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Pointer Passing Interfaces

1. Overview

Three new "_pointer()" interfaces were added to SQLite 3.20.0 (2017-08-01):

Questions and confusion quickly arose on the mailing lists about the purpose behind these new interfaces, why they were introduced, and what problem they solve. This essay attempts to answer those questions and clear up the confusion.

2. A Brief History Of Pointer Passing In SQLite

It is sometimes convenient for SQLite extensions to communicate non-SQL values between subcomponents or between the extension and the application. Some examples:

The traditional way of communicating this information was to transform a C-language pointer into a BLOB or a 64-bit integer, then move that BLOB or integer through SQLite using the usual interfaces like sqlite3_bind_blob(), sqlite3_result_blob(), sqlite3_value_blob() or the integer equivalents.

2.1. Upping The Threat Level

Passing around pointers as if they were integers or BLOBs is easy, effective, and works well in an environment where the application components are all friendly toward one another. However, passing pointers as integers and BLOBs allows hostile SQL text to forge invalid pointers that can carry out mischief.

For example, the first argument to the snippet() function is supposed to be a special column of the FTS3 table that contains a pointer to an fts3cursor object that contains information about the current full text search match. That pointer was formerly passed as a BLOB. For example, if the FTS3 table is named "t1" and has a column named "cx", one might write:

SELECT snippet(t1) FROM t1 WHERE cx MATCH $pattern;

But if a hacker is able to run arbitrary SQL, he might run a slightly different query, like this:

SELECT hex(t1) FROM t1 WHERE cx MATCH $pattern;

Because the pointer is passed in the t1 column of the t1 table as a BLOB (in older versions of SQLite), such a query would have shown the value of the pointer in hex. The attacker could then modify that pointer to try to get the snippet() function to modify memory in some other part of the application address space instead of the fts3cursor object it was supposed to be operating on:

SELECT snippet(x'6092310100000000') FROM t1 WHERE cx MATCH $pattern;

Historically, this was not considered a threat. The argument was that if a hostile agent is able to inject arbitrary SQL text into the application, then that agent is already in full control of the application, so letting the hostile agent forge a pointer does not give the agent any new capability.

For most cases, it is true that potential attackers have no way of injecting arbitrary SQL, and so most uses of SQLite are immune to the attack above. But there are some notable exceptions. To wit:

2.2. Preventing Forged Pointers

The first attempt at closing security gaps in pointer passing was to prevent pointer values from being forged. This was accomplished by having the sender attach a subtype to each pointer using sqlite3_result_subtype() and having the receiver verify that subtype using sqlite3_value_subtype() and reject pointers that had an incorrect subtype. Since there is no way to attach a subtype to a result using pure SQL, this prevents pointers from being forged using SQL. The only way to send a pointer is to use C code. If an attacker can set a subtype, then he is also able to forge a pointer without the help of SQLite.

Using subtypes to identify valid pointers prevented the WebSQL exploit. But it turned out to be an incomplete solution.

2.3. Pointer Leaks

The use of subtypes on pointers prevented pointer forgery using pure SQL. But subtypes do nothing to prevent an attacker from reading the values of pointers. In other words, subtypes on pointer values prevent attacks using SQL statements like this:

SELECT snippet(x'6092310100000000') FROM t1 WHERE cx MATCH $pattern;

The BLOB argument to snippet() does not have the correct subtype, so the snippet function ignores it, makes no changes to any data structures, and harmlessly returns NULL.

But the use of subtypes does nothing to prevent the value of a pointer from being read using SQL code like this:

SELECT hex(t1) FROM t1 WHERE cx MATCH $pattern;

What harm could come of that, you ask? The SQLite developers (including this author) wondered the same thing. But then security researchers pointed out that knowledge of pointers can help attackers to circumvent address-space randomization defenses. This is called a "pointer leak". A pointer leak is not itself a vulnerability, but it can aid an attacker in effectively exploiting other vulnerabilities.

