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 (to sign/authenticate pointers)
- a call operand bundle (to authenticate called pointers)
The current implementation leverages the Armv8.3-A PAuth/Pointer Authentication Code instructions in the AArch64 backend. This support is used to implement the Darwin arm64e ABI, as well as the PAuth ABI Extension to ELF.
LLVM IR Representation¶
Intrinsics¶
These intrinsics are provided by LLVM to expose pointer authentication operations.
‘llvm.ptrauth.sign
’¶
Syntax:¶
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:¶
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:¶
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).
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:¶
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 different 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:¶
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:¶
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.
Operand Bundle¶
Function pointers used as indirect call targets can be signed when
materialized, and authenticated before calls. This can be accomplished
with the `llvm.ptrauth.auth
<#llvm-ptrauth-auth>`__ intrinsic,
feeding its result to an indirect call.
However, that exposes the intermediate, unauthenticated pointer, e.g.,
if it gets spilled to the stack. An attacker can then overwrite the
pointer in memory, negating the security benefit provided by pointer
authentication. To prevent that, the ptrauth
operand bundle may be
used: it guarantees that the intermediate call target is kept in a
register and never stored to memory. This hardening benefit is similar
to that provided by `llvm.ptrauth.resign
<#llvm-ptrauth-resign>`__).
Concretely:
define void @f(void ()* %fp) {
call void %fp() [ "ptrauth"(i32 <key>, i64 <data>) ]
ret void
}
is functionally equivalent to:
define void @f(void ()* %fp) {
%fp_i = ptrtoint void ()* %fp to i64
%fp_auth = call i64 @llvm.ptrauth.auth(i64 %fp_i, i32 <key>, i64 <data>)
%fp_auth_p = inttoptr i64 %fp_auth to void ()*
call void %fp_auth_p()
ret void
}
but with the added guarantee that %fp_i
, %fp_auth
, and
%fp_auth_p
are not stored to (and reloaded from) memory.
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 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 aMOVK
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.