[llvm] 68854f4 - [IR] Define ptrauth intrinsics.
Ahmed Bougacha via llvm-commits
llvm-commits at lists.llvm.org
Sun Nov 14 08:05:44 PST 2021
Author: Ahmed Bougacha
Date: 2021-11-14T07:59:00-08:00
New Revision: 68854f4e572afec802299e36b2db71dfc4cf2f27
URL: https://github.com/llvm/llvm-project/commit/68854f4e572afec802299e36b2db71dfc4cf2f27
DIFF: https://github.com/llvm/llvm-project/commit/68854f4e572afec802299e36b2db71dfc4cf2f27.diff
LOG: [IR] Define ptrauth intrinsics.
This defines the new `@llvm.ptrauth.` pointer authentication intrinsics:
sign, auth, strip, blend, and sign_generic, documented in PointerAuth.md.
Pointer Authentication is a mechanism by which certain pointers are
signed. When a pointer gets signed, a cryptographic hash of its value
and other values (pepper and salt) is stored in unused bits of that
pointer.
Before the pointer is used, it needs to be authenticated, i.e., have its
signature checked. This prevents pointer values of unknown origin from
being used to replace the signed pointer value.
sign and auth provide the core operations. strip removes the ptrauth
bits from a signed pointer without checking them. sign_generic allows
signing non-pointer values. Finally, blend combines salt values
("discriminators") to derive more targeted and less reusable ones.
In later patches, we implement primary backend support for these
intrinsics using the AArch64 PAuth feature, and build on that to
implement the arm64e Darwin ABI and ELF PAuth ABI Extension in clang.
For more details, see the docs page, as well as our llvm-dev RFC:
http://lists.llvm.org/pipermail/llvm-dev/2019-October/136091.html
or our 2019 Developers' Meeting talk.
Differential Revision: https://reviews.llvm.org/D90868
Added:
llvm/docs/PointerAuth.md
Modified:
llvm/docs/LangRef.rst
llvm/docs/Reference.rst
llvm/include/llvm/IR/Intrinsics.td
Removed:
################################################################################
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 3a5bc9c199d55..7540a7776b57f 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -17616,6 +17616,13 @@ The LLVM exception handling intrinsics (which all start with
``llvm.eh.`` prefix), are described in the `LLVM Exception
Handling <ExceptionHandling.html#format-common-intrinsics>`_ document.
+Pointer Authentication Intrinsics
+---------------------------------
+
+The LLVM pointer authentication intrinsics (which all start with
+``llvm.ptrauth.`` prefix), are described in the `Pointer Authentication
+<PointerAuth.html#intrinsics>`_ document.
+
.. _int_trampoline:
Trampoline Intrinsics
diff --git a/llvm/docs/PointerAuth.md b/llvm/docs/PointerAuth.md
new file mode 100644
index 0000000000000..d62d051eca3d2
--- /dev/null
+++ b/llvm/docs/PointerAuth.md
@@ -0,0 +1,260 @@
+# Pointer Authentication
+
+## Introduction
+
+Pointer Authentication is a mechanism by which certain pointers are signed.
+When a pointer gets signed, a cryptographic hash of its value and other values
+(pepper and salt) is stored in unused bits of that pointer.
+
+Before the pointer is used, it needs to be authenticated, i.e., have its
+signature checked. This prevents pointer values of unknown origin from being
+used to replace the signed pointer value.
+
+At the IR level, it is represented using a [set of intrinsics](#intrinsics)
+(to sign/authenticate pointers).
+
+The current implementation leverages the
+[Armv8.3-A PAuth/Pointer Authentication Code](#armv8-3-a-pauth-pointer-authentication-code)
+instructions in the [AArch64 backend](#aarch64-support).
+This support is used to implement the Darwin arm64e ABI, as well as the
+[PAuth ABI Extension to ELF](https://github.com/ARM-software/abi-aa/blob/main/pauthabielf64/pauthabielf64.rst).
+
+
+## LLVM IR Representation
+
+### Intrinsics
+
+These intrinsics are provided by LLVM to expose pointer authentication
+operations.