3. The New Pointer-Passing Interfaces

Allowing extension components to pass private information to one another securely and without introducing pointer leaks requires new interfaces:

To SQL, the values created by sqlite3_bind_pointer() and sqlite3_result_pointer() are indistinguishable from NULL. An SQL statement that tries to use the hex() function to read the value of a pointer will get an SQL NULL answer. The only way to discover whether or not a value has an associated pointer is to use the sqlite3_value_pointer() interface with the appropriate type string T.

Pointer values read by sqlite3_value_pointer() cannot be generated by pure SQL. Hence, it is not possible for SQL to forge pointers.

Pointer values generated by sqlite3_bind_pointer() and sqlite3_result_pointer() cannot be read by pure SQL. Hence, it is not possible for SQL to leak the value of pointers.

In this way the new pointer-passing interface seems to solve all of the security problems associated with passing pointer values from one extension to another in SQLite.

3.1. Pointer Types

The "pointer type" in the last parameter to sqlite3_bind_pointer(), sqlite3_result_pointer(), and sqlite3_value_pointer() is used to prevent pointers intended for one extension from being redirected to a different extension. For example, without the use of pointer types, an attacker could still get access to pointer information in a system that included both the FTS3 and the CARRAY extension using SQL like this:

SELECT ca.value FROM t1, carray(t1,10) AS ca WHERE cx MATCH $pattern

In the statement above, the FTS3 cursor pointer generated by the MATCH operator is send into the carray() table-valued function instead of its intended recipient snippet(). The carray() function treats the pointer as a pointer to an array of integers and returns each integer one by one, thus leaking the content of the FTS3 cursor object. Since the FTS3 cursor object contains pointers to other objects, the statement above would be a pointer leak.

Except, the statement above does not work, thanks to pointer types. The pointer generated by the MATCH operator has a type of "fts3cursor" but the carray() function expects to receives a pointer of type "carray". Because the pointer type on the sqlite3_result_pointer() does not match the pointer type on the sqlite3_value_pointer() call, sqlite3_value_pointer() returns NULL in carray() and thus signals the CARRAY extension that it has been passed an invalid pointer.

3.1.1. Pointer types are static strings

Pointer types are static strings, which ideally should be string literals embedded directly in the SQLite API call, not parameters passed in from other functions. Consideration was given to using integer values as the pointer type, but static strings provides a much larger name space which reduces the chance of accidental type-name collisions between unrelated extensions.

By "static string", we mean a zero-terminated array of bytes that is fixed and unchanging for the life of the program. In other words, the pointer type string should be a string constant. In constrast, a "dynamic string" is a zero-terminated array of bytes that is held in memory allocated from the heap, and which must be freed to avoid a memory leak. Do not use dynamic strings as the pointer type string.

Multiple commentators have expressed a desire to use dynamic strings for the pointer type, and to have SQLite take ownership of the type strings and to automatically free the type string when it has finished using it. That design is rejected for the following reasons:

  1. The pointer type is not intended to be flexible and dynamic. The pointer type is intended to be a design-time constant. Applications should not synthesize pointer type strings at run-time. Providing support for dynamic pointer type strings would lead developers to misuse the pointer-passing interfaces by creating run-time synthesized pointer type strings. Requiring the pointer type strings to be static encourages developers to do the right thing by choosing fixed pointer type names at design-time and encoding those names as constant strings.

  2. All string values at the SQL level in SQLite are dynamic strings. Requiring type strings to be static makes it difficult to create an application-defined SQL function that can synthesize a pointer of an arbitrary type. We do not want users to create such SQL functions, since such functions would compromise the security of the system. Thus, the requirement to use static strings helps to defend that the integrity of the pointer-passing interfaces against ill-designed SQL functions. The static string requirement is not a perfect defense, since a sophisticated programmer can code around it, and a novice program can simply take the memory leak. But by stating that the pointer type string must be static, we hope to encourage developers who might otherwise use a dynamic string for the pointer type to think more carefully about the problem and avoid introducing security issues.

