[PATCH v1 0/5] KVM: arm64: Enforce MTE disablement at EL2

[Fuad Tabba]

pKVM never exposes MTE to protected guests (pVM), but we must also
ensure a malicious host cannot use MTE to attack the hypervisor or a
pVM.

If MTE is supported by the hardware (and is enabled at EL3), it remains
available to lower exception levels by default. Disabling it in the host
kernel (e.g., via 'arm64.nomte') only stops the kernel from advertising
the feature; it does not physically disable MTE in the hardware.

In this scenario, a malicious host could still access tags in pages
donated to a guest using MTE instructions (e.g., STG and LDG), bypassing
the kernel's configuration.

To prevent this, explicitly disable MTE at EL2 (by clearing HCR_EL2.ATA)
when the host has MTE disabled. This causes any MTE instruction usage to
generate a Data Abort (trap) to the hypervisor.

Additionally, to faithfully mimic hardware that does not support MTE,
trap accesses to MTE system registers (e.g., GCR_EL1) and inject an
Undefined Instruction exception back to the host.

This logic is applied in all non-VHE modes. For non-protected modes,
this remains beneficial as it prevents unpredictable behavior caused by
accessing allocation tags when the system considers them disabled.

Note that this ties into my other outgoing patch series [1], which also
has some MTE-related fixes, but is not dependent on it.

Based on Linux 6.18-rc7

Cheers,
/fuad

[1] https://lore.kernel.org/all/20251118103807.707500-1-tabba@google.com/

Fuad Tabba (4):
  arm64: Remove dead code resetting HCR_EL2 for pKVM
  arm64: Clear HCR_EL2.ATA when MTE is not supported or disabled
  arm64: Inject UNDEF when accessing MTE sysregs with MTE disabled
  KVM: arm64: Use kvm_has_mte() in pKVM trap initialization

Quentin Perret (1):
  KVM: arm64: Refactor enter_exception64()

 arch/arm64/include/asm/kvm_arm.h     |   2 +-
 arch/arm64/include/asm/kvm_emulate.h |   5 ++
 arch/arm64/kernel/head.S             |   2 +-
 arch/arm64/kvm/arm.c                 |   4 ++
 arch/arm64/kvm/hyp/exception.c       | 100 ++++++++++++++++-----------
 arch/arm64/kvm/hyp/nvhe/hyp-init.S   |   5 --
 arch/arm64/kvm/hyp/nvhe/hyp-main.c   |  44 ++++++++++++
 arch/arm64/kvm/hyp/nvhe/pkvm.c       |   2 +-
 8 files changed, 114 insertions(+), 50 deletions(-)


base-commit: ac3fd01e4c1efce8f2c054cdeb2ddd2fc0fb150d
-- 
2.52.0.487.g5c8c507ade-goog