+
+
+#### '``llvm.ptrauth.sign``'
+
+##### Syntax:
+
+```llvm
+declare i64 @llvm.ptrauth.sign(i64 <value>, i32 <key>, i64 <discriminator>)
+```
+
+##### Overview:
+
+The '``llvm.ptrauth.sign``' intrinsic signs a raw pointer.
+
+
+##### Arguments:
+
+The ``value`` argument is the raw pointer value to be signed.
+The ``key`` argument is the identifier of the key to be used to generate the
+signed value.
+The ``discriminator`` argument is the additional diversity data to be used as a
+discriminator (an integer, an address, or a blend of the two).
+
+##### Semantics:
+
+The '``llvm.ptrauth.sign``' intrinsic implements the `sign`_ operation.
+It returns a signed value.
+
+If ``value`` is already a signed value, the behavior is undefined.
+
+If ``value`` is not a pointer value for which ``key`` is appropriate, the
+behavior is undefined.
+
+
+#### '``llvm.ptrauth.auth``'
+
+##### Syntax:
+
+```llvm
+declare i64 @llvm.ptrauth.auth(i64 <value>, i32 <key>, i64 <discriminator>)
+```
+
+##### Overview:
+
+The '``llvm.ptrauth.auth``' intrinsic authenticates a signed pointer.
+
+##### Arguments:
+
+The ``value`` argument is the signed pointer value to be authenticated.
+The ``key`` argument is the identifier of the key that was used to generate
+the signed value.
+The ``discriminator`` argument is the additional diversity data to be used as a
+discriminator.
+
+##### Semantics:
+
+The '``llvm.ptrauth.auth``' intrinsic implements the `auth`_ operation.
+It returns a raw pointer value.
+If ``value`` does not have a correct signature for ``key`` and ``discriminator``,
+the intrinsic traps in a target-specific way.
+
+
+#### '``llvm.ptrauth.strip``'
+
+##### Syntax:
+
+```llvm
+declare i64 @llvm.ptrauth.strip(i64 <value>, i32 <key>)
+```
+
+##### Overview:
+
+The '``llvm.ptrauth.strip``' intrinsic strips the embedded signature out of a
+possibly-signed pointer.
+
+
+##### Arguments:
+
+The ``value`` argument is the signed pointer value to be stripped.
+The ``key`` argument is the identifier of the key that was used to generate
+the signed value.
+
+##### Semantics:
+
+The '``llvm.ptrauth.strip``' intrinsic implements the `strip`_ operation.
+It returns a raw pointer value. It does **not** check that the
+signature is valid.
+
+``key`` should identify a key that is appropriate for ``value``, as defined
+by the target-specific [keys](#key)).
+
+If ``value`` is a raw pointer value, it is returned as-is (provided the ``key``
+is appropriate for the pointer).
+
+If ``value`` is not a pointer value for which ``key`` is appropriate, the
+behavior is target-specific.
+
+If ``value`` is a signed pointer value, but ``key`` does not identify the
+same key that was used to generate ``value``, the behavior is
+target-specific.
+
+
+#### '``llvm.ptrauth.resign``'
+
+##### Syntax:
+
+```llvm
+declare i64 @llvm.ptrauth.resign(i64 <value>,
+ i32 <old key>, i64 <old discriminator>,
+ i32 <new key>, i64 <new discriminator>)
+```
+
+##### Overview:
+
+The '``llvm.ptrauth.resign``' intrinsic re-signs a signed pointer using
+a
diff erent key and diversity data.
+
+##### Arguments:
+
+The ``value`` argument is the signed pointer value to be authenticated.
+The ``old key`` argument is the identifier of the key that was used to generate
+the signed value.
+The ``old discriminator`` argument is the additional diversity data to be used
+as a discriminator in the auth operation.
+The ``new key`` argument is the identifier of the key to use to generate the
+resigned value.
+The ``new discriminator`` argument is the additional diversity data to be used
+as a discriminator in the sign operation.
+
+##### Semantics:
+
+The '``llvm.ptrauth.resign``' intrinsic performs a combined `auth`_ and `sign`_
+operation, without exposing the intermediate raw pointer.
+It returns a signed pointer value.
+If ``value`` does not have a correct signature for ``old key`` and
+``old discriminator``, the intrinsic traps in a target-specific way.
