341 lines
10 KiB
Markdown
341 lines
10 KiB
Markdown
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# Pointer Authentication
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## Introduction
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Pointer Authentication is a mechanism by which certain pointers are signed.
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When a pointer gets signed, a cryptographic hash of its value and other values
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(pepper and salt) is stored in unused bits of that pointer.
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Before the pointer is used, it needs to be authenticated, i.e., have its
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signature checked. This prevents pointer values of unknown origin from being
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used to replace the signed pointer value.
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At the IR level, it is represented using:
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* a [set of intrinsics](#intrinsics) (to sign/authenticate pointers)
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* a [call operand bundle](#operand-bundle) (to authenticate called pointers)
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The current implementation leverages the
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[Armv8.3-A PAuth/Pointer Authentication Code](#armv8-3-a-pauth-pointer-authentication-code)
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instructions in the [AArch64 backend](#aarch64-support).
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This support is used to implement the Darwin arm64e ABI, as well as the
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[PAuth ABI Extension to ELF](https://github.com/ARM-software/abi-aa/blob/main/pauthabielf64/pauthabielf64.rst).
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## LLVM IR Representation
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### Intrinsics
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These intrinsics are provided by LLVM to expose pointer authentication
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operations.
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#### '`llvm.ptrauth.sign`'
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##### Syntax:
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```llvm
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declare i64 @llvm.ptrauth.sign(i64 <value>, i32 <key>, i64 <discriminator>)
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```
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##### Overview:
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The '`llvm.ptrauth.sign`' intrinsic signs a raw pointer.
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##### Arguments:
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The `value` argument is the raw pointer value to be signed.
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The `key` argument is the identifier of the key to be used to generate the
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signed value.
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The `discriminator` argument is the additional diversity data to be used as a
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discriminator (an integer, an address, or a blend of the two).
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##### Semantics:
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The '`llvm.ptrauth.sign`' intrinsic implements the `sign`_ operation.
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It returns a signed value.
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If `value` is already a signed value, the behavior is undefined.
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If `value` is not a pointer value for which `key` is appropriate, the
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behavior is undefined.
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#### '`llvm.ptrauth.auth`'
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##### Syntax:
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```llvm
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declare i64 @llvm.ptrauth.auth(i64 <value>, i32 <key>, i64 <discriminator>)
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```
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##### Overview:
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The '`llvm.ptrauth.auth`' intrinsic authenticates a signed pointer.
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##### Arguments:
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The `value` argument is the signed pointer value to be authenticated.
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The `key` argument is the identifier of the key that was used to generate
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the signed value.
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The `discriminator` argument is the additional diversity data to be used as a
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discriminator.
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##### Semantics:
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The '`llvm.ptrauth.auth`' intrinsic implements the `auth`_ operation.
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It returns a raw pointer value.
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If `value` does not have a correct signature for `key` and `discriminator`,
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the intrinsic traps in a target-specific way.
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#### '`llvm.ptrauth.strip`'
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##### Syntax:
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```llvm
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declare i64 @llvm.ptrauth.strip(i64 <value>, i32 <key>)
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```
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##### Overview:
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The '`llvm.ptrauth.strip`' intrinsic strips the embedded signature out of a
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possibly-signed pointer.
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##### Arguments:
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The `value` argument is the signed pointer value to be stripped.
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The `key` argument is the identifier of the key that was used to generate
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the signed value.
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##### Semantics:
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The '`llvm.ptrauth.strip`' intrinsic implements the `strip`_ operation.
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It returns a raw pointer value. It does **not** check that the
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signature is valid.
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`key` should identify a key that is appropriate for `value`, as defined
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by the target-specific [keys](#keys)).
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If `value` is a raw pointer value, it is returned as-is (provided the `key`
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is appropriate for the pointer).
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If `value` is not a pointer value for which `key` is appropriate, the
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behavior is target-specific.
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If `value` is a signed pointer value, but `key` does not identify the
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same key that was used to generate `value`, the behavior is
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target-specific.
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#### '`llvm.ptrauth.resign`'
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##### Syntax:
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```llvm
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declare i64 @llvm.ptrauth.resign(i64 <value>,
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i32 <old key>, i64 <old discriminator>,
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i32 <new key>, i64 <new discriminator>)
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```
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##### Overview:
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The '`llvm.ptrauth.resign`' intrinsic re-signs a signed pointer using
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a different key and diversity data.
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##### Arguments:
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The `value` argument is the signed pointer value to be authenticated.
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The `old key` argument is the identifier of the key that was used to generate
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the signed value.
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The `old discriminator` argument is the additional diversity data to be used
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as a discriminator in the auth operation.
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The `new key` argument is the identifier of the key to use to generate the
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resigned value.
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The `new discriminator` argument is the additional diversity data to be used
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as a discriminator in the sign operation.
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##### Semantics:
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The '`llvm.ptrauth.resign`' intrinsic performs a combined `auth`_ and `sign`_
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operation, without exposing the intermediate raw pointer.
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It returns a signed pointer value.
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If `value` does not have a correct signature for `old key` and
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`old discriminator`, the intrinsic traps in a target-specific way.
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#### '`llvm.ptrauth.sign_generic`'
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##### Syntax:
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```llvm
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declare i64 @llvm.ptrauth.sign_generic(i64 <value>, i64 <discriminator>)
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```
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##### Overview:
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The '`llvm.ptrauth.sign_generic`' intrinsic computes a generic signature of
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arbitrary data.
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##### Arguments:
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The `value` argument is the arbitrary data value to be signed.
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The `discriminator` argument is the additional diversity data to be used as a
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discriminator.
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##### Semantics:
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The '`llvm.ptrauth.sign_generic`' intrinsic computes the signature of a given
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combination of value and additional diversity data.
