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

prctl

A process can control its own userspace DEXCR value using the PR_PPC_GET_DEXCR and PR_PPC_SET_DEXCR pair of prctl(2) commands. These calls have the form:

prctl(PR_PPC_GET_DEXCR, unsigned long which, 0, 0, 0);
prctl(PR_PPC_SET_DEXCR, unsigned long which, unsigned long ctrl, 0, 0);

The possible ‘which’ and ‘ctrl’ values are as follows. Note there is no relation between the ‘which’ value and the DEXCR aspect’s index.

prctl() which	Aspect name	Aspect index
PR_PPC_DEXCR_SBHE	Speculative Branch Hint Enable (SBHE)	0
PR_PPC_DEXCR_IBRTPD	Indirect Branch Recurrent Target Prediction Disable (IBRTPD)	3
PR_PPC_DEXCR_SRAPD	Subroutine Return Address Prediction Disable (SRAPD)	4
PR_PPC_DEXCR_NPHIE	Non-Privileged Hash Instruction Enable (NPHIE)	5

prctl() ctrl	Meaning
PR_PPC_DEXCR_CTRL_EDITABLE	This aspect can be configured with PR_PPC_SET_DEXCR (get only)
PR_PPC_DEXCR_CTRL_SET	This aspect is set / set this aspect
PR_PPC_DEXCR_CTRL_CLEAR	This aspect is clear / clear this aspect
PR_PPC_DEXCR_CTRL_SET_ONEXEC	This aspect will be set after exec / set this aspect after exec
PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC	This aspect will be clear after exec / clear this aspect after exec

Note that

• which is a plain value, not a bitmask. Aspects must be worked with individually.

• ctrl is a bitmask. PR_PPC_GET_DEXCR returns both the current and onexec configuration. For example, PR_PPC_GET_DEXCR may return PR_PPC_DEXCR_CTRL_EDITABLE | PR_PPC_DEXCR_CTRL_SET | PR_PPC_DEXCR_CTRL_CLEAR_ONEXEC. This would indicate the aspect is currently set, it will be cleared when you run exec, and you can change this with the PR_PPC_SET_DEXCR prctl.

• The set/clear terminology refers to setting/clearing the bit in the DEXCR. For example:

  prctl(PR_PPC_SET_DEXCR, PR_PPC_DEXCR_IBRTPD, PR_PPC_DEXCR_CTRL_SET, 0, 0);
  

  will set the IBRTPD aspect bit in the DEXCR, causing indirect branch prediction to be disabled.

• The status returned by PR_PPC_GET_DEXCR represents what value the process would like applied. It does not include any alternative overrides, such as if the hypervisor is enforcing the aspect be set. To see the true DEXCR state software should read the appropriate SPRs directly.

• The aspect state when starting a process is copied from the parent’s state on fork(2). The state is reset to a fixed value on execve(2). The PR_PPC_SET_DEXCR prctl() can control both of these values.

• The *_ONEXEC controls do not change the current process’s DEXCR.

Use PR_PPC_SET_DEXCR with one of PR_PPC_DEXCR_CTRL_SET or PR_PPC_DEXCR_CTRL_CLEAR to edit a given aspect.

Common error codes for both getting and setting the DEXCR are as follows:

Error	Meaning
EINVAL	The DEXCR is not supported by the kernel.
ENODEV	The aspect is not recognised by the kernel or not supported by the hardware.

PR_PPC_SET_DEXCR may also report the following error codes:

Error	Meaning
EINVAL	The ctrl value contains unrecognised flags.
EINVAL	The ctrl value contains mutually conflicting flags (e.g., PR_PPC_DEXCR_CTRL_SET | PR_PPC_DEXCR_CTRL_CLEAR)
EPERM	This aspect cannot be modified with prctl() (check for the PR_PPC_DEXCR_CTRL_EDITABLE flag with PR_PPC_GET_DEXCR).
EPERM	The process does not have sufficient privilege to perform the operation. For example, clearing NPHIE on exec is a privileged operation (a process can still clear its own NPHIE aspect without privileges).

This interface allows a process to control its own DEXCR aspects, and also set the initial DEXCR value for any children in its process tree (up to the next child to use an *_ONEXEC control). This allows fine-grained control over the default value of the DEXCR, for example allowing containers to run with different default values.

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()).