Re: [BUG] KVM: NULL pointer dereference in kvm_tdp_mmu_map under memory pressure

[punixcorn]

Following up with additional analysis from gdb.

The crash is at spte.h:263 in to_shadow_page(), not at the
tdp_mmu_alloc_sp() path as initially suspected.

  (gdb) list *(kvm_tdp_mmu_map+0x471)
  0x79451 is in kvm_tdp_mmu_map (mmu/spte.h:263)
  return (struct kvm_mmu_page *)page_private(page);

The crash location suggests page_private() is returning 0 for the
parent shadow page in tdp_mmu_init_child_sp(). The exact cause is
unclear. Sharing for maintainer review.

My earlier speculative fix (checking sp == NULL) was incorrect.
I am not familiar enough with the KVM MMU internals to propose a
correct fix. Sharing this in case it helps maintainers narrow down
the root cause.