[PATCH] KVM: x86: Don't leave APF half-enabled on bad APF data GPA

[Ethan Yang <ethan.yang.kernel@gmail.com>]

kvm_pv_enable_async_pf() updates vcpu->arch.apf.msr_en_val before
initializing the APF data gfn_to_hva cache. If userspace provides an
invalid GPA, kvm_gfn_to_hva_cache_init() fails, but msr_en_val stays
enabled and leaves APF state half-initialized.

Later APF paths can then try to use the empty cache and trigger
WARN_ON() in kvm_read_guest_offset_cached().

Determine the new APF enabled state from the incoming MSR value, do cache
initialization first on the enable path, and commit msr_en_val only after
successful initialization. Keep the disable path behavior unchanged.

Reported-by: syzbot+bc0e18379a290e5edfe4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bc0e18379a290e5edfe4
Link: https://lore.kernel.org/r/aHfD3MczrDpzDX9O@google.com
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ethan Yang <ethan.yang.kernel@gmail.com>
---
 arch/x86/kvm/x86.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fd1c4a36b59..c355bbd36c3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1041,11 +1041,15 @@ bool kvm_require_dr(struct kvm_vcpu *vcpu, int dr)
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_require_dr);
 
-static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+static bool kvm_pv_async_pf_enabled_data(u64 data)
 {
 	u64 mask = KVM_ASYNC_PF_ENABLED | KVM_ASYNC_PF_DELIVERY_AS_INT;
+	return (data & mask) == mask;
+}
 
-	return (vcpu->arch.apf.msr_en_val & mask) == mask;
+static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu)
+{
+	return kvm_pv_async_pf_enabled_data(vcpu->arch.apf.msr_en_val);
 }
 
 static inline u64 pdptr_rsvd_bits(struct kvm_vcpu *vcpu)
@@ -3645,17 +3649,20 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 	if (!lapic_in_kernel(vcpu))
 		return data ? 1 : 0;
 
+	bool new_enabled = kvm_pv_async_pf_enabled_data(data);
+
+	if (new_enabled &&
+		kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
+			sizeof(u64)))
+		return 1;
 	vcpu->arch.apf.msr_en_val = data;
 
-	if (!kvm_pv_async_pf_enabled(vcpu)) {
+	if (!new_enabled) {
 		kvm_clear_async_pf_completion_queue(vcpu);
 		kvm_async_pf_hash_reset(vcpu);
 		return 0;
 	}
 
-	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
-					sizeof(u64)))
-		return 1;
 
 	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
 	vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
-- 
2.47.3