mm/huge_memory: use correct flags for device private PMD entry

In the Linux kernel, the following vulnerability has been resolved:

mm/huge_memory: use correct flags for device private PMD entry

Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
device-private entries") updated set_pmd_migration_entry() to use
pmdp_huge_get_and_clear() in the softleaf case, but made no further
adjustments to the function itself.

Therefore this function continues to incorrectly use pmd_write(),
pmd_soft_dirty() and pmd_uffd_wp() to determine whether the installed
migration entry should be marked writable, softdirty or uffd-wp
respectively.

Whilst all are incorrect, the most problematic of these is pmd_write(), as
this can lead to corrupted rmap state.

On x86-64 _PAGE_SWP_SOFT_DIRTY is aliased to _PAGE_RW. So calling
pmd_write() on a softleaf will return the softdirty state encoded in the
entry, assuming CONFIG_MEM_SOFT_DIRTY was enabled.

This was observed when running the hmm.hmm_device_private.anon_write_child
selftest:

1. The test faults in a range then migrates it such that a device-private
THP range is established.

2. The parent then migrates it to a device-private writable PMD entry whose
folio is entirely AnonExclusive with entire_mapcount=1, softdirty set
(accidentally correct write state).

3. The parent forks and the PMD entries are set to device-private read only
entries, entire_mapcount=2, softdirty still set.

4. [BUG] The child writes to the range then migrates to RAM - intending to
install non-writable migration entries - but replacing parent and child
PMD mappings with WRITABLE entries due to misinterpreting the softdirty
bit.

5. In remove_migration_pmd(), if !softleaf_is_migration_read(entry) we
set the RMAP_EXCLUSIVE flag when calling folio_add_anon_rmap_pmd() for
both parent and child, which are therefore AnonExclusive.

6. [SPLAT] Child sets migrated folio entire_mapcount=1, parent sets
entire_mapcount=2 and we end up with an AnonExclusive folio with
entire_mapcount=2! Assert fires in __folio_add_anon_rmap():

VM_WARN_ON_FOLIO(folio_test_large(folio) &&
folio_entire_mapcount(folio) > 1 &&
PageAnonExclusive(cur_page), folio)

This patch fixes the issue by correctly referencing the softleaf entry
fields for writable, softdirty and uffd-wp in set_pmd_migration_entry().

It also only updates A/D flags if the entry is present as these are
otherwise not meaningful for a softleaf entry.

This patch also flips the if (!present) { ... } else { ... } logic in
set_pmd_migration_entry() so it is easier to understand, and adds some
comments to make things clearer.

I was able to bisect this to commit 775465fd26a3 ("lib/test_hmm: add zone
device private THP test infrastructure") which first exposes this bug as
it was the commit that permitted test_hmm to generate the test.

However commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
device-private entries") is the commit that actually enabled this
behaviour.