[PATCH] KVM: nSVM: Enter guest mode before initializing nested NPT MMU

Sean Christopherson
2025-01-30T01:08:31Z

When preparing vmcb02 for nested VMRUN (or state restore), "enter" guest
mode prior to initializing the MMU for nested NPT so that guest_mode is
set in the MMU's role. KVM's model is that all L2 MMUs are tagged with
guest_mode, as the behavior of hypervisor MMUs tends to be significantly
different than kernel MMUs.

Practically speaking, the bug is relatively benign, as KVM only directly
queries role.guest_mode in kvm_mmu_free_guest_mode_roots(), which SVM
doesn't use, and in paths that are optimizations (mmu_page_zap_pte() and
shadow_mmu_try_split_huge_pages()).

And while the role is incorprated into shadow page usage, because nested
NPT requires KVM to be using NPT for L1, reusing shadow pages across L1
and L2 is impossible as L1 MMUs will always have direct=1, while L2 MMUs
will have direct=0.

Hoist the TLB processing and setting of HF_GUEST_MASK to the beginning
of the flow instead of forcing guest_mode in the MMU, as nothing in
nested_vmcb02_prepare_control() between the old and new locations touches
TLB flush requests or HF_GUEST_MASK, i.e. there's no reason to present
inconsistent vCPU state to the MMU.

Fixes: 69cb877487de ("KVM: nSVM: move MMU setup to nested_prepare_vmcb_control")
Cc: stable@vger.kernel.org
Reported-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/mmu.c | 2 +-
arch/x86/kvm/svm/nested.c | 10 +++++-----
2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 74fa38ebddbf..3ff55a18347d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5524,7 +5524,7 @@ void kvm_init_shadow_npt_mmu(struct kvm_vcpu *vcpu, unsigned long cr0,
union kvm_mmu_page_role root_role;

/* NPT requires CR0.PG=1. */
- WARN_ON_ONCE(cpu_role.base.direct);
+ WARN_ON_ONCE(cpu_role.base.direct || !cpu_role.base.guest_mode);

root_role = cpu_role.base;
root_role.level = kvm_mmu_get_tdp_level(vcpu);
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index d77b094d9a4d..04c375bf1ac2 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -646,6 +646,11 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
u32 pause_count12;
u32 pause_thresh12;

+ nested_svm_transition_tlb_flush(vcpu);
+
+ /* Enter Guest-Mode */
+ enter_guest_mode(vcpu);
+
/*
* Filled at exit: exit_code, exit_code_hi, exit_info_1, exit_info_2,
* exit_int_info, exit_int_info_err, next_rip, insn_len, insn_bytes.
@@ -762,11 +767,6 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
}
}

- nested_svm_transition_tlb_flush(vcpu);
-
- /* Enter Guest-Mode */
- enter_guest_mode(vcpu);
-
/*
* Merge guest and host intercepts - must be called with vcpu in
* guest-mode to take effect.

base-commit: eb723766b1030a23c38adf2348b7c3d1409d11f0
--
2.48.1.262.g85cc9f2d1e-goog