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[PAuthABI] Allow GNU program properties as a PAuth ABI marking schema
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A new program property GNU_PROPERTY_AARCH64_FEATURE_PAUTH is defined in
section .note.gnu.property that can be used as an alternative method for
signing PAuthABI-compatible ELF files.At least in the Bionic linker,
this is easier to implement: the property (and the existing
.note.gnu.property section) can be trivially extracted from a DSO, and
is already happening for other properties (mainly
GNU_PROPERTY_AARCH64_FEATURE_1_AND to check for BTI and PAC-ret). In
contrast, a new section would require traversing all the sections in the
ELF and checking for the section name, which is more complicated to
implement and possibly slower.

Unfortunately, .note.gnu.property is not guaranteed to be understood by
all ELF-based platforms, so we still keep the new section
.note.AARCH64-PAUTH-ABI-tag as the default marking schema.
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pbarrio committed Sep 1, 2021
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Expand Up @@ -14,7 +14,7 @@
.. _LSB: https://refspecs.linuxfoundation.org/LSB_1.2.0/gLSB/noteabitag.html
.. _SCO-ELF: http://www.sco.com/developers/gabi/
.. _TLSDESC: http://www.fsfla.org/~lxoliva/writeups/TLS/paper-lk2006.pdf

.. _LINUX_ABI: https://github.com/hjl-tools/linux-abi/wiki
.. footer::

###Page###
Expand Down Expand Up @@ -254,6 +254,8 @@ This document refers to, or is referred to by, the following documents.
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| `GABI_SHT_RELR <https://groups.google.com/d/msg/generic-abi/bX460iggiKg/YT2RrjpMAwAJ>`_ | ELF GABI Google Groups | Proposal for a new section type SHT_RELR |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+
| LINUX_ABI_ | https://github.com/hjl-tools/linux-abi/wiki | Linux Extensions to gABI |
+-----------------------------------------------------------------------------------------+-------------------------------------------------------------+--------------------------------------------------------------------------+

Terms and Abbreviations
-----------------------
Expand Down Expand Up @@ -832,10 +834,20 @@ language to signing schema is expected to evolve over time. Even if
the low-level ELF extensions remain constant, a change to the
high-level language mapping may result in incompatible ELF files.

Every relocatable object, executable and shared library that uses the
PAuth ABI ELF extensions must have a section named
``.note.AARCH64-PAUTH-ABI-tag`` of type ``SHT_NOTE``. This section is
structured as a note section as documented in SCO-ELF_.
This document defines a default ELF marking schema and a base
compatibility model. Platforms may define their own ELF marking and
compatibility model that replace or extend the default ones. `Appendix
Alternative ELF Marking Using GNU Program Properties`_ defines an
alternative marking schema for platforms that support the
``.note.gnu.property`` section.

Default Marking Schema
----------------------

A new section named ``.note.AARCH64-PAUTH-ABI-tag`` of type
``SHT_NOTE`` is defined. This section is structured as a note section
as documented in SCO-ELF_, and its attribute flag ``SHF_ALLOC`` must
be set.

The name field (``namesz`` / ``name``) contains the string "ARM". The
type field shall be 1, and the ``descsz`` field must be at least 16.
Expand All @@ -846,17 +858,28 @@ identified for the first word. When ``descsz`` is larger than 16 the
remainder of the contents of desc are defined by the (platform id,
version number).

This ABI does not determine the format of the platform identifier. Arm
reserves the platform id 0 for a bare-metal platform.
The following values of the platform id are reserved:

The (platform id, version number) of (0, 0) is reserved as an invalid
combination. The program cannot be run when pointer authentication is
enabled.
+-----------+-----------+
| Platform | Hex value |
+===========+===========+
| Invalid | 0x0 |
+-----------+-----------+
| Baremetal | 0x1 |
+-----------+-----------+

The version id in ``.note.AARCH64-PAUTH-ABI-tag`` is not directly
The version number in ``.note.AARCH64-PAUTH-ABI-tag`` is not directly
related to the version number of this document. It is controlled by
the object-producer based on the signing schema that have been used
for pointers.
the object-producer based on the signing schema that has been used for
pointers.

