[PATCH mm-hotfixes] mm/huge_memory: use correct flags for device private PMD entry

[Lorenzo Stoakes]

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.

Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
---
 mm/huge_memory.c | 45 +++++++++++++++++++++++++++++++++------------
 1 file changed, 33 insertions(+), 12 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bf9b480bb3b0..79463c709c98 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4982,7 +4982,7 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
 	struct vm_area_struct *vma = pvmw->vma;
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long address = pvmw->address;
-	bool anon_exclusive;
+	bool anon_exclusive, present, writable, softdirty, uffd_wp;
 	pmd_t pmdval;
 	swp_entry_t entry;
 	pmd_t pmdswp;
@@ -4990,12 +4990,26 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
 	if (!(pvmw->pmd && !pvmw->pte))
 		return 0;

-	flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
-	if (unlikely(!pmd_present(*pvmw->pmd)))
-		pmdval = pmdp_huge_get_and_clear(vma->vm_mm, address, pvmw->pmd);
-	else
+	present = pmd_present(*pvmw->pmd);
+	if (likely(present)) {
+		flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
+
 		pmdval = pmdp_invalidate(vma, address, pvmw->pmd);

+		writable = pmd_write(pmdval);
+		softdirty = pmd_soft_dirty(pmdval);
+		uffd_wp = pmd_uffd_wp(pmdval);
+	} else {
+		softleaf_t old_entry;
+
+		pmdval = pmdp_huge_get_and_clear(vma->vm_mm, address, pvmw->pmd);
+		old_entry = softleaf_from_pmd(pmdval);
+
+		writable = softleaf_is_device_private_write(old_entry);
+		softdirty = pmd_swp_soft_dirty(pmdval);
+		uffd_wp = pmd_swp_uffd_wp(pmdval);
+	}
+
 	/* See folio_try_share_anon_rmap_pmd(): invalidate PMD first. */
 	anon_exclusive = folio_test_anon(folio) && PageAnonExclusive(page);
 	if (anon_exclusive && folio_try_share_anon_rmap_pmd(folio, page)) {
@@ -5003,24 +5017,31 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
 		return -EBUSY;
 	}

-	if (pmd_dirty(pmdval))
-		folio_mark_dirty(folio);
-	if (pmd_write(pmdval))
+	/* Determine type of migration entry. */
+	if (writable)
 		entry = make_writable_migration_entry(page_to_pfn(page));
 	else if (anon_exclusive)
 		entry = make_readable_exclusive_migration_entry(page_to_pfn(page));
 	else
 		entry = make_readable_migration_entry(page_to_pfn(page));
-	if (pmd_young(pmdval))
+
+	/* Set A/D bits as necessary. */
+	if (present && pmd_young(pmdval))
 		entry = make_migration_entry_young(entry);
-	if (pmd_dirty(pmdval))
+	if (present && pmd_dirty(pmdval)) {
+		folio_mark_dirty(folio);
 		entry = make_migration_entry_dirty(entry);
+	}
+
+	/* Set PMD. */
 	pmdswp = swp_entry_to_pmd(entry);
-	if (pmd_soft_dirty(pmdval))
+	if (softdirty)
 		pmdswp = pmd_swp_mksoft_dirty(pmdswp);
-	if (pmd_uffd_wp(pmdval))
+	if (uffd_wp)
 		pmdswp = pmd_swp_mkuffd_wp(pmdswp);
 	set_pmd_at(mm, address, pvmw->pmd, pmdswp);
+
+	/* Migration entry installed: cleanup rmap, folio. */
 	folio_remove_rmap_pmd(folio, page, vma);
 	folio_put(folio);
 	trace_set_migration_pmd(address, pmd_val(pmdswp));
--
2.54.0