[PATCH v5 2/7] KVM: arm64: Top up the memcache for pKVM permission faults

[Fuad Tabba <fuad.tabba@linux.dev>]

A permission fault normally only relaxes a leaf, so user_mem_abort()
skips the memcache top-up. Under pKVM such a fault can instead collapse
pages into a block. That needs a fresh pkvm_mapping object, and without
it cache->mapping is NULL, so pkvm_pgtable_stage2_map() dereferences NULL
and faults the host under mmu_lock. Staging only the object is not
enough: the hypervisor requires kvm_mmu_cache_min_pages in the memcache
even for the allocation-free install, so under memcache pressure the
collapse returns -ENOMEM and trips the WARN_ON(ret) in
pkvm_pgtable_stage2_map() where a non-pKVM guest succeeds.

Top up the full memcache for pKVM permission faults so both the mapping
object and the min-pages are staged before mmu_lock.

Fixes: db14091d8f75 ("KVM: arm64: Stage-2 huge mappings for np-guests")
Reported-by: Bradley Morgan <include@grrlz.net>
Link: https://lore.kernel.org/all/20260623161545.EA08E1F000E9@smtp.kernel.org/
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/mmu.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 6c941aaa10c6..4d7c9bdcef00 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2114,10 +2114,14 @@ static int user_mem_abort(const struct kvm_s2_fault_desc *s2fd)
 	 * 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.
+	 * Under pKVM a permission fault can also collapse pages into a block,
+	 * which needs a fresh mapping object, and the hypervisor requires the
+	 * min-pages memcache even when the install allocates nothing.
 	 */
 	memcache = get_mmu_memcache(s2fd->vcpu);
 	if (!perm_fault || (memslot_is_logging(s2fd->memslot) &&
-			    kvm_is_write_fault(s2fd->vcpu))) {
+			    kvm_is_write_fault(s2fd->vcpu)) ||
+	    is_protected_kvm_enabled()) {
 		ret = topup_mmu_memcache(s2fd->vcpu, memcache);
 		if (ret)
 			return ret;
-- 
2.39.5