[PATCH] 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))), and page_folio()
performs READ_ONCE(page->compound_head) -- 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 dax_disassociate_entry().  Apply the same
treatment to dax_busy_page(), which has the identical pattern but
was not touched by the prior fix.

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>
Signed-off-by: Souvik Banerjee <souvik@amlalabs.com>
---
 fs/dax.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 6d175cd47a99..6878473265bb 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -505,21 +505,23 @@ 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