  3. Having SQLite take ownership of the type strings would impose a performance cost on all applications, even applications that do not use the pointer-passing interfaces. SQLite passes values around as instances of the sqlite3_value object. That object has a destructor, which because of the fact that sqlite3_value objects are used for nearly everything, is invoked frequently. If the destructor needs to check to see if there is a pointer type string that needs to be freed, that is a few extra CPU cycles that need to be burned on each call to the destructor. Those cycles add up. We would be willing to bear the cost of the extra CPU cycles if pointer-passing was a commonly used programming paradigm, but pointer-passing is rare, and so it seems unwise to impose a run-time cost on billions and billions of applications that do not use pointer passing just for convenience of a few applications that do.

If you feel that you need dynamic pointer type strings in your application, that is a strong indicator that you are misusing the pointer-passing interface. Your intended use may be unsafe. Please rethink your design. Determine if you really need to be passing pointers through SQL in the first place. Or perhaps find a different mechanism other than the pointer-passing interfaces described by this article.

3.2. Destructor Functions

The last parameter to the sqlite3_bind_pointer() and sqlite3_result_pointer() routines is a pointer to a procedure used to dispose of the P pointer once SQLite has finished with it. This pointer can be NULL, in which case no destructor is called.

When the D parameter is not NULL, that means that ownership of the pointer is being transferred to SQLite. SQLite will take responsibility for freeing resources associated with the pointer when it has finished using the pointer. If the D parameter is NULL, that means that ownership of the pointer remains with the caller and the caller is responsible for disposing of the pointer.

Note that the destructor function D is for the pointer value P, not for the type string T. The type string T should be a static string with an infinite lifetime.

If ownership of the pointer is passed into SQLite by providing a non-NULL D parameter to sqlite3_bind_pointer() or sqlite3_result_pointer() then the ownership remains with SQLite until the object is destroyed. There is no way to transfer ownership out of SQLite and back into the application again.

4. Restrictions On The Use of Pointer Values

The pointers that piggy-back on SQL NULL values using the sqlite3_bind_pointer(), sqlite3_result_pointer(), and sqlite3_value_pointer() interface are transient and ephemeral. The pointers are never written into the database. The pointers will not survive sorting. The latter fact is why there is no sqlite3_column_pointer() interface, since it is impossible to predict whether or not the query planner will insert a sort operation prior to returning a value from a query, so it would be impossible to know if a pointer value inserted into a query by sqlite3_bind_pointer() or sqlite3_result_pointer() would survive through to the result set.

Pointer values must flow directly from their producer into their consumer, with no intermediate operators or functions. Any transformation of a pointer value destroys the pointer and transforms the value into an ordinary SQL NULL.

Both the pointer and the pointer type parameter to the sqlite3_bind_pointer() and sqlite3_result_pointer() interfaces are "owned" by the caller. In other words, the caller is responsible for ensuring that both values remain valid until after the last access via sqlite3_value_pointer().

5. Summary

Key take-aways from this essay:

  1. The internet is an increasingly hostile place. These day, developers should assume that attackers will find a way to execute arbitrary SQL in an application. Applications should be designed to prevent the execution of arbitrary SQL from escalating into a more severe exploit.

  2. A few SQLite extensions benefit from passing pointers:

    • The FTS3 MATCH operator passes pointers into snippet(), offsets(), and matchinfo().
    • The carray table-valued function needs to accept a pointer to an array of C-language values from the application.
    • The remember() extension needs a pointer to a C-language integer variable in which to remember the value it passes.
    • Applications need to receive a pointer to the "fts5_api" object in order to add extensions, such as custom tokenizers, to the FTS5 extension.
  3. Pointers should never be exchanged by encoding them as some other SQL datatype, such as integers or BLOBs. Instead, use the interfaces designed to facilitate secure pointer passing: sqlite3_bind_pointer(), sqlite3_result_pointer(), and sqlite3_value_pointer().

  4. The use of pointer-passing is an advanced technique that should be used infrequently and cautiously. Pointer-passing should not be used haphazardly or carelessly. Pointer-passing is a sharp tool that can leave deep scars if misused.

  5. The "pointer type" string which is the last parameter to each of the pointer-passing interfaces should be a distinct, application-specific string literal that appears directly in the API call. The pointer type should not be a parameter passed in from a higher-level function.

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