Pointer Authentication

Introduction

Pointer authentication is a technology which offers strong probabilistic protection against exploiting a broad class of memory bugs to take control of program execution. When adopted consistently in a language ABI, it provides a form of relatively fine-grained control flow integrity (CFI) check that resists both return-oriented programming (ROP) and jump-oriented programming (JOP) attacks.

While pointer authentication can be implemented purely in software, direct hardware support (e.g. as provided by Armv8.3 PAuth) can dramatically improve performance and code size. Similarly, while pointer authentication can be implemented on any architecture, taking advantage of the (typically) excess addressing range of a target with 64-bit pointers minimizes the impact on memory performance and can allow interoperation with existing code (by disabling pointer authentication dynamically). This document will generally attempt to present the pointer authentication feature independent of any hardware implementation or ABI. Considerations that are implementation-specific are clearly identified throughout.

Note that there are several different terms in use:

• Pointer authentication is a target-independent language technology.

• PAuth (sometimes referred to as PAC, for Pointer Authentication Codes) is an AArch64 architecture extension that provides hardware support for pointer authentication. Additional extensions either modify some of the PAuth instruction behavior (notably FPAC), or provide new instruction variants (PAuth_LR).

• Armv8.3 is an AArch64 architecture revision that makes PAuth mandatory.

• arm64e is a specific ABI (not yet fully stable) for implementing pointer authentication using PAuth on certain Apple operating systems.

This document serves four purposes:

• It describes the basic ideas of pointer authentication.

• It documents several language extensions that are useful on targets using pointer authentication.

• It will eventually present a theory of operation for the security mitigation, describing the basic requirements for correctness, various weaknesses in the mechanism, and ways in which programmers can strengthen its protections (including recommendations for language implementors).

• It will eventually document the language ABIs currently used for C, C++, Objective-C, and Swift on arm64e, although these are not yet stable on any target.

Basic Concepts

The simple address of an object or function is a raw pointer. A raw pointer can be signed to produce a signed pointer. A signed pointer can be then authenticated in order to verify that it was validly signed and extract the original raw pointer. These terms reflect the most likely implementation technique: computing and storing a cryptographic signature along with the pointer.

An abstract signing key is a name which refers to a secret key which is used to sign and authenticate pointers. The concrete key value for a particular name is consistent throughout a process.

A discriminator is an arbitrary value used to diversify signed pointers so that one validly-signed pointer cannot simply be copied over another. A discriminator is simply opaque data of some implementation-defined size that is included in the signature as a salt (see Discriminators for details.)

Nearly all aspects of pointer authentication use just these two primary operations:

• sign(raw_pointer, key, discriminator) produces a signed pointer given a raw pointer, an abstract signing key, and a discriminator.

• auth(signed_pointer, key, discriminator) produces a raw pointer given a signed pointer, an abstract signing key, and a discriminator.

auth(sign(raw_pointer, key, discriminator), key, discriminator) must succeed and produce raw_pointer. auth applied to a value that was ultimately produced in any other way is expected to fail, which halts the program either:

• immediately, on implementations that enforce auth success (e.g., when using compiler-generated auth failure checks, or Armv8.3 with the FPAC extension), or

• when the resulting pointer value is used, on implementations that don’t.

However, regardless of the implementation’s handling of auth failures, it is permitted for auth to fail to detect that a signed pointer was not produced in this way, in which case it may return anything; this is what makes pointer authentication a probabilistic mitigation rather than a perfect one.

There are two secondary operations which are required only to implement certain intrinsics in <ptrauth.h>:

• strip(signed_pointer, key) produces a raw pointer given a signed pointer and a key without verifying its validity, unlike auth. This is useful for certain kinds of tooling, such as crash backtraces; it should generally not be used in the basic language ABI except in very careful ways.

• sign_generic(value) produces a cryptographic signature for arbitrary data, not necessarily a pointer. This is useful for efficiently verifying that non-pointer data has not been tampered with.

Whenever any of these operations is called for, the key value must be known statically. This is because the layout of a signed pointer may vary according to the signing key. (For example, in Armv8.3, the layout of a signed pointer depends on whether Top Byte Ignore (TBI) is enabled, which can be set independently for I and D keys.)