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It returns a full signature value (as opposed to a signed pointer value, with
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an embedded partial signature).
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As opposed to [`llvm.ptrauth.sign`](#llvm-ptrauth-sign), it does not interpret
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`value` as a pointer value. Instead, it is an arbitrary data value.
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#### '`llvm.ptrauth.blend`'
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##### Syntax:
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```llvm
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declare i64 @llvm.ptrauth.blend(i64 <address discriminator>, i64 <integer discriminator>)
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```
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##### Overview:
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The '`llvm.ptrauth.blend`' intrinsic blends a pointer address discriminator
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with a small integer discriminator to produce a new "blended" discriminator.
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##### Arguments:
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The `address discriminator` argument is a pointer value.
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The `integer discriminator` argument is a small integer, as specified by the
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target.
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##### Semantics:
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The '`llvm.ptrauth.blend`' intrinsic combines a small integer discriminator
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with a pointer address discriminator, in a way that is specified by the target
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implementation.
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### Operand Bundle
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Function pointers used as indirect call targets can be signed when materialized,
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and authenticated before calls. This can be accomplished with the
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[`llvm.ptrauth.auth`](#llvm-ptrauth-auth) intrinsic, feeding its result to
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an indirect call.
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However, that exposes the intermediate, unauthenticated pointer, e.g., if it
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gets spilled to the stack. An attacker can then overwrite the pointer in
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memory, negating the security benefit provided by pointer authentication.
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To prevent that, the `ptrauth` operand bundle may be used: it guarantees that
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the intermediate call target is kept in a register and never stored to memory.
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This hardening benefit is similar to that provided by
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[`llvm.ptrauth.resign`](#llvm-ptrauth-resign)).
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Concretely:
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```llvm
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define void @f(void ()* %fp) {
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call void %fp() [ "ptrauth"(i32 <key>, i64 <data>) ]
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ret void
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}
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```
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is functionally equivalent to:
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```llvm
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define void @f(void ()* %fp) {
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%fp_i = ptrtoint void ()* %fp to i64
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%fp_auth = call i64 @llvm.ptrauth.auth(i64 %fp_i, i32 <key>, i64 <data>)
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%fp_auth_p = inttoptr i64 %fp_auth to void ()*
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call void %fp_auth_p()
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ret void
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}
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```
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but with the added guarantee that `%fp_i`, `%fp_auth`, and `%fp_auth_p`
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are not stored to (and reloaded from) memory.
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## AArch64 Support
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AArch64 is currently the only architecture with full support of the pointer
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authentication primitives, based on Armv8.3-A instructions.
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### Armv8.3-A PAuth Pointer Authentication Code
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The Armv8.3-A architecture extension defines the PAuth feature, which provides
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support for instructions that manipulate Pointer Authentication Codes (PAC).
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#### Keys
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5 keys are supported by the PAuth feature.
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Of those, 4 keys are interchangeably usable to specify the key used in IR
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constructs:
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* `ASIA`/`ASIB` are instruction keys (encoded as respectively 0 and 1).
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* `ASDA`/`ASDB` are data keys (encoded as respectively 2 and 3).
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`ASGA` is a special key that cannot be explicitly specified, and is only ever
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used implicitly, to implement the
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[`llvm.ptrauth.sign_generic`](#llvm-ptrauth-sign-generic) intrinsic.
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#### Instructions
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The IR [Intrinsics](#intrinsics) described above map onto these
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instructions as such:
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* [`llvm.ptrauth.sign`](#llvm-ptrauth-sign): `PAC{I,D}{A,B}{Z,SP,}`
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* [`llvm.ptrauth.auth`](#llvm-ptrauth-auth): `AUT{I,D}{A,B}{Z,SP,}`
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* [`llvm.ptrauth.strip`](#llvm-ptrauth-strip): `XPAC{I,D}`
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* [`llvm.ptrauth.blend`](#llvm-ptrauth-blend): The semantics of the blend
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operation are specified by the ABI. In both the ELF PAuth ABI Extension and
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arm64e, it's a `MOVK` into the high 16 bits. Consequently, this limits
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the width of the integer discriminator used in blends to 16 bits.
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* [`llvm.ptrauth.sign_generic`](#llvm-ptrauth-sign-generic): `PACGA`
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* [`llvm.ptrauth.resign`](#llvm-ptrauth-resign): `AUT*+PAC*`. These are
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represented as a single pseudo-instruction in the backend to guarantee that
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the intermediate raw pointer value is not spilled and attackable.
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#### Assembly Representation
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At the assembly level, authenticated relocations are represented
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using the `@AUTH` modifier:
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```asm
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.quad _target@AUTH(<key>,<discriminator>[,addr])
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```
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where:
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* `key` is the Armv8.3-A key identifier (`ia`, `ib`, `da`, `db`)
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* `discriminator` is the 16-bit unsigned discriminator value
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* `addr` signifies that the authenticated pointer is address-discriminated
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(that is, that the relocation's target address is to be blended into the
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`discriminator` before it is used in the sign operation.
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For example:
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```asm
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_authenticated_reference_to_sym:
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.quad _sym@AUTH(db,0)
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_authenticated_reference_to_sym_addr_disc:
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.quad _sym@AUTH(ia,12,addr)
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```
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#### ELF Object File Representation
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At the object file level, authenticated relocations are represented
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using the `R_AARCH64_AUTH_ABS64` relocation kind (with value `0xE100`).
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The signing schema is encoded in the place of relocation to be applied
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as follows:
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```
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| 63 | 62 | 61:60 | 59:48 | 47:32 | 31:0 |
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| ----------------- | -------- | -------- | -------- | ------------- | ------------------- |
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| address diversity | reserved | key | reserved | discriminator | reserved for addend |
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```
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