1.. SPDX-License-Identifier: GPL-2.0-or-later
2
3==========================================
4DEXCR (Dynamic Execution Control Register)
5==========================================
6
7Overview
8========
9
10The DEXCR is a privileged special purpose register (SPR) introduced in
11PowerPC ISA 3.1B (Power10) that allows per-cpu control over several dynamic
12execution behaviours. These behaviours include speculation (e.g., indirect
13branch target prediction) and enabling return-oriented programming (ROP)
14protection instructions.
15
16The execution control is exposed in hardware as up to 32 bits ('aspects') in
17the DEXCR. Each aspect controls a certain behaviour, and can be set or cleared
18to enable/disable the aspect. There are several variants of the DEXCR for
19different purposes:
20
21DEXCR
22    A privileged SPR that can control aspects for userspace and kernel space
23HDEXCR
24    A hypervisor-privileged SPR that can control aspects for the hypervisor and
25    enforce aspects for the kernel and userspace.
26UDEXCR
27    An optional ultravisor-privileged SPR that can control aspects for the ultravisor.
28
29Userspace can examine the current DEXCR state using a dedicated SPR that
30provides a non-privileged read-only view of the userspace DEXCR aspects.
31There is also an SPR that provides a read-only view of the hypervisor enforced
32aspects, which ORed with the userspace DEXCR view gives the effective DEXCR
33state for a process.
34
35
36Configuration
37=============
38
39The DEXCR is currently unconfigurable. All threads are run with the
40NPHIE aspect enabled.
41
42
43coredump and ptrace
44===================
45
46The userspace values of the DEXCR and HDEXCR (in this order) are exposed under
47``NT_PPC_DEXCR``. These are each 64 bits and readonly, and are intended to
48assist with core dumps. The DEXCR may be made writable in future. The top 32
49bits of both registers (corresponding to the non-userspace bits) are masked off.
50
51If the kernel config ``CONFIG_CHECKPOINT_RESTORE`` is enabled, then
52``NT_PPC_HASHKEYR`` is available and exposes the HASHKEYR value of the process
53for reading and writing. This is a tradeoff between increased security and
54checkpoint/restore support: a process should normally have no need to know its
55secret key, but restoring a process requires setting its original key. The key
56therefore appears in core dumps, and an attacker may be able to retrieve it from
57a coredump and effectively bypass ROP protection on any threads that share this
58key (potentially all threads from the same parent that have not run ``exec()``).
59