[PATCH v3 0/4] KVM: arm64: Enforce MTE disablement at EL2

[Fuad Tabba]

Changes since v2 [1]:
- Trap accesses to GMID_EL1 by setting `HCR_EL2.TID5` when MTE is
  disabled (Marc)
- Dropped patch refactoring `enter_exception64()` in favor of open-coded
  logic in `inject_undef64()` in patch 3/4 (Marc)
- Based on Linux 6.19-rc6

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

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.

The ability to disable MTE in the host kernel is used by some systems,
such as Android, so that the physical memory otherwise used as tag
storage can be used for other things (i.e. treated just like the rest of
memory). 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` and setting `HCR_EL2.TID5`) when the host has MTE
disabled. This causes MTE instructions to trap to the hypervisor.

Cheers,
/fuad

[1] https://lore.kernel.org/all/20251211113828.370370-1-tabba@google.com/

Fuad Tabba (4):
  KVM: arm64: Remove dead code resetting HCR_EL2 for pKVM
  KVM: arm64: Trap MTE access and discovery when MTE is disabled
  KVM: arm64: Inject UNDEF when accessing MTE sysregs with MTE disabled
  KVM: arm64: Use kvm_has_mte() in pKVM trap initialization

 arch/arm64/include/asm/kvm_arm.h   |  2 +-
 arch/arm64/kernel/head.S           |  2 +-
 arch/arm64/kvm/arm.c               |  6 +++
 arch/arm64/kvm/hyp/nvhe/hyp-init.S |  5 ---
 arch/arm64/kvm/hyp/nvhe/hyp-main.c | 67 ++++++++++++++++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/pkvm.c     |  2 +-
 6 files changed, 76 insertions(+), 8 deletions(-)


base-commit: 24d479d26b25bce5faea3ddd9fa8f3a6c3129ea7
-- 
2.52.0.457.g6b5491de43-goog