[PATCH 6/8] KVM: arm64: Propagate stage-2 map failure on host->guest donation

[Fuad Tabba <tabba@google.com>]

__pkvm_host_donate_guest() flips the host stage-2 PTE for the donated
page to a non-valid annotation (KVM_HOST_INVALID_PTE_TYPE_DONATION,
owner = PKVM_ID_GUEST) via host_stage2_set_owner_metadata_locked()
and then calls kvm_pgtable_stage2_map() to install the matching guest
stage-2 mapping. 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. WARN_ON of a reachable failure at
EL2 is a panic primitive, not a debug aid.

kvm_pgtable_stage2_map() can fail in reachable ways even at PAGE_SIZE
granularity: __pkvm_host_donate_guest() verifies PKVM_NOPAGE for the
guest IPA before the map, meaning no valid stage-2 entry exists. The
walker must allocate new page-table pages from the vcpu memcache to
install the mapping, returning -ENOMEM if exhausted. The host
controls the vcpu memcache via the topup interface, so an
under-provisioned donation request converts a recoverable error into
a fatal hyp panic.

Capture the stage-2 map return value and propagate it. The walker
may have installed partial leaf entries for the IPA before failing,
so unmap the range to clear them; otherwise the guest would retain
stage-2 access to a page the host is about to reclaim as
PKVM_PAGE_OWNED. Then roll back the host stage-2 mutation: the only
forward mutation is host_stage2_set_owner_metadata_locked() flipping
the host vmemmap from PKVM_PAGE_OWNED to PKVM_NOPAGE and the host
stage-2 PTE from idmap to invalid+annotation.
host_stage2_set_owner_locked(_, _, PKVM_ID_HOST) restores both.

The rollback calls host_stage2_set_owner_locked() under WARN_ON.
This is the correct use: host_stage2_set_owner_metadata_locked()
just wrote the host leaf PTE as an invalid+annotation entry, so the
reverse idmap rewrite cannot require new page-table allocation — it
rewrites the leaf in-place. The WARN_ON asserts an impossible state
under correct EL2 execution, semantically distinct from the misuse
being fixed.

Fixes: 1e579adca177 ("KVM: arm64: Introduce __pkvm_host_donate_guest()")
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 7044913a0758..b8c57a95e9bf 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -1391,9 +1391,30 @@ int __pkvm_host_donate_guest(u64 pfn, u64 gfn, struct pkvm_hyp_vcpu *vcpu)
 	meta = host_stage2_encode_gfn_meta(vm, gfn);
 	WARN_ON(host_stage2_set_owner_metadata_locked(phys, PAGE_SIZE,
 						      PKVM_ID_GUEST, meta));
-	WARN_ON(kvm_pgtable_stage2_map(&vm->pgt, ipa, PAGE_SIZE, phys,
-				       pkvm_mkstate(KVM_PGTABLE_PROT_RWX, PKVM_PAGE_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_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 installed in the
+		 * guest stage-2. Tear them down before rolling back the host
+		 * stage-2; otherwise the guest would retain access to a page
+		 * the host is about to reclaim as PKVM_PAGE_OWNED.
+		 */
+		kvm_pgtable_stage2_unmap(&vm->pgt, ipa, PAGE_SIZE);
+
+		/*
+		 * Roll back the donation annotation applied above by
+		 * host_stage2_set_owner_metadata_locked() (host vmemmap
+		 * PKVM_NOPAGE -> PKVM_PAGE_OWNED, host stage-2 PTE
+		 * invalid+annotation -> idmap). The leaf PTE was just
+		 * installed by the forward call, so reinstating the idmap
+		 * rewrites it without needing fresh page-table pages from
+		 * host_s2_pool.
+		 */
+		WARN_ON(host_stage2_set_owner_locked(phys, PAGE_SIZE, PKVM_ID_HOST));
+	}
 
 unlock:
 	guest_unlock_component(vm);
-- 
2.54.0.545.g6539524ca2-goog