The (platform id, version number) value of (0, 0) is reserved exclusively
for the purpose of representing an invalid combination.

If a file contains a section named ``.note.AARCH64-PAUTH-ABI-tag``,
it must observe the entirety of the rules in this default marking
schema. Generating such section with a platform-specific schema is
forbidden.

Base Compatibility Model
------------------------
Expand All @@ -872,20 +895,32 @@ compatibility.
pointers were signed in a compatible way with the default signing
rules for tuple (platform id, version number).

* The result of a successful combination of
``.note.AArch64-PAUTH-ABI-tag`` sections is a single
``.note.AArch64-PAUTH-ABI-tag`` section containing the (platform id,
version number) tuple. The result of an unsuccesful combination must
be either a single ``.note.AArch64-PAUTH-ABI-tag`` section containing
an invalid (platform id, version number) of (0, 0), or no
``.note.AArch64-PAUTH-ABI-tag`` section written to the output file.

* The static linker may fault the combination of relocatable
objects that contain ``.note.AARCH64-PAUTH-ABI-tag`` sections with
incompatible (platform id, version number) tuples. If an ELF file is
produced, the output ``.note.AARCH64-PAUTH-ABI-tag`` must have the
invalid (platform id, version number) of (0, 0).
incompatible (platform id, version number) tuples.

* A dynamic loader may disable pointer authentication, or fault the
program with an error message, in case it encounters an incompatible
ELF file. A file is incompatible with the loader in any of the
following cases:

* The combination of relocatable objects with
``.note.AARCH64-PAUTH-ABI-tag`` and relocatable objects without a
``.note.AARCH64-PAUTH-ABI-tag`` is not defined by this ABI.
* If section ``.note.AARCH64-PAUTH-ABI-tag`` is missing.

* A dynamic loader that encounters a (platform id, version number)
that it does not recognize, or if the (platform id, version number)
is (0,0), it must disable pointer authentication for the process or
give an error message.
* If the (platform id, version number) tuple is not recognized.

* If the (platform id, version number) is (0,0).

The combination of relocatable objects with
``.note.AARCH64-PAUTH-ABI-tag`` and relocatable objects without a
``.note.AARCH64-PAUTH-ABI-tag`` is not defined by this ABI.

Platforms may replace the base compatibility model with a
platform-specific model.
Expand Down Expand Up @@ -1196,3 +1231,34 @@ Some observations:
* When not dynamic linking a static linker may choose to encode the
pointer signing information in a custom encoding understood by the
start-up code used.

Appendix Alternative ELF Marking Using GNU Program Properties
=============================================================

If a platform supports section ``.note.gnu.property``, this can be used
as the base for an alternative schema. The format of this section is
defined in LINUX_ABI_.

The following processor-specific program property types are defined:

+----------------------------------------+------------+
| Name | Value |
+========================================+============+
| GNU\_PROPERTY\_AARCH64\_FEATURE\_PAUTH | 0xc0000001 |
+----------------------------------------+------------+

The format of the data in ``pr_data`` is at least two 64-bit words,
the first being a platform identifier, and the second being a version
number specific to the platform identified in the first word.
Consequently, the ``pr_datasz`` field must be at least 16. When
``pr_datasz`` is larger than 16, the remainder of the contents of
``pr_data`` are specific to the (platform id, version number). If
``pr_datasz`` is not a multiple of 8, ``pr_padding`` must be added so
that PR_DATASZ + PR_PADDING is a multiple of 8 and the property
remains 8-byte aligned.

The rules for the fields are the same as for the default marking
schema: this ABI does not define the exact format of the platform and
version identifiers, but reserves the combination of (platform,
version) equal to (0,0) to represent an ELF file incompatible with
this ABI.

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