[PATCH v2 0/2] KVM: x86: Syzkaller nested_run_pending defense

[Sean Christopherson]

Stop playing a losing game of whack-a-mole with syzkaller over
nested_run_pending, and simply treat the flag as "untrusted" if userspace gains
control of the vCPU.  I.e. don't WARN if userspace _may_ have stuffed vCPU
state to force an impossible VM-Exit.

The signal:noise ratio for nested_run_pending WARNs when syzkaller is fuzzing
nVMX/nSVM is awful, and playing whack-a-mole with syzkaller is doing more harm
that good at this point.  While it's possible syzkaller could find an actual
KVM bug by stuffing vCPU state, if the _only_ symptom is the nested_run_pending
WARN, then in theory the bug should also be hittable by stuffing L2 state prior
to nested VM-Enter.

v2:
 - Put the state transition in kvm_x86_vcpu_pre_run(). [Yosry]
 - Add Yosry's review.

v1: https://lore.kernel.org/all/20260310224546.2582118-1-seanjc@xxxxxxxxxx

Sean Christopherson (1):
  KVM: x86: Suppress WARNs on nested_run_pending after userspace exit

Yosry Ahmed (1):
  KVM: x86: Move nested_run_pending to kvm_vcpu_arch

 arch/x86/include/asm/kvm_host.h | 15 ++++++++++
 arch/x86/kvm/svm/nested.c       | 23 ++++++++-------
 arch/x86/kvm/svm/svm.c          | 16 +++++------
 arch/x86/kvm/svm/svm.h          |  4 ---
 arch/x86/kvm/vmx/nested.c       | 50 +++++++++++++++++----------------
 arch/x86/kvm/vmx/vmx.c          | 16 +++++------
 arch/x86/kvm/vmx/vmx.h          |  3 --
 arch/x86/kvm/x86.c              |  7 +++++
 arch/x86/kvm/x86.h              | 10 +++++++
 9 files changed, 87 insertions(+), 57 deletions(-)


base-commit: bfd7f4adc1230373c25e1b787a6f1ee407eb0656
-- 
2.53.0.851.ga537e3e6e9-goog