[PATCH 0/7] IBRS patch series

Tim Chen
2018-01-04T18:18:04Z

This patch series enables the basic detection and usage of x86 indirect
branch speculation feature. It enables the indirect branch restricted
speculation (IBRS) on kernel entry and disables it on exit.
It enumerates the indirect branch prediction barrier (IBPB).

The x86 IBRS feature requires corresponding microcode support.
It mitigates the variant 2 vulnerability described in
https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html

If IBRS is set, near returns and near indirect jumps/calls will not
allow their predicted target address to be controlled by code that
executed in a less privileged prediction mode before the IBRS mode was
last written with a value of 1 or on another logical processor so long
as all RSB entries from the previous less privileged prediction mode
are overwritten.

Setting of IBPB ensures that earlier code's behavior does not control later
indirect branch predictions. It is used when context switching to new
untrusted address space. Unlike IBRS, IBPB is a command MSR
and does not retain its state.

Speculation on Skylake and later requires these patches ("dynamic IBRS")
be used instead of retpoline[1]. If you are very paranoid or you run on
a CPU where IBRS=1 is cheaper, you may also want to run in "IBRS always"
mode.

See: https://docs.google.com/document/d/e/2PACX-1vSMrwkaoSUBAFc6Fjd19F18c1O9pudkfAY-7lGYGOTN8mc9ul-J6pWadcAaBJZcVA7W_3jlLKRtKRbd/pub

More detailed description of IBRS is described in the first patch.

It is applied on top of the page table isolation changes.

A run time and boot time control of the IBRS feature is provided

There are 2 ways to control IBRS

1. At boot time
noibrs kernel boot parameter will disable IBRS usage

Otherwise if the above parameters are not specified, the system
will enable ibrs and ibpb usage if the cpu supports it.

2. At run time
echo 0 > /sys/kernel/debug/ibrs_enabled will turn off IBRS
echo 1 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in kernel
echo 2 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in both userspace and kernel (IBRS always)

[1] https://lkml.org/lkml/2018/1/4/174

Tim Chen (7):
x86/feature: Detect the x86 feature to control Speculation
x86/enter: MACROS to set/clear IBRS
x86/enter: Use IBRS on syscall and interrupts
x86/idle: Disable IBRS entering idle and enable it on wakeup
x86: Use IBRS for firmware update path
x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature
x86/microcode: Recheck IBRS features on microcode reload

Documentation/admin-guide/kernel-parameters.txt | 4 +
arch/x86/entry/entry_64.S | 24 +++
arch/x86/entry/entry_64_compat.S | 9 +
arch/x86/include/asm/apm.h | 6 +
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/efi.h | 16 +-
arch/x86/include/asm/msr-index.h | 7 +
arch/x86/include/asm/mwait.h | 19 ++
arch/x86/include/asm/spec_ctrl.h | 253 ++++++++++++++++++++++++
arch/x86/kernel/cpu/Makefile | 1 +
arch/x86/kernel/cpu/microcode/core.c | 6 +
arch/x86/kernel/cpu/scattered.c | 11 ++
arch/x86/kernel/cpu/spec_ctrl.c | 124 ++++++++++++
arch/x86/kernel/process.c | 9 +-
14 files changed, 486 insertions(+), 4 deletions(-)
create mode 100644 arch/x86/include/asm/spec_ctrl.h
create mode 100644 arch/x86/kernel/cpu/spec_ctrl.c

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2.9.4