[PATCH] KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()

[PATCH] KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()

[Sean Christopherson]

Wrap kvm_vcpu_exit_request()'s load of vcpu->mode with READ_ONCE() to
ensure the variable is re-loaded from memory, as there is no guarantee the
caller provides the necessary annotations to ensure KVM sees a fresh value,
e.g. the VM-Exit fastpath could theoretically reuse the pre-VM-Enter value.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 70219e406987..d38fdb5f90dd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2124,8 +2124,9 @@ EXPORT_SYMBOL_GPL(kvm_emulate_monitor);
 static inline bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu)
 {
 	xfer_to_guest_mode_prepare();
-	return vcpu->mode == EXITING_GUEST_MODE || kvm_request_pending(vcpu) ||
-		xfer_to_guest_mode_work_pending();
+
+	return READ_ONCE(vcpu->mode) == EXITING_GUEST_MODE ||
+	       kvm_request_pending(vcpu) || xfer_to_guest_mode_work_pending();
 }
 
 /*

base-commit: 15e1c3d65975524c5c792fcd59f7d89f00402261
-- 
2.46.0.295.g3b9ea8a38a-goog

Re: [PATCH] KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()

[Sean Christopherson]

On Wed, 28 Aug 2024 16:20:13 -0700, Sean Christopherson wrote:
> Wrap kvm_vcpu_exit_request()'s load of vcpu->mode with READ_ONCE() to
> ensure the variable is re-loaded from memory, as there is no guarantee the
> caller provides the necessary annotations to ensure KVM sees a fresh value,
> e.g. the VM-Exit fastpath could theoretically reuse the pre-VM-Enter value.

Applied to kvm-x86 misc, thanks!

[1/1] KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()
      https://github.com/kvm-x86/linux/commit/ba50bb4a9fb5

--
https://github.com/kvm-x86/linux/tree/next

Re: [PATCH] KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()

[Sean Christopherson]

On Thu, Oct 31, 2024, Sean Christopherson wrote:
> On Wed, 28 Aug 2024 16:20:13 -0700, Sean Christopherson wrote:
> > Wrap kvm_vcpu_exit_request()'s load of vcpu->mode with READ_ONCE() to
> > ensure the variable is re-loaded from memory, as there is no guarantee the
> > caller provides the necessary annotations to ensure KVM sees a fresh value,
> > e.g. the VM-Exit fastpath could theoretically reuse the pre-VM-Enter value.
> 
> Applied to kvm-x86 misc, thanks!
> 
> [1/1] KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()
>       https://github.com/kvm-x86/linux/commit/ba50bb4a9fb5

FYI, I rebased misc to v6.12-rc5, as patches in another series had already been
taken through the tip tree.  New hash:

[1/1] KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()
      https://github.com/kvm-x86/linux/commit/3ffe874ea3eb