Linux: add basic unprivileged namespace support.

sandbox/linux/services/credentials.cc

Index: sandbox/linux/services/credentials.cc
diff --git a/sandbox/linux/services/credentials.cc b/sandbox/linux/services/credentials.cc
index a6387d2785c722c0b78c8725259991ee95cfd243..0af5a42b06ed3dabcacac66a237a9078e7902385 100644
--- a/sandbox/linux/services/credentials.cc
+++ b/sandbox/linux/services/credentials.cc
@@ -4,11 +4,17 @@
 
 #include "sandbox/linux/services/credentials.h"
 
+#include <errno.h>
 #include <stdio.h>
 #include <sys/capability.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 {
 
@@ -32,6 +38,101 @@ struct CapTextFreeDeleter {
 // 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 {
+    DCHECK(f);
+    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;
+
+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"));
+  PCHECK(f);
+  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) {
+  DCHECK(result);
+  *result = false;
+
+  COMPILE_ASSERT((base::is_same<base::PlatformThreadId, int>::value),
+                 TidIsAnInt);
+  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";
+    return;
+  }
+
+  // CWD is essentially an implicit file descriptor, so be careful to not leave
+  // it behind.
+  PCHECK(0 == chdir("/"));
+
+  *result = true;
+  return;
+}
+
+// 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;
+  chrooter.message_loop()->PostTask(FROM_HERE,
+      base::Bind(&ChrootToThreadFdInfo, chrooter.thread_id(), &is_chrooted));
+  // Make sure our task has run before committing the return value.
+  chrooter.Stop();
+  return is_chrooted;
+}
+
 }  // namespace.
 
 namespace sandbox {
@@ -42,13 +143,15 @@ Credentials::Credentials() {
 Credentials::~Credentials() {
 }
 
-void Credentials::DropAllCapabilities() {
+bool Credentials::DropAllCapabilities() {
   ScopedCap cap(cap_init());
   CHECK(cap);
   PCHECK(0 == cap_set_proc(cap.get()));
+  // We never let this function fail.
+  return true;
 }
 
-bool Credentials::HasAnyCapability() {
+bool Credentials::HasAnyCapability() const {
   ScopedCap current_cap(cap_get_proc());
   CHECK(current_cap);
   ScopedCap empty_cap(cap_init());
@@ -56,7 +159,7 @@ bool Credentials::HasAnyCapability() {
   return cap_compare(current_cap.get(), empty_cap.get()) != 0;
 }
 
-scoped_ptr<std::string> Credentials::GetCurrentCapString() {
+scoped_ptr<std::string> Credentials::GetCurrentCapString() const {
   ScopedCap current_cap(cap_get_proc());
   CHECK(current_cap);
   ScopedCapText cap_text(cap_to_text(current_cap.get(), NULL));
@@ -64,4 +167,39 @@ scoped_ptr<std::string> Credentials::GetCurrentCapString() {
   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.
+  DCHECK(GetRESIds(NULL, NULL));
+  const char kGidMapFile[] = "/proc/self/gid_map";
+  const char kUidMapFile[] = "/proc/self/uid_map";
+  CHECK(WriteToIdMapFile(kGidMapFile, gid));
+  CHECK(WriteToIdMapFile(kUidMapFile, uid));
+  DCHECK(GetRESIds(NULL, NULL));
+  return true;
+}
+
+bool Credentials::DropFileSystemAccess() {
+  return ChrootToSafeEmptyDir();
+}
+
 }  // namespace sandbox.