linux-kernel - [PATCH] KVM: x86/mmu: Precisely invalidate MMU root

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Message-ID: <20240408231115.1387279-1-seanjc@google.com>
Date: Mon,  8 Apr 2024 16:11:15 -0700
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, 
	syzbot+dc308fcfcd53f987de73@...kaller.appspotmail.com, 
	Phi Nguyen <phind.uet@...il.com>
Subject: [PATCH] KVM: x86/mmu: Precisely invalidate MMU root_role during CPUID update

Set kvm_mmu_page_role.invalid to mark the various MMU root_roles invalid
during CPUID update in order to force a refresh, instead of zeroing out
the entire role.  This fixes a bug where kvm_mmu_free_roots() incorrectly
thinks a root is indirect, i.e. not a TDP MMU, due to "direct" being
zeroed, which in turn causes KVM to take mmu_lock for write instead of
read.

Note, paving over the entire role was largely unintentional, commit
7a458f0e1ba1 ("KVM: x86/mmu: remove extended bits from mmu_role, rename
field") simply missed that "invalid" could be set.

Fixes: 576a15de8d29 ("KVM: x86/mmu: Free TDP MMU roots while holding mmy_lock for read")
Reported-by: syzbot+dc308fcfcd53f987de73@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/0000000000009b38080614c49bdb@google.com
Cc: Phi Nguyen <phind.uet@...il.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 992e651540e8..1eadefdb4bf2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5576,9 +5576,9 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	 * that problem is swept under the rug; KVM's CPUID API is horrific and
 	 * it's all but impossible to solve it without introducing a new API.
 	 */
-	vcpu->arch.root_mmu.root_role.word = 0;
-	vcpu->arch.guest_mmu.root_role.word = 0;
-	vcpu->arch.nested_mmu.root_role.word = 0;
+	vcpu->arch.root_mmu.root_role.invalid = 1;
+	vcpu->arch.guest_mmu.root_role.invalid = 1;
+	vcpu->arch.nested_mmu.root_role.invalid = 1;
 	vcpu->arch.root_mmu.cpu_role.ext.valid = 0;
 	vcpu->arch.guest_mmu.cpu_role.ext.valid = 0;
 	vcpu->arch.nested_mmu.cpu_role.ext.valid = 0;

base-commit: 8cb4a9a82b21623dbb4b3051dd30d98356cf95bc
-- 
2.44.0.478.gd926399ef9-goog