Note for API designers and language implementors

These are the primitive operations of pointer authentication, provided for clarity of description. They are not suitable either as high-level interfaces or as primitives in a compiler IR because they expose raw pointers. Raw pointers require special attention in the language implementation to avoid the accidental creation of exploitable code sequences.

The following details are all implementation-defined:

• the nature of a signed pointer

• the size of a discriminator

• the number and nature of the signing keys

• the implementation of the sign, auth, strip, and sign_generic operations

While the use of the terms “sign” and “signed pointer” suggest the use of a cryptographic signature, other implementations may be possible. See Alternative implementations for an exploration of implementation options.

Implementation example: Armv8.3

Readers may find it helpful to know how these terms map to Armv8.3 PAuth:

• A signed pointer is a pointer with a signature stored in the otherwise-unused high bits. The kernel configures the address width based on the system’s addressing needs, and enables TBI for I or D keys as needed. The bits above the address bits and below the TBI bits (if enabled) are unused. The signature width then depends on this addressing configuration.

• A discriminator is a 64-bit integer. Constant discriminators are 16-bit integers. Blending a constant discriminator into an address consists of replacing the top 16 bits of the pointer containing the address with the constant. Pointers used for blending purposes should only have address bits, since higher bits will be at least partially overwritten with the constant discriminator.

• There are five 128-bit signing-key registers, each of which can only be directly read or set by privileged code. Of these, four are used for signing pointers, and the fifth is used only for sign_generic. The key data is simply a pepper added to the hash, not an encryption key, and so can be initialized using random data.

• sign computes a cryptographic hash of the pointer, discriminator, and signing key, and stores it in the high bits as the signature. auth removes the signature, computes the same hash, and compares the result with the stored signature. strip removes the signature without authenticating it. While aut* instructions do not themselves trap on failure in Armv8.3 PAuth, they do with the later optional FPAC extension. An implementation can also choose to emulate this trapping behavior by emitting additional instructions around aut*.

• sign_generic corresponds to the pacga instruction, which takes two 64-bit values and produces a 64-bit cryptographic hash. Implementations of this instruction are not required to produce meaningful data in all bits of the result.

Discriminators

A discriminator is arbitrary extra data which alters the signature calculated for a pointer. When two pointers are signed differently — either with different keys or with different discriminators — an attacker cannot simply replace one pointer with the other.

To use standard cryptographic terminology, a discriminator acts as a salt in the signing of a pointer, and the key data acts as a pepper. That is, both the discriminator and key data are ultimately just added as inputs to the signing algorithm along with the pointer, but they serve significantly different roles. The key data is a common secret added to every signature, whereas the discriminator is a value that can be derived from the context in which a specific pointer is signed. However, unlike a password salt, it’s important that discriminators be independently derived from the circumstances of the signing; they should never simply be stored alongside a pointer. Discriminators are then re-derived in authentication operations.

The intrinsic interface in <ptrauth.h> allows an arbitrary discriminator value to be provided, but can only be used when running normal code. The discriminators used by language ABIs must be restricted to make it feasible for the loader to sign pointers stored in global memory without needing excessive amounts of metadata. Under these restrictions, a discriminator may consist of either or both of the following:

• The address at which the pointer is stored in memory. A pointer signed with a discriminator which incorporates its storage address is said to have address diversity. In general, using address diversity means that a pointer cannot be reliably copied by an attacker to or from a different memory location. However, an attacker may still be able to attack a larger call sequence if they can alter the address through which the pointer is accessed. Furthermore, some situations cannot use address diversity because of language or other restrictions.

• A constant integer, called a constant discriminator. A pointer signed with a non-zero constant discriminator is said to have constant diversity. If the discriminator is specific to a single declaration, it is said to have declaration diversity; if the discriminator is specific to a type of value, it is said to have type diversity. For example, C++ v-tables on arm64e sign their component functions using a hash of their method names and signatures, which provides declaration diversity; similarly, C++ member function pointers sign their invocation functions using a hash of the member pointer type, which provides type diversity.

