QEMU 2.11.1 and making use of Spectre/Meltdown mitigation for KVM guests

14 Feb 2018 — by Michael Roth

A previous post detailed how QEMU/KVM might be affected by Spectre/Meltdown attacks, and what the plan was to mitigate them in QEMU 2.11.1 (and eventually QEMU 2.12).

QEMU 2.11.1 is now available, and contains the aforementioned mitigation functionality for x86 guests, along with additional mitigation functionality for pseries and s390x guests (ARM guests do not currently require additional QEMU patches). However, enabling this functionality requires additional configuration beyond just updating QEMU, which we want to address with this post.

Please note that QEMU/KVM has at least the same requirements as other unprivileged processes running on the host with regard to Spectre/Meltdown mitigation. What is being addressed here is enabling a guest operating system to enable the same (or similar) mitigations to protect itself from unprivileged guest processes running under the guest operating system. Thus, the patches/requirements listed here are specific to that goal and should not be regarded as the full set of requirements to enable mitigations on the host side (though in some cases there is some overlap between the two with regard to required patches/etc).

Also please note that this is a best-effort from the QEMU/KVM community, and these mitigations rely on a mix of additional kernel/firmware/microcode updates that are in some cases not yet available publicly, or may not yet be implemented in some distros, so users are highly encouraged to consult with their respective vendors/distros to confirm whether all the required components are in place. We do our best to highlight the requirements here, but this may not be an exhaustive list.

Enabling mitigation features for x86 KVM guests

Note: these mitigations are known to cause some performance degradation for certain workloads (whether used on host or guest), and for some Intel architectures alternative solutions like retpoline-based kernels may be available which may provide similar levels of mitigation with reduced performance impact. Please check with your distro/vendor to see what options are available to you.

For x86 guests there are 2 additional CPU flags associated with Spectre/Meltdown mitigation: spec-ctrl, and ibpb:

  • spec-ctrl: exposes Indirect Branch Restricted Speculation (IBRS)
  • ibpb: exposes Indirect Branch Prediction Barriers

These flags expose additional functionality made available through new microcode updates for certain Intel/AMD processors that can be used to mitigate various attack vectors related to Spectre. (Meltdown mitigation via KPTI does not require additional CPU functionality or microcode, and does not require an updated QEMU, only the related guest/host kernel patches).

Utilizing this functionality requires guest/host kernel updates, as well as microcode updates for Intel and recent AMD processors. The status of these kernel patches upstream is still in flux, but most supported distros have some form of the patches that is sufficient to make use of the functionality. The current status/availability of microcode updates depends on your CPU architecture/model. Please check with your vendor/distro to confirm these prerequisites are available/installed.

Generally, for Intel CPUs with updated microcode, spec-ctrl will enable both IBRS and IBPB functionality. For AMD EPYC processors, ibpb can be used to enable IBPB specifically, and is thought to be sufficient by itself for that particular architecture.

These flags can be set in a similar manner as other CPU flags, i.e.:

qemu-system-x86_64 -cpu qemu64,+spec-ctrl,... ...
qemu-system-x86_64 -cpu IvyBridge,+spec-ctrl,... ...
qemu-system-x86_64 -cpu EPYC,+ibpb,... ...

Additionally, for management stacks that lack support for setting specific CPU flags, a set of new CPU types have been added which enable the appropriate CPU flags automatically:

qemu-system-x86_64 -cpu Nehalem-IBRS ...
qemu-system-x86_64 -cpu Westmere-IBRS ...
qemu-system-x86_64 -cpu SandyBridge-IBRS ...
qemu-system-x86_64 -cpu IvyBridge-IBRS ...
qemu-system-x86_64 -cpu Haswell-IBRS ...
qemu-system-x86_64 -cpu Haswell-noTSX-IBRS ...
qemu-system-x86_64 -cpu Broadwell-IBRS ...
qemu-system-x86_64 -cpu Broadwell-noTSX-IBRS ...
qemu-system-x86_64 -cpu Skylake-Client-IBRS ...
qemu-system-x86_64 -cpu Skylake-Server-IBRS ...
qemu-system-x86_64 -cpu EPYC-IBPB ...

