Until now, the page table walker counted increments to the PA and IPA
of a walk in two separate places. While the PA is incremented as soon as
a leaf PTE is installed in stage2_map_walker_try_leaf(), the IPA is
actually bumped in the generic table walker context. Critically,
__kvm_pgtable_visit() rereads the PTE after the LEAF callback returns
to work out if a table or leaf was installed, and only bumps the IPA for
a leaf PTE.

This arrangement worked fine when we handled faults behind the write lock,
as the walker had exclusive access to the stage-2 page tables. However,
commit 1577cb58 ("KVM: arm64: Handle stage-2 faults in parallel")
started handling all stage-2 faults behind the read lock, opening up a
race where a walker could increment the PA but not the IPA of a walk.
Nothing good ensues, as the walker starts mapping with the incorrect
IPA -> PA relationship.

For example, assume that two vCPUs took a data abort on the same IPA.
One observes that dirty logging is disabled, and the other observed that
it is enabled:

  vCPU attempting PMD mapping		  vCPU attempting PTE mapping
  ======================================  =====================================
  /* install PMD */
  stage2_make_pte(ctx, leaf);
  data->phys += granule;
  					  /* replace PMD with a table */
  					  stage2_try_break_pte(ctx, data->mmu);
					  stage2_make_pte(ctx, table);
  /* table is observed */
  ctx.old = READ_ONCE(*ptep);
  table = kvm_pte_table(ctx.old, level);

  /*
   * map walk continues w/o incrementing
   * IPA.
   */
   __kvm_pgtable_walk(..., level + 1);

Bring an end to the whole mess by using the IPA as the single source of
truth for how far along a walk has gotten. Work out the correct PA to
map by calculating the IPA offset from the beginning of the walk and add
that to the starting physical address.

Cc: stable@vger.kernel.org
Fixes: 1577cb58 ("KVM: arm64: Handle stage-2 faults in parallel")
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230421071606.1603916-2-oliver.upton@linux.dev