The implementation may need to restrict constant discriminators to be significantly smaller than the full size of a discriminator. For example, on arm64e, constant discriminators are only 16-bit values. This is believed to not significantly weaken the mitigation, since collisions remain uncommon.

The algorithm for blending a constant discriminator with a storage address is implementation-defined.

Signing Schemas

Correct use of pointer authentication requires the signing code and the authenticating code to agree about the signing schema for the pointer:

• the abstract signing key with which the pointer should be signed and

• an algorithm for computing the discriminator.

As described in the section above on Discriminators, in most situations, the discriminator is produced by taking a constant discriminator and optionally blending it with the storage address of the pointer. In these situations, the signing schema breaks down even more simply:

• the abstract signing key,

• a constant discriminator, and

• whether to use address diversity.

It is important that the signing schema be independently derived at all signing and authentication sites. Preferably, the schema should be hard-coded everywhere it is needed, but at the very least, it must not be derived by inspecting information stored along with the pointer.

Language Features

There is currently one main pointer authentication language feature:

• The language provides the <ptrauth.h> intrinsic interface for manually signing and authenticating pointers in code. These can be used in circumstances where very specific behavior is required.

Language Extensions

Feature Testing

Whether the current target uses pointer authentication can be tested for with a number of different tests.

• __has_feature(ptrauth_intrinsics) is true if <ptrauth.h> provides its normal interface. This may be true even on targets where pointer authentication is not enabled by default.

<ptrauth.h>

This header defines the following types and operations:

ptrauth_key

This enum is the type of abstract signing keys. In addition to defining the set of implementation-specific signing keys (for example, Armv8.3 defines ptrauth_key_asia), it also defines some portable aliases for those keys. For example, ptrauth_key_function_pointer is the key generally used for C function pointers, which will generally be suitable for other function-signing schemas.

In all the operation descriptions below, key values must be constant values corresponding to one of the implementation-specific abstract signing keys from this enum.

ptrauth_extra_data_t

This is a typedef of a standard integer type of the correct size to hold a discriminator value.

In the signing and authentication operation descriptions below, discriminator values must have either pointer type or integer type. If the discriminator is an integer, it will be coerced to ptrauth_extra_data_t.

ptrauth_blend_discriminator

ptrauth_blend_discriminator(pointer, integer)

Produce a discriminator value which blends information from the given pointer and the given integer.

Implementations may ignore some bits from each value, which is to say, the blending algorithm may be chosen for speed and convenience over theoretical strength as a hash-combining algorithm. For example, arm64e simply overwrites the high 16 bits of the pointer with the low 16 bits of the integer, which can be done in a single instruction with an immediate integer.

pointer must have pointer type, and integer must have integer type. The result has type ptrauth_extra_data_t.

ptrauth_string_discriminator

ptrauth_string_discriminator(string)

Compute a constant discriminator from the given string.

string must be a string literal of char character type. The result has type ptrauth_extra_data_t.

The result value is never zero and always within range for both the __ptrauth qualifier and ptrauth_blend_discriminator.

This can be used in constant expressions.

ptrauth_strip

ptrauth_strip(signedPointer, key)

Given that signedPointer matches the layout for signed pointers signed with the given key, extract the raw pointer from it. This operation does not trap and cannot fail, even if the pointer is not validly signed.

ptrauth_sign_constant

ptrauth_sign_constant(pointer, key, discriminator)

Return a signed pointer for a constant address in a manner which guarantees a non-attackable sequence.

pointer must be a constant expression of pointer type which evaluates to a non-null pointer. key must be a constant expression of type ptrauth_key. discriminator must be a constant expression of pointer or integer type; if an integer, it will be coerced to ptrauth_extra_data_t. The result will have the same type as pointer.

This can be used in constant expressions.

ptrauth_sign_unauthenticated

ptrauth_sign_unauthenticated(pointer, key, discriminator)

Produce a signed pointer for the given raw pointer without applying any authentication or extra treatment. This operation is not required to have the same behavior on a null pointer that the language implementation would.

