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

[Dave Jiang <dave.jiang@intel.com>]

On 5/1/26 4:39 PM, 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))), 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>

Reviewed-by: Dave Jiang <dave.jiang@intel.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