+
+#### '``llvm.ptrauth.sign_generic``'
+
+##### Syntax:
+
+```llvm
+declare i64 @llvm.ptrauth.sign_generic(i64 <value>, i64 <discriminator>)
+```
+
+##### Overview:
+
+The '``llvm.ptrauth.sign_generic``' intrinsic computes a generic signature of
+arbitrary data.
+
+##### Arguments:
+
+The ``value`` argument is the arbitrary data value to be signed.
+The ``discriminator`` argument is the additional diversity data to be used as a
+discriminator.
+
+##### Semantics:
+
+The '``llvm.ptrauth.sign_generic``' intrinsic computes the signature of a given
+combination of value and additional diversity data.
+
+It returns a full signature value (as opposed to a signed pointer value, with
+an embedded partial signature).
+
+As opposed to [``llvm.ptrauth.sign``](#llvm-ptrauth-sign), it does not interpret
+``value`` as a pointer value. Instead, it is an arbitrary data value.
+
+
+#### '``llvm.ptrauth.blend``'
+
+##### Syntax:
+
+```llvm
+declare i64 @llvm.ptrauth.blend(i64 <address discriminator>, i64 <integer discriminator>)
+```
+
+##### Overview:
+
+The '``llvm.ptrauth.blend``' intrinsic blends a pointer address discriminator
+with a small integer discriminator to produce a new "blended" discriminator.
+
+##### Arguments:
+
+The ``address discriminator`` argument is a pointer value.
+The ``integer discriminator`` argument is a small integer, as specified by the
+target.
+
+##### Semantics:
+
+The '``llvm.ptrauth.blend``' intrinsic combines a small integer discriminator
+with a pointer address discriminator, in a way that is specified by the target
+implementation.
+
+
+## AArch64 Support
+
+AArch64 is currently the only architecture with full support of the pointer
+authentication primitives, based on Armv8.3-A instructions.
+
+### Armv8.3-A PAuth Pointer Authentication Code
+
+The Armv8.3-A architecture extension defines the PAuth feature, which provides
+support for instructions that manipulate Pointer Authentication Codes (PAC).
+
+#### Keys
+
+5 keys are supported by the PAuth feature.
+
+Of those, 4 keys are interchangeably usable to specify the key used in IR
+constructs:
+* ``ASIA``/``ASIB`` are instruction keys (encoded as respectively 0 and 1).
+* ``ASDA``/``ASDB`` are data keys (encoded as respectively 2 and 3).
+
+``ASGA`` is a special key that cannot be explicitly specified, and is only ever
+used implicitly, to implement the
+[``llvm.ptrauth.sign_generic``](#llvm-ptrauth-sign-generic) intrinsic.
+
+#### Instructions
+
+The IR [Intrinsics](#intrinsics) described above map onto these
+instructions as such:
+* [``llvm.ptrauth.sign``](#llvm-ptrauth-sign): ``PAC{I,D}{A,B}{Z,SP,}``
+* [``llvm.ptrauth.auth``](#llvm-ptrauth-auth): ``AUT{I,D}{A,B}{Z,SP,}``
+* [``llvm.ptrauth.strip``](#llvm-ptrauth-strip): ``XPAC{I,D}``
+* [``llvm.ptrauth.blend``](#llvm-ptrauth-blend): The semantics of the blend
+ operation are specified by the ABI. In both the ELF PAuth ABI Extension and
+ arm64e, it's a ``MOVK`` into the high 16 bits. Consequently, this limits
+ the width of the integer discriminator used in blends to 16 bits.
+* [``llvm.ptrauth.sign_generic``](#llvm-ptrauth-sign-generic): ``PACGA``
+* [``llvm.ptrauth.resign``](#llvm-ptrauth-resign): ``AUT*+PAC*``. These are
+ represented as a single pseudo-instruction in the backend to guarantee that
+ the intermediate raw pointer value is not spilled and attackable.
diff --git a/llvm/docs/Reference.rst b/llvm/docs/Reference.rst
index 662f1ec650fcf..d10fc8f23f735 100644
--- a/llvm/docs/Reference.rst
+++ b/llvm/docs/Reference.rst
@@ -34,6 +34,7 @@ LLVM and API reference documentation.
