KVM: MMU: only write protect mappings at pagetable level

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If a pagetable contains a writeable large spte, all of its sptes will be
write protected, including non-leaf ones, leading to endless pagefaults.

Do not write protect pages above PT_PAGE_TABLE_LEVEL, as the spte fault
paths assume non-leaf sptes are writable.

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index c3853d5..c716ff8 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3442,6 +3442,9 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 		if (!test_bit(slot, sp->slot_bitmap))
 			continue;
 
+		if (sp->role.level != PT_PAGE_TABLE_LEVEL)
+			continue;
+
 		pt = sp->spt;
 		for (i = 0; i < PT64_ENT_PER_PAGE; ++i)
 			/* avoid RMW */
--
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