[RFC PATCH 0/6] Protect VMM from speculative attacks using x86 PKRU

Hi,

This RFC patch series introduces a hardware-assisted memory protection
mechanism in QEMU using x86 Protection Keys for Userspace (PKRU / MPK)
to complete the Address Space Isolation ecosystem by protecting the VMM
from guest-to-host speculative execution attacks.

In a guest-to-host speculative side-channel attack against QEMU, a
malicious guest mistrains the VMM branch predictor to execute a
speculative gadget in host mode. This gadget speculatively reads a
host secret and uses that secret as an index to touch guest RAMBlocks,
leaving an observable microarchitectural footprint in the CPU cache.
Existing mitigations like issuing IBPB on every VM-exit incur high
overhead, while guest_memfd cannot protect memory that must stay mapped.

To break this covert channel, we protect guest RAMBlocks using a
dedicated pkey (pkey_mprotect).

- During normal QEMU host userspace execution (I/O handling, timers,
  event loops), we lock this pkey by setting PKEY_DISABLE_ACCESS in
  the PKRU register via WRPKRU. Because CPU pipelines natively enforce
  PKRU restrictions during out-of-order execution, any speculative
  gadget in host mode attempting to touch guest RAMBlocks is blocked by
  the MMU at the hardware level, leaving zero footprint in the cache.

- Before entering KVM (KVM_RUN), we temporarily unlock the pkey, and
  immediately re-lock it upon returning to QEMU userspace.

- To gracefully handle legitimate host accesses to guest RAMBlocks
  when locked, a SEGV_PKUERR signal handler unlocks the pkey in the
  interrupted thread's xstate and issues an IBPB (PR_SPEC_INDIRECT_BRANCH)
  to prevent speculative leakage before resuming execution.

Comments and feedback on this approach are very welcome!

To: [email protected]
Cc: Paolo Bonzini <[email protected]>
Cc: Zhao Liu <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Philippe Mathieu-Daudé <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: [email protected]
Cc: James Houghton <[email protected]>
Cc: Mingwei Zhang <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Brendan Jackman <[email protected]>
Cc: Reiji Watanabe <[email protected]>
Cc: Jacky Li <[email protected]>

Signed-off-by: Jacky Li <[email protected]>
---
Jacky Li (6):
      [RFC PATCH 1/6] x86: Introduce basic PKRU hardware wrappers
      [RFC PATCH 2/6] kvm: Add guest memory Pkey initialization
      [RFC PATCH 3/6] physmem: Tag guest RAMBlocks with Protection Key
      [RFC PATCH 4/6] kvm: Lock guest RAMBlocks via PKRU during host userspace execution
      [RFC PATCH 5/6] x86: Add xstate parsing and PKRU offset detection
      [RFC PATCH 6/6] kvm: Implement SIGSEGV sentinel for Pkey recovery

 MAINTAINERS               |   1 +
 accel/kvm/kvm-all.c       |   7 +-
 include/exec/cpu-common.h |   4 +
 include/qemu/mmap-alloc.h |   5 +
 system/physmem.c          |  22 ++-
 system/vl.c               |   1 +
 util/meson.build          |   2 +-
 util/mmap-alloc.c         |   3 +-
 util/pkey.c               | 386 ++++++++++++++++++++++++++++++++++++++++++++++
 9 files changed, 424 insertions(+), 7 deletions(-)
---
base-commit: fa19879df1658f96ac07365fca8835b7decd6995
change-id: 20260729-feature-pkey-dev-c8a026db85b4

Best regards,
-- 
Jacky Li <[email protected]>