KVM: TDX: Do not retry locally when the retry is caused by invalid memslot

Avoid local retries within the TDX EPT violation handler if a retry is
triggered by faulting in an invalid memslot, indicating that the memslot
is undergoing a removal process.  Faulting in a GPA from an invalid
memslot will never succeed, and holding SRCU prevents memslot deletion
from succeeding, i.e. retrying when the memslot is being actively deleted
will lead to (breakable) deadlock.

Opportunistically export kvm_vcpu_gfn_to_memslot() to allow for a per-vCPU
lookup (which, strictly speaking, is unnecessary since TDX doesn't support
SMM, but aligns the TDX code with the MMU code).

Fixes: b0327bb ("KVM: TDX: Retry locally in TDX EPT violation handler on RET_PF_RETRY")
Reported-by: Reinette Chatre <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]
[Yan: Wrote patch log, comment, fixed a minor error, function export]
Signed-off-by: Yan Zhao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[sean: massage changelog, relocate and tweak comment]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

arch/x86/kvm/vmx/tdx.c

@@ -2002,6 +2002,8 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
 	 * handle retries locally in their EPT violation handlers.
 	 */
 	while (1) {
+		struct kvm_memory_slot *slot;
+
 		ret = __vmx_handle_ept_violation(vcpu, gpa, exit_qual);
 
 		if (ret != RET_PF_RETRY || !local_retry)
@@ -2015,6 +2017,15 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
 			break;
 		}
 
+		/*
+		 * Bail if the memslot is invalid, i.e. is being deleted, as
+		 * faulting in will never succeed and this task needs to drop
+		 * SRCU in order to let memslot deletion complete.
+		 */
+		slot = kvm_vcpu_gfn_to_memslot(vcpu, gpa_to_gfn(gpa));
+		if (slot && slot->flags & KVM_MEMSLOT_INVALID)
+			break;
+
 		cond_resched();
 	}
 	return ret;

virt/kvm/kvm_main.c

@@ -2661,6 +2661,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
 
 	return NULL;
 }
+EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
 
 bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
 {