[PATCH v2] fs/dax: check for empty/zero entries before calling pfn_to_page()

[Souvik Banerjee <souvik@amlalabs.com>]

Commit 98c183a4fccf ("fs/dax: don't disassociate zero page entries")
added zero/empty-entry early returns to dax_associate_entry() and
dax_disassociate_entry(), but placed them *after* the
`struct folio *folio = dax_to_folio(entry);` line.  dax_to_folio()
expands to page_folio(pfn_to_page(dax_to_pfn(entry))), which calls
_compound_head() and performs READ_ONCE(page->compound_info) -- a real
dereference of the struct page pointer derived from a bogus PFN
extracted from the empty/zero XA value.

On systems where vmemmap covers all of RAM that dereference reads
garbage and is harmless: the early return then discards the result.
On virtio-pmem with altmap (vmemmap stored inside the device), only
the real device PFN range is mapped, so the dereference triggers a
kernel paging fault from the truncate / invalidate path and from the
PMD-downgrade branch of dax_iomap_pte_fault when an entry is being
freed:

  Unable to handle kernel paging request at
  virtual address ffff_fdff_bf00_0008 (vmemmap region)
  Call trace:
   dax_disassociate_entry.isra.0+0x20/0x50
   dax_iomap_pte_fault
   dax_iomap_fault
   erofs_dax_fault

Close the residual gap by moving the dax_to_folio() call after the
zero/empty guard in both dax_associate_entry() and
dax_disassociate_entry().  Apply the same treatment to dax_busy_page(),
which has the identical pattern but was not touched by the prior fix.
dax_associate_entry() is reachable with a zero entry via
dax_insert_entry() -> dax_associate_entry(new_entry, ...), where
new_entry can carry DAX_ZERO_PAGE (built by dax_make_entry() in
dax_load_hole() / dax_pmd_load_hole()).  dax_disassociate_entry() and
dax_busy_page() additionally see DAX_EMPTY entries created by
grab_mapping_entry().

The remaining users of dax_to_folio() / dax_to_pfn() in fs/dax.c are
either guarded or only reachable on real-PFN entries, so this exhausts
the anti-pattern.

Fixes: 98c183a4fccf ("fs/dax: don't disassociate zero page entries")
Fixes: 38607c62b34b ("fs/dax: properly refcount fs dax pages")
Cc: stable@vger.kernel.org # v6.15+
Cc: Alistair Popple <apopple@nvidia.com>
Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Souvik Banerjee <souvik@amlalabs.com>
---
Changes in v2:
  - Also fix dax_associate_entry() (Suggested-by: David Hildenbrand,
    confirmed by Alistair Popple).  The same anti-pattern existed there:
    dax_to_folio(entry) ran before the zero/empty guard.  new_entry on
    that path can carry DAX_ZERO_PAGE via dax_load_hole() /
    dax_pmd_load_hole(), so the dereference reads a struct page derived
    from the zero-page PFN before the early return discards it.
  - Audited remaining dax_to_folio() / dax_to_pfn() call sites in fs/dax.c;
    no further instances of the pattern.
  - Updated the page_folio() expansion in the commit message to refer to
    the current field name (page->compound_info via _compound_head()).

v1: https://lore.kernel.org/all/20260501233933.2614302-1-souvik@amlalabs.com/

 fs/dax.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 6d175cd47a99..4bca6e2bc342 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -480,11 +480,12 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 				unsigned long address, bool shared)
 {
 	unsigned long size = dax_entry_size(entry), index;
-	struct folio *folio = dax_to_folio(entry);
+	struct folio *folio;
 
 	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry))
 		return;
 
+	folio = dax_to_folio(entry);
 	index = linear_page_index(vma, address & ~(size - 1));
 	if (shared && (folio->mapping || dax_folio_is_shared(folio))) {
 		if (folio->mapping)
@@ -505,21 +506,23 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 				bool trunc)
 {
-	struct folio *folio = dax_to_folio(entry);
+	struct folio *folio;
 
 	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry))
 		return;
 
+	folio = dax_to_folio(entry);
 	dax_folio_put(folio);
 }
 
 static struct page *dax_busy_page(void *entry)
 {
-	struct folio *folio = dax_to_folio(entry);
+	struct folio *folio;
 
 	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry))
 		return NULL;
 
+	folio = dax_to_folio(entry);
 	if (folio_ref_count(folio) - folio_mapcount(folio))
 		return &folio->page;
 	else
-- 
2.51.1