With these settings enabled, guests may still require additional configuration to enable IBRS/IBPB, which may vary somewhat from one distro to another. For RHEL guests, the following resource may be useful:

With regard to migration compatibility, spec-ctrl/ibrs (or the corresponding CPU type) should be set the same on both source/target to maintain compatibility. Thus, guests will need to be rebooted to make use of the new features.

Enabling mitigation features for pseries KVM guests

For pseries guests there are 3 tri-state -machine options/capabilities relating to Spectre/Meltdown mitigation: cap-cfpc, cap-sbbc, cap-ibs, which each correspond to a set of host machine capabilities advertised by the KVM kernel module in new/patched host kernels that can be used to mitigate various aspects of Spectre/Meltdown:

  • cap-cfpc: Cache Flush on Privilege Change
  • cap-sbbc: Speculation Barrier Bounds Checking
  • cap-ibs: Indirect Branch Serialisation

Each option can be set to one of “broken”, “workaround”, or “fixed”, which correspond, respectively, to instructing the guest whether the host is vulnerable, has OS-level workarounds available, or has hardware/firmware that does not require OS-level workarounds. Based on these options, QEMU will perform checks to validate whether the specified settings are available on the current host and pass these settings on to the guest kernel. At a minimum, any setting other than “broken” will require a host kernel that has some form of the following patches:

commit 3214d01f139b7544e870fc0b7fcce8da13c1cb51
KVM: PPC: Book3S: Provide information about hardware/firmware CVE workarounds

commit 191eccb1580939fb0d47deb405b82a85b0379070
powerpc/pseries: Add H_GET_CPU_CHARACTERISTICS flags & wrapper

and whether a host will support “workaround” and “fixed” settings for each option will depend on the hardware/firmware level of the host system.

In turn, to make use of “workaround” or “fixed” settings for each option, the guest kernel will require at least the following set of patches:

These are available upstream and have been backported to a number of stable kernels. Please check with your vendor/distro to confirm the required hardware/firmware and guest kernel patches are available/installed.

All three options, cap-cfpc, cap-sbbc, and cap-ibs default to “broken” to maintain compatibility with previous versions of QEMU and unpatched host kernels. To enable them you must start QEMU with the desired mitigation strategy specified explicitly. For example:

qemu-system-ppc64 ... \
  -machine pseries-2.11,cap-cfpc=workaround,cap-sbbc=workaround,cap-ibs=fixed

With regard to migration compatibility, setting any of these features to a value other than “broken” will require an identical setting for that option on the source/destination guest. To enable these settings your guests will need to be rebooted at some point.

Enabling mitigation features for s390x KVM guests

For s390x guests there are 2 CPU feature bits relating to Spectre/Meltdown:

  • bpb: Branch prediction blocking
  • ppa15: PPA15 is installed

bpb requires a host kernel patched with:

commit 35b3fde6203b932b2b1a5b53b3d8808abc9c4f60
KVM: s390: wire up bpb feature

and both bpb and ppa15 require a firmware with the appropriate support level as well as guest kernel patches to enable the functionality within guests. Please check with your distro/vendor to confirm.

Both bpb and ppa15 are enabled by default when using “-cpu host” and when the host kernels supports these facilities. For other CPU models, the flags have to be set manually. For example:

qemu-system-s390x -M s390-ccw-virtio-2.11 ... \
  -cpu zEC12,bpb=on,ppa15=on

With regard to migration, enabling bpb or ppa15 feature flags requires that the source/target also has those flags enabled. Since this is enabled by default for ‘-cpu host’ (when available on the host), you must ensure that bpb=off,ppa15=off is used if you wish to maintain migration compatibility with existing guests when using ‘-cpu host’, or take steps to reboot guests with bpb/ppa15 enabled prior to migration.