[PATCH 7/8] KVM: arm64: Propagate stage-2 map failure on guest->host share

[Fuad Tabba]

__pkvm_guest_share_host() updates the guest stage-2 PTE for a
guest-OWNED page to PKVM_PAGE_SHARED_OWNED via
kvm_pgtable_stage2_map() and then transitions the host vmemmap and
stage-2 PTE to PKVM_PAGE_SHARED_BORROWED. The map's return value was
wrapped in WARN_ON() and otherwise discarded.

At EL2 in nVHE/pKVM, WARN_ON() is not warn-and-continue: it expands
to a BRK that enters the invalid-host-el2 vector and branches to
hyp_panic(), declared __noreturn.

__pkvm_guest_share_host() calls get_valid_guest_pte() before the
map, which verifies that a valid last-level (PAGE_SIZE) leaf PTE
already exists for the IPA. Because the leaf and all intermediate
tables are in place, the subsequent kvm_pgtable_stage2_map()
replacing it cannot fail via -ENOMEM: no block to split, no new
tables to install. The failure path is not currently reachable.

Nevertheless, WARN_ON() on any fallible call is the wrong pattern at
EL2: if the get_valid_guest_pte() precondition were ever relaxed, or
the walker gained a new failure mode, the WARN_ON would convert a
recoverable error into a fatal hyp panic. Capture the return value
and propagate it. The unmap() is kept as a defensive guard for the
currently unreachable failure path; no host-side unwinding is needed
since the host vmemmap and stage-2 update is the next step and is
correctly skipped on error.

Fixes: 03313efed5e2 ("KVM: arm64: Implement the MEM_SHARE hypercall for protected VMs")
Signed-off-by: Fuad Tabba <tabba at google.com>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index b8c57a95e9bf..6fb546af699f 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -979,10 +979,23 @@ int __pkvm_guest_share_host(struct pkvm_hyp_vcpu *vcpu, u64 gfn)
 	if (__host_check_page_state_range(phys, PAGE_SIZE, PKVM_NOPAGE))
 		goto unlock;
 
-	ret = 0;
-	WARN_ON(kvm_pgtable_stage2_map(&vm->pgt, ipa, PAGE_SIZE, phys,
-				       pkvm_mkstate(KVM_PGTABLE_PROT_RWX, PKVM_PAGE_SHARED_OWNED),
-				       &vcpu->vcpu.arch.pkvm_memcache, 0));
+	ret = kvm_pgtable_stage2_map(&vm->pgt, ipa, PAGE_SIZE, phys,
+				     pkvm_mkstate(KVM_PGTABLE_PROT_RWX, PKVM_PAGE_SHARED_OWNED),
+				     &vcpu->vcpu.arch.pkvm_memcache, 0);
+	if (ret) {
+		/*
+		 * Stage-2 map can fail mid-walk (e.g. -ENOMEM from the
+		 * memcache), leaving partial leaf entries in the guest
+		 * stage-2 transitioned to PKVM_PAGE_SHARED_OWNED. Tear
+		 * them down so the host does not see a partially-shared
+		 * mapping it has not yet acknowledged via the host
+		 * stage-2 update below. No host bookkeeping needs
+		 * unwinding here: the only mutation prior to the failed
+		 * map is the (now-discarded) guest stage-2 update itself.
+		 */
+		kvm_pgtable_stage2_unmap(&vm->pgt, ipa, PAGE_SIZE);
+		goto unlock;
+	}
 	WARN_ON(__host_set_page_state_range(phys, PAGE_SIZE, PKVM_PAGE_SHARED_BORROWED));
 unlock:
 	guest_unlock_component(vm);
-- 
2.54.0.545.g6539524ca2-goog