SERVER-29319 Check $db in explain

3 files changed, 23 insertions, 0 deletions

diff --git a/jstests/core/explain_db_mismatch.js b/jstests/core/explain_db_mismatch.js
new file mode 100644
index 00000000000..13d54cae77f
--- /dev/null
+++ b/jstests/core/explain_db_mismatch.js
@@ -0,0 +1,7 @@
+// Ensure that explain command errors if the inner command has a $db field that doesn't match the
+// outer command.
+(function() {
+    assert.commandFailedWithCode(
+        db.runCommand({explain: {find: 'some_collection', $db: 'not_my_db'}}),
+        ErrorCodes.InvalidNamespace);
+}());
diff --git a/src/mongo/db/commands/explain_cmd.cpp b/src/mongo/db/commands/explain_cmd.cpp
index e55da5b2974..92f56e864f5 100644
--- a/src/mongo/db/commands/explain_cmd.cpp
+++ b/src/mongo/db/commands/explain_cmd.cpp
@@ -133,6 +133,14 @@ public:
         // This is the nested command which we are explaining.
         BSONObj explainObj = cmdObj.firstElement().Obj();
 
+        if (auto innerDb = explainObj["$db"]) {
+            uassert(ErrorCodes::InvalidNamespace,
+                    str::stream() << "Mismatched $db in explain command. Expected " << dbname
+                                  << " but got "
+                                  << innerDb.checkAndGetStringData(),
+                    innerDb.checkAndGetStringData() == dbname);
+        }
+
         Command* commToExplain = Command::findCommand(explainObj.firstElementFieldName());
         if (NULL == commToExplain) {
             mongoutils::str::stream ss;
diff --git a/src/mongo/s/commands/cluster_explain_cmd.cpp b/src/mongo/s/commands/cluster_explain_cmd.cpp
index 2fa4184a5c0..c21a9db40b0 100644
--- a/src/mongo/s/commands/cluster_explain_cmd.cpp
+++ b/src/mongo/s/commands/cluster_explain_cmd.cpp
@@ -120,6 +120,14 @@ public:
         // Command::explain() method.
         const BSONObj explainObj = ([&] {
             const auto innerObj = cmdObj.firstElement().Obj();
+            if (auto innerDb = innerObj["$db"]) {
+                uassert(ErrorCodes::InvalidNamespace,
+                        str::stream() << "Mismatched $db in explain command. Expected " << dbName
+                                      << " but got "
+                                      << innerDb.checkAndGetStringData(),
+                        innerDb.checkAndGetStringData() == dbName);
+            }
+
             BSONObjBuilder bob;
             bob.appendElements(innerObj);
             for (auto outerElem : cmdObj) {