[PATCH v3 2/3] KVM: arm64: top up pKVM mapping cache for permission faults

[Bradley Morgan <include@grrlz.net>]

Permission faults normally only relax an existing leaf, so the fault path
does not top up the memcache.

With pKVM, a permission fault can also replace page mappings with a
PMD mapping. That path needs a fresh pkvm_mapping object, and can
dereference a NULL cache->mapping if the cache was not topped up.

Allocate just that object for pKVM permission faults.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/all/20260623161545.EA08E1F000E9@smtp.kernel.org/ [1]

Fixes: db14091d8f75 ("KVM: arm64: Stage-2 huge mappings for np-guests")
Cc: stable@vger.kernel.org
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 arch/arm64/kvm/mmu.c | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 6c941aaa10c6..3f57f6825a33 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1177,17 +1177,26 @@ void free_hyp_memcache(struct kvm_hyp_memcache *mc)
 	__free_hyp_memcache(mc, hyp_mc_free_fn, kvm_host_va, mc);
 }
 
+static int topup_hyp_memcache_mapping(struct kvm_hyp_memcache *mc)
+{
+	if (mc->mapping)
+		return 0;
+
+	mc->mapping = kzalloc_obj(struct pkvm_mapping,
+				  GFP_KERNEL_ACCOUNT);
+	return mc->mapping ? 0 : -ENOMEM;
+}
+
 int topup_hyp_memcache(struct kvm_hyp_memcache *mc, unsigned long min_pages)
 {
+	int ret;
+
 	if (!is_protected_kvm_enabled())
 		return 0;
 
-	if (!mc->mapping) {
-		mc->mapping = kzalloc_obj(struct pkvm_mapping,
-					  GFP_KERNEL_ACCOUNT);
-		if (!mc->mapping)
-			return -ENOMEM;
-	}
+	ret = topup_hyp_memcache_mapping(mc);
+	if (ret)
+		return ret;
 
 	return __topup_hyp_memcache(mc, min_pages, hyp_mc_alloc_fn,
 				    kvm_host_pa, mc);
@@ -2113,7 +2122,9 @@ static int user_mem_abort(const struct kvm_s2_fault_desc *s2fd)
 	 * Permission faults just need to update the existing leaf entry,
 	 * and so normally don't require allocations from the memcache. The
 	 * only exception to this is when dirty logging is enabled at runtime
-	 * and a write fault needs to collapse a block entry into a table.
+	 * and a write fault needs to collapse a block entry into a table. With
+	 * pKVM, they may still need a fresh mapping object if the fault turns
+	 * page entries into a block entry.
 	 */
 	memcache = get_mmu_memcache(s2fd->vcpu);
 	if (!perm_fault || (memslot_is_logging(s2fd->memslot) &&
@@ -2121,6 +2132,10 @@ static int user_mem_abort(const struct kvm_s2_fault_desc *s2fd)
 		ret = topup_mmu_memcache(s2fd->vcpu, memcache);
 		if (ret)
 			return ret;
+	} else if (is_protected_kvm_enabled()) {
+		ret = topup_hyp_memcache_mapping(memcache);
+		if (ret)
+			return ret;
 	}
 
 	/*
-- 
2.53.0