MIRLangRef
OptBisect
PDB/index
+ PointerAuth
ScudoHardenedAllocator
MemTagSanitizer
Security
@@ -208,5 +209,9 @@ Additional Topics
:doc:`Coroutines`
LLVM support for coroutines.
+:doc:`PointerAuth`
+ A description of pointer authentication, its LLVM IR representation, and its
+ support in the backend.
+
:doc:`YamlIO`
A reference guide for using LLVM's YAML I/O library.
diff --git a/llvm/include/llvm/IR/Intrinsics.td b/llvm/include/llvm/IR/Intrinsics.td
index 9c51a2f2b7ea3..637e6d8f6cf5f 100644
--- a/llvm/include/llvm/IR/Intrinsics.td
+++ b/llvm/include/llvm/IR/Intrinsics.td
@@ -1850,6 +1850,61 @@ def int_experimental_vector_splice : DefaultAttrsIntrinsic<[llvm_anyvector_ty],
llvm_i32_ty],
[IntrNoMem, ImmArg<ArgIndex<2>>]>;
+
+//===----------------- Pointer Authentication Intrinsics ------------------===//
+//
+
+// Sign an unauthenticated pointer using the specified key and discriminator,
+// passed in that order.
+// Returns the first argument, with some known bits replaced with a signature.
+def int_ptrauth_sign : Intrinsic<[llvm_i64_ty],
+ [llvm_i64_ty, llvm_i32_ty, llvm_i64_ty],
+ [IntrNoMem, ImmArg<ArgIndex<1>>]>;
+
+// Authenticate a signed pointer, using the specified key and discriminator.
+// Returns the first argument, with the signature bits removed.
+// The signature must be valid.
+def int_ptrauth_auth : Intrinsic<[llvm_i64_ty],
+ [llvm_i64_ty, llvm_i32_ty, llvm_i64_ty],
+ [IntrNoMem,ImmArg<ArgIndex<1>>]>;
+
+// Authenticate a signed pointer and resign it.
+// The second (key) and third (discriminator) arguments specify the signing
+// schema used for authenticating.
+// The fourth and fifth arguments specify the schema used for signing.
+// The signature must be valid.
+// This is a combined form of @llvm.ptrauth.sign and @llvm.ptrauth.auth, with
+// an additional integrity guarantee on the intermediate value.
+def int_ptrauth_resign : Intrinsic<[llvm_i64_ty],
+ [llvm_i64_ty, llvm_i32_ty, llvm_i64_ty,
+ llvm_i32_ty, llvm_i64_ty],
+ [IntrNoMem, ImmArg<ArgIndex<1>>,
+ ImmArg<ArgIndex<3>>]>;
+
+// Strip the embedded signature out of a signed pointer.
+// The second argument specifies the key.
+// This behaves like @llvm.ptrauth.auth, but doesn't require the signature to
+// be valid.
+def int_ptrauth_strip : Intrinsic<[llvm_i64_ty],
+ [llvm_i64_ty, llvm_i32_ty],
+ [IntrNoMem, ImmArg<ArgIndex<1>>]>;
+
+// Blend a small integer discriminator with an address discriminator, producing
+// a new discriminator value.
+def int_ptrauth_blend : Intrinsic<[llvm_i64_ty],
+ [llvm_i64_ty, llvm_i64_ty],
+ [IntrNoMem]>;
+
+// Compute the signature of a value, using a given discriminator.
+// This
diff ers from @llvm.ptrauth.sign in that it doesn't embed the computed
+// signature in the pointer, but instead returns the signature as a value.
+// That allows it to be used to sign non-pointer data: in that sense, it is
+// generic. There is no generic @llvm.ptrauth.auth: instead, the signature
+// can be computed using @llvm.ptrauth.sign_generic, and compared with icmp.
+def int_ptrauth_sign_generic : Intrinsic<[llvm_i64_ty],
+ [llvm_i64_ty, llvm_i64_ty],
+ [IntrNoMem]>;
+
//===----------------------------------------------------------------------===//
//===----------------------------------------------------------------------===//
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