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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/11/26 23:40, Souvik Banerjee wrote:
> 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>

Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David