From 65d6c884bfbd38235659e6df193345e5ad874043 Mon Sep 17 00:00:00 2001 From: Benjamin Gray Date: Mon, 19 Jun 2023 17:36:27 +1000 Subject: [PATCH] Documentation: Document PowerPC kernel DEXCR interface Describe the DEXCR and document how to configure it. Signed-off-by: Benjamin Gray Signed-off-by: Michael Ellerman Link: https://msgid.link/20230616034846.311705-9-bgray@linux.ibm.com --- Documentation/powerpc/dexcr.rst | 58 +++++++++++++++++++++++++++++++++++++++++ Documentation/powerpc/index.rst | 1 + 2 files changed, 59 insertions(+) create mode 100644 Documentation/powerpc/dexcr.rst diff --git a/Documentation/powerpc/dexcr.rst b/Documentation/powerpc/dexcr.rst new file mode 100644 index 0000000..615a631f --- /dev/null +++ b/Documentation/powerpc/dexcr.rst @@ -0,0 +1,58 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +========================================== +DEXCR (Dynamic Execution Control Register) +========================================== + +Overview +======== + +The DEXCR is a privileged special purpose register (SPR) introduced in +PowerPC ISA 3.1B (Power10) that allows per-cpu control over several dynamic +execution behaviours. These behaviours include speculation (e.g., indirect +branch target prediction) and enabling return-oriented programming (ROP) +protection instructions. + +The execution control is exposed in hardware as up to 32 bits ('aspects') in +the DEXCR. Each aspect controls a certain behaviour, and can be set or cleared +to enable/disable the aspect. There are several variants of the DEXCR for +different purposes: + +DEXCR + A privileged SPR that can control aspects for userspace and kernel space +HDEXCR + A hypervisor-privileged SPR that can control aspects for the hypervisor and + enforce aspects for the kernel and userspace. +UDEXCR + An optional ultravisor-privileged SPR that can control aspects for the ultravisor. + +Userspace can examine the current DEXCR state using a dedicated SPR that +provides a non-privileged read-only view of the userspace DEXCR aspects. +There is also an SPR that provides a read-only view of the hypervisor enforced +aspects, which ORed with the userspace DEXCR view gives the effective DEXCR +state for a process. + + +Configuration +============= + +The DEXCR is currently unconfigurable. All threads are run with the +NPHIE aspect enabled. + + +coredump and ptrace +=================== + +The userspace values of the DEXCR and HDEXCR (in this order) are exposed under +``NT_PPC_DEXCR``. These are each 64 bits and readonly, and are intended to +assist with core dumps. The DEXCR may be made writable in future. The top 32 +bits of both registers (corresponding to the non-userspace bits) are masked off. + +If the kernel config ``CONFIG_CHECKPOINT_RESTORE`` is enabled, then +``NT_PPC_HASHKEYR`` is available and exposes the HASHKEYR value of the process +for reading and writing. This is a tradeoff between increased security and +checkpoint/restore support: a process should normally have no need to know its +secret key, but restoring a process requires setting its original key. The key +therefore appears in core dumps, and an attacker may be able to retrieve it from +a coredump and effectively bypass ROP protection on any threads that share this +key (potentially all threads from the same parent that have not run ``exec()``). diff --git a/Documentation/powerpc/index.rst b/Documentation/powerpc/index.rst index 85e80e3..d33b554 100644 --- a/Documentation/powerpc/index.rst +++ b/Documentation/powerpc/index.rst @@ -15,6 +15,7 @@ powerpc cxl cxlflash dawr-power9 + dexcr dscr eeh-pci-error-recovery elf_hwcaps -- 2.7.4