linux-kernel - [PATCH 2/2] KVM: x86: Sanity check that kvm_has_noapic_vcpu is zero at module

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Date: Fri,  9 Feb 2024 14:20:47 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Li RongQing <lirongqing@...du.com>, Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 2/2] KVM: x86: Sanity check that kvm_has_noapic_vcpu is zero
 at module_exit()

WARN if kvm.ko is unloaded with an elevated kvm_has_noapic_vcpu to guard
against incorrect management of the key, e.g. to detect if KVM fails to
decrement the key in error paths.  Because kvm_has_noapic_vcpu is purely
an optimization, in all likelihood KVM could completely botch handling of
kvm_has_noapic_vcpu and no one would notice (which is a good argument for
deleting the key entirely, but that's a problem for another day).

Note, ideally the sanity check would be performance when kvm_usage_count
goes to zero, but adding an arch callback just for this sanity check isn't
at all worth doing.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 59119157bd20..779a5b32a5bf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -13878,9 +13878,6 @@ module_init(kvm_x86_init);

 static void __exit kvm_x86_exit(void)
 {
-	/*
-	 * If module_init() is implemented, module_exit() must also be
-	 * implemented to allow module unload.
-	 */
+	WARN_ON_ONCE(static_branch_unlikely(&kvm_has_noapic_vcpu));
 }
 module_exit(kvm_x86_exit);
-- 
2.43.0.687.g38aa6559b0-goog