[RFC PATCH] KVM: x86/mmu: fix general protection fault in kvm_mmu_uninit_tdp_mmu

[Pavel Skripkin <paskripkin@gmail.com>]

Syzbot reported GPF in kvm_mmu_uninit_tdp_mmu(), which is caused by
passing NULL pointer to flush_workqueue().

tdp_mmu_zap_wq is allocated via alloc_workqueue() which may fail. There
is no error hanling and kvm_mmu_uninit_tdp_mmu() return value is simply
ignored. Even all kvm_*_init_vm() functions are void, so the easiest
solution is to check that tdp_mmu_zap_wq is valid pointer before passing
it somewhere.

Fixes: 22b94c4b63eb ("KVM: x86/mmu: Zap invalidated roots via asynchronous worker")
Reported-and-tested-by: syzbot+717ed82268812a643b28@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index e7e7876251b3..b3e8ff7ac5b0 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -48,8 +48,10 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
 	if (!kvm->arch.tdp_mmu_enabled)
 		return;
 
-	flush_workqueue(kvm->arch.tdp_mmu_zap_wq);
-	destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
+	if (kvm->arch.tdp_mmu_zap_wq) {
+		flush_workqueue(kvm->arch.tdp_mmu_zap_wq);
+		destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
+	}
 
 	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_pages));
 	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_roots));
@@ -119,9 +121,11 @@ static void tdp_mmu_zap_root_work(struct work_struct *work)
 
 static void tdp_mmu_schedule_zap_root(struct kvm *kvm, struct kvm_mmu_page *root)
 {
-	root->tdp_mmu_async_data = kvm;
-	INIT_WORK(&root->tdp_mmu_async_work, tdp_mmu_zap_root_work);
-	queue_work(kvm->arch.tdp_mmu_zap_wq, &root->tdp_mmu_async_work);
+	if (kvm->arch.tdp_mmu_zap_wq) {
+		root->tdp_mmu_async_data = kvm;
+		INIT_WORK(&root->tdp_mmu_async_work, tdp_mmu_zap_root_work);
+		queue_work(kvm->arch.tdp_mmu_zap_wq, &root->tdp_mmu_async_work);
+	}
 }
 
 static inline bool kvm_tdp_root_mark_invalid(struct kvm_mmu_page *page)
-- 
2.35.1