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: [email protected] # v6.15+
Cc: Alistair Popple <[email protected]>
Signed-off-by: Souvik Banerjee <[email protected]>
---
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
