[PATCH v2 0/8] KVM: x86/hyperv: Fix racy usage of vcpu->arch.hyperv

[Sean Christopherson <seanjc@google.com>]

Fix a bug found by syzkaller (on a Google-internal kernel) where KVM consumes
a vCPU's HyperV structure before it's fully initialized, by concurrently
triggering PV TLB flushes (queues flushes into a vCPU's FIFO without holding
the vCPU's mutex) on a vCPU that is in the process of activating HyperV.

Harden against similar bugs by asserting the vcpu->mutex is held when using
the "normal" to_hv_vcpu(), same as we did for get_vmcs12() and
get_shadow_vmcs12() (also in response to cross-task races).  To avoid false
positives when creating a vCPU, initialize vcpu_idx to -1, and treat the vCPU
as unreachable (other than the caller, obviously) if its index is -1.

v2:
 - Init vcpu->vcpu_idx to -1, use that as a canary to detect the vCPU is
   unreachable, and allow accessing Hyper-V state if the vCPU is otherwise
   unreachable. [syzbot]

v1: https://lore.kernel.org/all/20260423140833.439512-1-seanjc@google.com

Sean Christopherson (8):
  KVM: x86/hyperv: Get target FIFO in hv_tlb_flush_enqueue(), not caller
  KVM: x86/hyperv: Check for NULL vCPU Hyper-V object in
    kvm_hv_get_tlb_flush_fifo()
  KVM: x86/hyperv: Ensure vCPU's Hyper-V object is initialized on
    cross-vCPU accesses
  KVM: Initialize a vCPU's index to '-1' while it's being created
  KVM: Move nVMX's lockdep logic for vcpu->mutex to a common helper
  KVM: x86: Treat a vCPU as unreachable if its index is invalid
  KVM: x86/hyperv: Assert vCPU's mutex is held in to_hv_vcpu()
  KVM: x86/hyperv: Use {READ,WRITE}_ONCE for cross-task synic->active
    accesses

 arch/x86/kvm/hyperv.c     | 64 +++++++++++++++++++++------------------
 arch/x86/kvm/hyperv.h     | 25 +++++++++++++--
 arch/x86/kvm/vmx/nested.h |  6 ++--
 include/linux/kvm_host.h  |  7 +++++
 virt/kvm/kvm_main.c       |  2 ++
 5 files changed, 67 insertions(+), 37 deletions(-)


base-commit: c1f7303302927f9cbf4efedf70f0512cde168c65
-- 
2.54.0.1136.gdb2ca164c4-goog