This is a treacherous operation that can easily result in signing oracles. Programs should use it seldom and carefully.

ptrauth_auth_and_resign

ptrauth_auth_and_resign(pointer, oldKey, oldDiscriminator, newKey, newDiscriminator)

Authenticate that pointer is signed with oldKey and oldDiscriminator and then resign the raw-pointer result of that authentication with newKey and newDiscriminator.

pointer must have pointer type. The result will have the same type as pointer. This operation is not required to have the same behavior on a null pointer that the language implementation would.

The code sequence produced for this operation must not be directly attackable. However, if the discriminator values are not constant integers, their computations may still be attackable. In the future, Clang should be enhanced to guaranteed non-attackability if these expressions are safely-derived.

ptrauth_auth_data

ptrauth_auth_data(pointer, key, discriminator)

Authenticate that pointer is signed with key and discriminator and remove the signature.

pointer must have object pointer type. The result will have the same type as pointer. This operation is not required to have the same behavior on a null pointer that the language implementation would.

In the future when Clang makes safe derivation guarantees, the result of this operation should be considered safely-derived.

ptrauth_sign_generic_data

ptrauth_sign_generic_data(value1, value2)

Computes a signature for the given pair of values, incorporating a secret signing key.

This operation can be used to verify that arbitrary data has not been tampered with by computing a signature for the data, storing that signature, and then repeating this process and verifying that it yields the same result. This can be reasonably done in any number of ways; for example, a library could compute an ordinary checksum of the data and just sign the result in order to get the tamper-resistance advantages of the secret signing key (since otherwise an attacker could reliably overwrite both the data and the checksum).

value1 and value2 must be either pointers or integers. If the integers are larger than uintptr_t then data not representable in uintptr_t may be discarded.

The result will have type ptrauth_generic_signature_t, which is an integer type. Implementations are not required to make all bits of the result equally significant; in particular, some implementations are known to not leave meaningful data in the low bits.

Alternative Implementations

Signature Storage

It is not critical for the security of pointer authentication that the signature be stored “together” with the pointer, as it is in Armv8.3. An implementation could just as well store the signature in a separate word, so that the sizeof a signed pointer would be larger than the sizeof a raw pointer.

Storing the signature in the high bits, as Armv8.3 does, has several trade-offs:

• Disadvantage: there are substantially fewer bits available for the signature, weakening the mitigation by making it much easier for an attacker to simply guess the correct signature.

• Disadvantage: future growth of the address space will necessarily further weaken the mitigation.

• Advantage: memory layouts don’t change, so it’s possible for pointer-authentication-enabled code (for example, in a system library) to efficiently interoperate with existing code, as long as pointer authentication can be disabled dynamically.

• Advantage: the size of a signed pointer doesn’t grow, which might significantly increase memory requirements, code size, and register pressure.

• Advantage: the size of a signed pointer is the same as a raw pointer, so generic APIs which work in types like void * (such as dlsym) can still return signed pointers. This means that clients of these APIs will not require insecure code in order to correctly receive a function pointer.

Hashing vs. Encrypting Pointers

Armv8.3 implements sign by computing a cryptographic hash and storing that in the spare bits of the pointer. This means that there are relatively few possible values for the valid signed pointer, since the bits corresponding to the raw pointer are known. Together with an auth oracle, this can make it computationally feasible to discover the correct signature with brute force. (The implementation should of course endeavor not to introduce auth oracles, but this can be difficult, and attackers can be devious.)

If the implementation can instead encrypt the pointer during sign and decrypt it during auth, this brute-force attack becomes far less feasible, even with an auth oracle. However, there are several problems with this idea:

• It’s unclear whether this kind of encryption is even possible without increasing the storage size of a signed pointer. If the storage size can be increased, brute-force atacks can be equally well mitigated by simply storing a larger signature.

• It would likely be impossible to implement a strip operation, which might make debuggers and other out-of-process tools far more difficult to write, as well as generally making primitive debugging more challenging.

• Implementations can benefit from being able to extract the raw pointer immediately from a signed pointer. An Armv8.3 processor executing an auth-and-load instruction can perform the load and auth in parallel; a processor which instead encrypted the pointer would be forced to perform these operations serially.