ARM64 CPU has zero register which is read-only, with a value of 0.
However, KVM currently incorrectly recognizes it being SP (because
Rt == 31, and in struct user_pt_regs 'regs' array is followed by SP),
resulting in invalid value being read, or even SP corruption on write.

The problem has been discovered by performing an operation

 *((volatile int *)reg) = 0;

which compiles as "str xzr, [xx]", and resulted in strange values being

v3 => v4:
- Unwrapped assignment in patch 0003

v2 => v3:
- Brought back some const modifiers in unaffected functions

v1 => v2:
- Changed type of transfer value to u64 and store it directly in
  struct sys_reg_params instead of a pointer
- Use lower_32_bits()/upper_32_bits() where appropriate
- Fixed wrong usage of 'Rt' instead of 'Rt2' in kvm_handle_cp_64(),
  overlooked in v1
- Do not write value back when reading

Pavel Fedin (4):
  KVM: arm64: Correctly handle zero register during MMIO
  KVM: arm64: Remove const from struct sys_reg_params
  KVM: arm64: Correctly handle zero register in system register accesses
  KVM: arm64: Get rid of old vcpu_reg()

 arch/arm/include/asm/kvm_emulate.h   |  12 ++++
 arch/arm/kvm/mmio.c                  |   5 +-
 arch/arm/kvm/psci.c                  |  20 +++---
 arch/arm64/include/asm/kvm_emulate.h |  18 +++--
 arch/arm64/kvm/handle_exit.c         |   2 +-
 arch/arm64/kvm/sys_regs.c            | 123 +++++++++++++++++------------------
 arch/arm64/kvm/sys_regs.h            |   8 +--
 arch/arm64/kvm/sys_regs_generic_v8.c |   4 +-
 8 files changed, 105 insertions(+), 87 deletions(-)


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