sandbox: Add StatOnlyWithIntermediateDirs()

Some use cases, such as some implementations of realpath(), require
calling stat() on all intermediate dirs of a path, but otherwise
do not need to open or read those directories.

Bug=b:112486795
Test=sandbox_linux_unittests passed

Change-Id: Iea0bad8be9e238a854de383ac6dc5124f462ead3
Reviewed-on: https://chromium-review.googlesource.com/1185376Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Drew Davenport <ddavenport@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585586}

sandbox/linux/syscall_broker/broker_file_permission.cc

@@ -204,8 +204,8 @@ bool BrokerFilePermission::CheckStat(const char* requested_filename,
   if (CheckAccessInternal(requested_filename, F_OK, file_to_access))
     return true;
 
-  // Allow stat() on leading directories if have create permission.
-  if (!allow_create_)
+  // Allow stat() on leading directories if have create or stat() permission.
+  if (!(allow_create_ || allow_stat_with_intermediates_))
     return false;
 
   // NOTE: ValidatePath proved requested_length != 0;
@@ -214,7 +214,10 @@ bool BrokerFilePermission::CheckStat(const char* requested_filename,
 
   // Special case for root: only one slash, otherwise must have a second
   // slash in the right spot to avoid substring matches.
+  // |allow_stat_with_intermediates_| can match on the full path, and
+  // |allow_create_| only matches a leading directory.
   if ((requested_length == 1 && requested_filename[0] == '/') ||
+      (allow_stat_with_intermediates_ && path_ == requested_filename) ||
       (requested_length < path_.length() &&
        memcmp(path_.c_str(), requested_filename, requested_length) == 0 &&
        path_.c_str()[requested_length] == '/')) {
@@ -236,13 +239,15 @@ BrokerFilePermission::BrokerFilePermission(const std::string& path,
                                            bool temporary_only,
                                            bool allow_read,
                                            bool allow_write,
-                                           bool allow_create)
+                                           bool allow_create,
+                                           bool allow_stat_with_intermediates)
     : path_(path),
       recursive_(recursive),
       temporary_only_(temporary_only),
       allow_read_(allow_read),
       allow_write_(allow_write),
-      allow_create_(allow_create) {
+      allow_create_(allow_create),
+      allow_stat_with_intermediates_(allow_stat_with_intermediates) {
   // Must have enough length for a '/'
   CHECK(path_.length() > 0) << GetErrorMessageForTests();
 

sandbox/linux/syscall_broker/broker_file_permission.h

@@ -25,40 +25,45 @@ class SANDBOX_EXPORT BrokerFilePermission {
   BrokerFilePermission& operator=(const BrokerFilePermission&) = default;
 
   static BrokerFilePermission ReadOnly(const std::string& path) {
-    return BrokerFilePermission(path, false, false, true, false, false);
+    return BrokerFilePermission(path, false, false, true, false, false, false);
   }
 
   static BrokerFilePermission ReadOnlyRecursive(const std::string& path) {
-    return BrokerFilePermission(path, true, false, true, false, false);
+    return BrokerFilePermission(path, true, false, true, false, false, false);
   }
 
   static BrokerFilePermission WriteOnly(const std::string& path) {
-    return BrokerFilePermission(path, false, false, false, true, false);
+    return BrokerFilePermission(path, false, false, false, true, false, false);
   }
 
   static BrokerFilePermission ReadWrite(const std::string& path) {
-    return BrokerFilePermission(path, false, false, true, true, false);
+    return BrokerFilePermission(path, false, false, true, true, false, false);
   }
 
   static BrokerFilePermission ReadWriteCreate(const std::string& path) {
-    return BrokerFilePermission(path, false, false, true, true, true);
+    return BrokerFilePermission(path, false, false, true, true, true, false);
   }
 
   static BrokerFilePermission ReadWriteCreateRecursive(
       const std::string& path) {
-    return BrokerFilePermission(path, true, false, true, true, true);
+    return BrokerFilePermission(path, true, false, true, true, true, false);
   }
 
   // Temporary files must always be newly created and do not confer rights to
   // use pre-existing files of the same name.
   static BrokerFilePermission ReadWriteCreateTemporary(
       const std::string& path) {
-    return BrokerFilePermission(path, false, true, true, true, true);
+    return BrokerFilePermission(path, false, true, true, true, true, false);
   }
 
   static BrokerFilePermission ReadWriteCreateTemporaryRecursive(
       const std::string& path) {
-    return BrokerFilePermission(path, true, true, true, true, true);
+    return BrokerFilePermission(path, true, true, true, true, true, false);
+  }
+
+  static BrokerFilePermission StatOnlyWithIntermediateDirs(
+      const std::string& path) {
+    return BrokerFilePermission(path, false, false, false, false, false, true);
   }
 
   // Returns true if |requested_filename| is allowed to be accessed
@@ -108,7 +113,8 @@ class SANDBOX_EXPORT BrokerFilePermission {
                        bool temporary_only,
                        bool allow_read,
                        bool allow_write,
-                       bool allow_create);
+                       bool allow_create,
+                       bool allow_stat_with_intermediates);
 
   // ValidatePath checks |path| and returns true if these conditions are met
   // * Greater than 0 length
@@ -138,6 +144,8 @@ class SANDBOX_EXPORT BrokerFilePermission {
   bool allow_read_;
   bool allow_write_;
   bool allow_create_;
+  // Allow stat() for |path| and all intermediate dirs.
+  bool allow_stat_with_intermediates_;
 };
 
 }  // namespace syscall_broker

sandbox/linux/syscall_broker/broker_file_permission_unittest.cc

@@ -260,6 +260,30 @@ TEST(BrokerFilePermission, ReadWriteCreateTemporaryRecursive) {
   // expected.
 }
 
+TEST(BrokerFilePermission, StatOnlyWithIntermediateDirs) {
+  const char kPath[] = "/tmp/good/path";
+  const char kLeading1[] = "/";
+  const char kLeading2[] = "/tmp";
+  const char kLeading3[] = "/tmp/good/path";
+  const char kTrailing[] = "/tmp/good/path/bad";
+
+  BrokerFilePermission perm =
+      BrokerFilePermission::StatOnlyWithIntermediateDirs(kPath);
+  // No open or access permission.
+  ASSERT_FALSE(perm.CheckOpen(kPath, O_RDONLY, NULL, NULL));
+  ASSERT_FALSE(perm.CheckOpen(kPath, O_WRONLY, NULL, NULL));
+  ASSERT_FALSE(perm.CheckOpen(kPath, O_RDWR, NULL, NULL));
+  ASSERT_FALSE(perm.CheckAccess(kPath, R_OK, NULL));
+  ASSERT_FALSE(perm.CheckAccess(kPath, W_OK, NULL));
+
+  // Stat for all leading paths, but not trailing paths.
+  ASSERT_TRUE(perm.CheckStat(kPath, NULL));
+  ASSERT_TRUE(perm.CheckStat(kLeading1, NULL));
+  ASSERT_TRUE(perm.CheckStat(kLeading2, NULL));
+  ASSERT_TRUE(perm.CheckStat(kLeading3, NULL));
+  ASSERT_FALSE(perm.CheckStat(kTrailing, NULL));
+}
+
 TEST(BrokerFilePermission, ValidatePath) {
   EXPECT_TRUE(BrokerFilePermissionTester::ValidatePath("/path"));
   EXPECT_TRUE(BrokerFilePermissionTester::ValidatePath("/"));