Linux sandbox: move CurrentProcessHasOpenDirectories

Move CurrentProcessHasOpenDirectories() to the Credentials class and rename
it to HasOpenDirectory().
Also add some unittests.

This is a re-land of https://codereview.chromium.org/85403011/.

BUG=312380
R=jorgelo@chromium.org, mmoss@google.com

Review URL: https://codereview.chromium.org/88243003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237390 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 121 insertions, 3 deletions

diff --git a/sandbox/linux/services/credentials.cc b/sandbox/linux/services/credentials.cc
index 0af5a42b..cea757c 100644
--- a/sandbox/linux/services/credentials.cc
+++ b/sandbox/linux/services/credentials.cc
@@ -4,9 +4,13 @@
 
 #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"
@@ -49,6 +53,15 @@ struct FILECloser {
 // TODO(jln): fix base/.
 typedef scoped_ptr<FILE, FILECloser> ScopedFILE;
 
+struct DIRCloser {
+  void operator()(DIR* d) const {
+    DCHECK(d);
+    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;
@@ -143,6 +156,51 @@ 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 not available, guess false.
+      // TODO(mostynb@opera.com): add a CHECK_EQ(ENOENT, errno); Figure out what
+      // other situations are here. http://crbug.com/314985
+      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));
+  CHECK(dir);
+
+  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) {
+      continue;
+    }
+
+    int fd_num;
+    CHECK(base::StringToInt(e.d_name, &fd_num));
+    if (fd_num == proc_fd || fd_num == proc_self_fd) {
+      continue;
+    }
+
+    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());
   CHECK(cap);
@@ -199,6 +257,9 @@ bool Credentials::MoveToNewUserNS() {
 }
 
 bool Credentials::DropFileSystemAccess() {
+  // Chrooting to a safe empty dir will only be safe if no directory file
+  // descriptor is available to the process.
+  DCHECK(!HasOpenDirectory(-1));
   return ChrootToSafeEmptyDir();
 }
 
diff --git a/sandbox/linux/services/credentials.h b/sandbox/linux/services/credentials.h
index 80b2ec1..c23db93 100644
--- a/sandbox/linux/services/credentials.h
+++ b/sandbox/linux/services/credentials.h
@@ -26,6 +26,20 @@ class Credentials {
   Credentials();
   ~Credentials();
 
+  // Checks whether the current process has any directory file descriptor open.
+  // Directory file descriptors are "capabilities" that would let a process use
+  // system calls such as openat() to bypass restrictions such as
+  // DropFileSystemAccess().
+  // Sometimes it's useful to call HasOpenDirectory() after file system access
+  // has been dropped. In this case, |proc_fd| should be a file descriptor to
+  // /proc. The file descriptor in |proc_fd| will be ignored by
+  // HasOpenDirectory() and remains owned by the caller. It is very important
+  // for the caller to close it.
+  // If /proc is available, |proc_fd| can be passed as -1.
+  // If |proc_fd| is -1 and /proc is not available, this function will return
+  // false.
+  bool HasOpenDirectory(int proc_fd);
+
   // Drop all capabilities in the effective, inheritable and permitted sets for
   // the current process.
   bool DropAllCapabilities();
@@ -52,9 +66,8 @@ class Credentials {
   // CAP_SYS_CHROOT can be acquired by using the MoveToNewUserNS() API.
   // Make sure to call DropAllCapabilities() after this call to prevent
   // escapes.
-  // To be secure, it's very important for this API to not be called with any
-  // directory file descriptor present. TODO(jln): integrate with
-  // crbug.com/269806 when available.
+  // To be secure, it's very important for this API to not be called while the
+  // process has any directory file descriptor open.
   bool DropFileSystemAccess();
 
  private:
diff --git a/sandbox/linux/services/credentials_unittest.cc b/sandbox/linux/services/credentials_unittest.cc
index da61cd5..9160bf7 100644
--- a/sandbox/linux/services/credentials_unittest.cc
+++ b/sandbox/linux/services/credentials_unittest.cc
@@ -5,14 +5,20 @@
 #include "sandbox/linux/services/credentials.h"
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <unistd.h>
 
+#include "base/file_util.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "sandbox/linux/tests/unit_tests.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
+using file_util::ScopedFD;
+
 namespace sandbox {
 
 namespace {
@@ -52,6 +58,44 @@ TEST(Credentials, CreateAndDestroy) {
   scoped_ptr<Credentials> cred2(new Credentials);
 }
 
+TEST(Credentials, HasOpenDirectory) {
+  Credentials creds;
+  // No open directory should exist at startup.
+  EXPECT_FALSE(creds.HasOpenDirectory(-1));
+  {
+    // Have a "/dev" file descriptor around.
+    int dev_fd = open("/dev", O_RDONLY | O_DIRECTORY);
+    ScopedFD dev_fd_closer(&dev_fd);
+    EXPECT_TRUE(creds.HasOpenDirectory(-1));
+  }
+  EXPECT_FALSE(creds.HasOpenDirectory(-1));
+}
+
+TEST(Credentials, HasOpenDirectoryWithFD) {
+  Credentials creds;
+
+  int proc_fd = open("/proc", O_RDONLY | O_DIRECTORY);
+  ScopedFD proc_fd_closer(&proc_fd);
+  ASSERT_LE(0, proc_fd);
+
+  // Don't pass |proc_fd|, an open directory (proc_fd) should
+  // be detected.
+  EXPECT_TRUE(creds.HasOpenDirectory(-1));
+  // Pass |proc_fd| and no open directory should be detected.
+  EXPECT_FALSE(creds.HasOpenDirectory(proc_fd));
+
+  {
+    // Have a "/dev" file descriptor around.
+    int dev_fd = open("/dev", O_RDONLY | O_DIRECTORY);
+    ScopedFD dev_fd_closer(&dev_fd);
+    EXPECT_TRUE(creds.HasOpenDirectory(proc_fd));
+  }
+
+  // The "/dev" file descriptor should now be closed, |proc_fd| is the only
+  // directory file descriptor open.
+  EXPECT_FALSE(creds.HasOpenDirectory(proc_fd));
+}
+
 SANDBOX_TEST(Credentials, DropAllCaps) {
   Credentials creds;
   CHECK(creds.DropAllCapabilities());