PG-1892 Do not use InternalKey for encrypted keys

It gets confusing when this structure is used for both encrypted and
decrypted key data. With this change it's obvious that any allocation of
InternalKey can potentially leak key material to swap files.

Also be more explicit with empty alignment in the TDEMapEntry struct as
multiple levels of anonymous structs to maintain alignment would get a
bit messy.

contrib/pg_tde/src/access/pg_tde_tdemap.c

@@ -54,30 +54,28 @@ typedef struct TDEFileHeader
 	TDESignedPrincipalKeyInfo signed_key_info;
 } TDEFileHeader;
 
-/* We do not need the dbOid since the entries are stored in a file per db */
+/*
+ * Feel free to use the unused fields for something, but beware that existing
+ * files may contain unexpected values here. Also be aware of alignment if
+ * changing any of the types as this struct is written/read directly from file.
+ *
+ * If changes are made, know that the first four fields are used as AAD when
+ * encrypting/decrypting existing keys from the key files, so any changes here
+ * might break existing clusters.
+ */
 typedef struct TDEMapEntry
 {
-	Oid			spcOid;
+	Oid			spcOid;			/* Part of AAD */
-	RelFileNumber relNumber;
+	RelFileNumber relNumber;	/* Part of AAD */
-	uint32		type;
+	uint32		type;			/* Part of AAD */
+	uint32		_unused3;		/* Part of AAD */
 
-	/*
+	uint8		encrypted_key_data[INTERNAL_KEY_LEN];
-	 * This anonymous struct is here to ensure the same alignment as before
+	uint8		key_base_iv[INTERNAL_KEY_IV_LEN];
-	 * the unused fields were removed from InternalKey.
+
-	 */
+	uint32		_unused1;		/* Will be 1 in existing files entries. */
-	struct
+	uint32		_unused4;
-	{
+	uint64		_unused2;		/* Will be 0 in existing files entries. */
-		InternalKey enc_key;
-
-		/*
-		 * These fields were added here to keep the file format the same after
-		 * some fields were removed from InternalKey. Feel free to use them
-		 * for something, but beware that existing files may contain
-		 * unexpected values here.
-		 */
-		uint32		_unused1;	/* Will be 1 in existing files entries. */
-		uint64		_unused2;	/* Will be 0 in existing files entries. */
-	};
 
 	/* IV and tag used when encrypting the key itself */
 	unsigned char entry_iv[MAP_ENTRY_IV_SIZE];
@@ -398,7 +396,7 @@ pg_tde_initialize_map_entry(TDEMapEntry *map_entry, const TDEPrincipalKey *princ
 	map_entry->spcOid = rlocator->spcOid;
 	map_entry->relNumber = rlocator->relNumber;
 	map_entry->type = MAP_ENTRY_TYPE_KEY;
-	map_entry->enc_key = *rel_key_data;
+	memcpy(map_entry->key_base_iv, rel_key_data->base_iv, INTERNAL_KEY_IV_LEN);
 
 	/*
 	 * We set these fields here so that existing file entries will be
@@ -414,9 +412,9 @@ pg_tde_initialize_map_entry(TDEMapEntry *map_entry, const TDEPrincipalKey *princ
 
 	AesGcmEncrypt(principal_key->keyData,
 				  map_entry->entry_iv, MAP_ENTRY_IV_SIZE,
-				  (unsigned char *) map_entry, offsetof(TDEMapEntry, enc_key),
+				  (unsigned char *) map_entry, offsetof(TDEMapEntry, encrypted_key_data),
 				  rel_key_data->key, INTERNAL_KEY_LEN,
-				  map_entry->enc_key.key,
+				  map_entry->encrypted_key_data,
 				  map_entry->aead_tag, MAP_ENTRY_AEAD_TAG_SIZE);
 }
 #endif
@@ -587,22 +585,22 @@ pg_tde_verify_principal_key_info(TDESignedPrincipalKeyInfo *signed_key_info, con
 static InternalKey *
 tde_decrypt_rel_key(const TDEPrincipalKey *principal_key, TDEMapEntry *map_entry)
 {
-	InternalKey *rel_key_data = palloc_object(InternalKey);
+	InternalKey *key = palloc_object(InternalKey);
 
 	Assert(principal_key);
 
-	*rel_key_data = map_entry->enc_key;
-
 	if (!AesGcmDecrypt(principal_key->keyData,
 					   map_entry->entry_iv, MAP_ENTRY_IV_SIZE,
-					   (unsigned char *) map_entry, offsetof(TDEMapEntry, enc_key),
+					   (unsigned char *) map_entry, offsetof(TDEMapEntry, encrypted_key_data),
-					   map_entry->enc_key.key, INTERNAL_KEY_LEN,
+					   map_entry->encrypted_key_data, INTERNAL_KEY_LEN,
-					   rel_key_data->key,
+					   key->key,
 					   map_entry->aead_tag, MAP_ENTRY_AEAD_TAG_SIZE))
 		ereport(ERROR,
 				errmsg("Failed to decrypt key, incorrect principal key or corrupted key file"));
 
-	return rel_key_data;
+	memcpy(key->base_iv, map_entry->key_base_iv, INTERNAL_KEY_IV_LEN);
+
+	return key;
 }
 
 /*