[PATCH v2 2/3] KVM: arm64: Avoid host/hyp share desync on unshare hypercall failure

[tabba at google.com]

unshare_pfn_hyp() erases the tracking node from hyp_shared_pfns
and frees it before invoking __pkvm_host_unshare_hyp. If the
hypercall fails (e.g. EL2 refcount still held, or page-state
mismatch), the host loses its record while EL2 still holds the
share, breaking later share/unshare attempts on the same pfn.

Invoke the hypercall first; erase and free only on success.

Document at the kvm_unshare_hyp() call site that the WARN_ON() is
left non-fatal: a failed unshare leaks the page (it stays shared
with the hypervisor) but breaks no isolation guarantee.

Fixes: 52b28657ebd7 ("KVM: arm64: pkvm: Unshare guest structs during teardown")
Reported-by: Sashiko (local):gemini-3.1-pro
Suggested-by: Vincent Donnefort <vdonnefort at google.com>
Signed-off-by: Fuad Tabba <tabba at google.com>
---
 arch/arm64/kvm/mmu.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 4a928fb003ff..e08503e89fc4 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -524,13 +524,17 @@ static int unshare_pfn_hyp(u64 pfn)
 		goto unlock;
 	}
 
-	this->count--;
-	if (this->count)
+	if (this->count > 1) {
+		this->count--;
+		goto unlock;
+	}
+
+	ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_hyp, pfn);
+	if (ret)
 		goto unlock;
 
 	rb_erase(&this->node, &hyp_shared_pfns);
 	kfree(this);
-	ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_hyp, pfn);
 unlock:
 	mutex_unlock(&hyp_shared_pfns_lock);
 
@@ -581,6 +585,11 @@ void kvm_unshare_hyp(void *from, void *to)
 	end = PAGE_ALIGN(__pa(to));
 	for (cur = start; cur < end; cur += PAGE_SIZE) {
 		pfn = __phys_to_pfn(cur);
+		/*
+		 * A failed unshare leaks the page: it stays shared with the
+		 * hypervisor and is no longer reusable for pKVM. No isolation
+		 * guarantee is broken, and this is not expected in practice.
+		 */
 		WARN_ON(unshare_pfn_hyp(pfn));
 	}
 }
-- 
2.54.0.929.g9b7fa37559-goog