrooted (eg into /proc/self/fdinfo) then
// the new root dir will not have directory listing permissions for us
// (hence EACCES). And if we do have this permission, then /proc won't
// exist anyway (hence ENOENT).
DPCHECK(errno == EACCES || errno == ENOENT)
<< "Unexpected failure when trying to open /proc/self/fd: ("
<< errno << ") " << strerror(errno);
// If not available, guess false.
return false;
CHECK_GE(proc_self_fd, 0);
// Ownership of proc_self_fd is transferred here, it must not be closed
// or modified afterwards except via dir.
ScopedDIR dir(fdopendir(proc_self_fd));
struct dirent e;
struct dirent* de;
while (!readdir_r(dir.get(), &e, &de) && de) {
if (strcmp(e.d_name, ".") == 0 || strcmp(e.d_name, "..") == 0) {
int fd_num;
CHECK(base::StringToInt(e.d_name, &fd_num));
if (fd_num == proc_fd || fd_num == proc_self_fd) {
struct stat s;
// It's OK to use proc_self_fd here, fstatat won't modify it.
CHECK(fstatat(proc_self_fd, e.d_name, &s, 0) == 0);
if (S_ISDIR(s.st_mode)) {
return true;
// No open unmanaged directories found.
return false;
bool Credentials::DropAllCapabilities() {
ScopedCap cap(cap_init());
PCHECK(0 == cap_set_proc(cap.get()));
// We never let this function fail.
return true;
bool Credentials::HasAnyCapability() const {
ScopedCap current_cap(cap_get_proc());
ScopedCap empty_cap(cap_init());
return cap_compare(current_cap.get(), empty_cap.get()) != 0;
scoped_ptr<std::string> Credentials::GetCurrentCapString() const {
ScopedCap current_cap(cap_get_proc());
ScopedCapText cap_text(cap_to_text(current_cap.get(), NULL));
return scoped_ptr<std::string> (new std::string(cap_text.get()));
bool Credentials::MoveToNewUserNS() {
uid_t uid;
gid_t gid;
if (!GetRESIds(&uid, &gid)) {
// If all the uids (or gids) are not equal to each other, the security
// model will most likely confuse the caller, abort.
DVLOG(1) << "uids or gids differ!";
return false;
int ret = unshare(CLONE_NEWUSER);
// EPERM can happen if already in a chroot. EUSERS if too many nested
// namespaces are used. EINVAL for kernels that don't support the feature.
// Valgrind will ENOSYS unshare().
PCHECK(!ret || errno == EPERM || errno == EUSERS || errno == EINVAL ||
errno == ENOSYS);
if (ret) {
VLOG(1) << "Looks like unprivileged CLONE_NEWUSER may not be available "
<< "on this kernel.";
return false;
// The current {r,e,s}{u,g}id is now an overflow id (c.f.
// /proc/sys/kernel/overflowuid). Setup the uid and gid maps.
const char kGidMapFile[] = "/proc/self/gid_map";
const char kUidMapFile[] = "/proc/self/uid_map";
CHECK(WriteToIdMapFile(kGidMapFile, gid));
CHECK(WriteToIdMapFile(kUidMapFile, uid));
return true;
bool Credentials::DropFileSystemAccess() {
// Chrooting to a safe empty dir will only be safe if no directory file
// descriptor is available to the process.
return ChrootToSafeEmptyDir();
} // namespace sandbox.