[v2,4/5] mm: Fix page reference leak in soft_offline_page()

The conversion to move pfn_to_online_page() internal to
soft_offline_page() missed that the get_user_pages() reference needs to
be dropped when pfn_to_online_page() fails.

When soft_offline_page() is handed a pfn_valid() &&
!pfn_to_online_page() pfn the kernel hangs at dax-device shutdown due to
a leaked reference.

Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 mm/memory-failure.c |   20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

On 12.01.21 10:34, Dan Williams wrote:
> The conversion to move pfn_to_online_page() internal to
> soft_offline_page() missed that the get_user_pages() reference needs to
> be dropped when pfn_to_online_page() fails.
> 
> When soft_offline_page() is handed a pfn_valid() &&
> !pfn_to_online_page() pfn the kernel hangs at dax-device shutdown due to
> a leaked reference.
> 
> Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  mm/memory-failure.c |   20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5a38e9eade94..78b173c7190c 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1885,6 +1885,12 @@ static int soft_offline_free_page(struct page *page)
>  	return rc;
>  }
>  
> +static void put_ref_page(struct page *page)
> +{
> +	if (page)
> +		put_page(page);
> +}
> +
>  /**
>   * soft_offline_page - Soft offline a page.
>   * @pfn: pfn to soft-offline
> @@ -1910,20 +1916,26 @@ static int soft_offline_free_page(struct page *page)
>  int soft_offline_page(unsigned long pfn, int flags)
>  {
>  	int ret;
> -	struct page *page;
>  	bool try_again = true;
> +	struct page *page, *ref_page = NULL;
> +
> +	WARN_ON_ONCE(!pfn_valid(pfn) && (flags & MF_COUNT_INCREASED));
>  
>  	if (!pfn_valid(pfn))
>  		return -ENXIO;
> +	if (flags & MF_COUNT_INCREASED)
> +		ref_page = pfn_to_page(pfn);
> +
>  	/* Only online pages can be soft-offlined (esp., not ZONE_DEVICE). */
>  	page = pfn_to_online_page(pfn);
> -	if (!page)
> +	if (!page) {
> +		put_ref_page(ref_page);
>  		return -EIO;
> +	}
>  
>  	if (PageHWPoison(page)) {
>  		pr_info("%s: %#lx page already poisoned\n", __func__, pfn);
> -		if (flags & MF_COUNT_INCREASED)
> -			put_page(page);
> +		put_ref_page(ref_page);
>  		return 0;
>  	}

Reviewed-by: David Hildenbrand <david@redhat.com>

On Tue, Jan 12, 2021 at 1:54 AM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Tue, Jan 12, 2021 at 01:34:58AM -0800, Dan Williams wrote:
> > The conversion to move pfn_to_online_page() internal to
> > soft_offline_page() missed that the get_user_pages() reference needs to
> > be dropped when pfn_to_online_page() fails.
>
> I would be more specific here wrt. get_user_pages (madvise).
> soft_offline_page gets called from more places besides madvise_*.

Sure.

>
> > When soft_offline_page() is handed a pfn_valid() &&
> > !pfn_to_online_page() pfn the kernel hangs at dax-device shutdown due to
> > a leaked reference.
> >
> > Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>
> LGTM, thanks for catching this:
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
>
> A nit below.
>
> > ---
> >  mm/memory-failure.c |   20 ++++++++++++++++----
> >  1 file changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index 5a38e9eade94..78b173c7190c 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -1885,6 +1885,12 @@ static int soft_offline_free_page(struct page *page)
> >       return rc;
> >  }
> >
> > +static void put_ref_page(struct page *page)
> > +{
> > +     if (page)
> > +             put_page(page);
> > +}
>
> I am not sure this warrants a function.
> I would probably go with "if (ref_page).." in the two corresponding places,
> but not feeling strong here.

I'll take another look, it felt cluttered...

>
> > +
> >  /**
> >   * soft_offline_page - Soft offline a page.
> >   * @pfn: pfn to soft-offline
> > @@ -1910,20 +1916,26 @@ static int soft_offline_free_page(struct page *page)
> >  int soft_offline_page(unsigned long pfn, int flags)
> >  {
> >       int ret;
> > -     struct page *page;
> >       bool try_again = true;
> > +     struct page *page, *ref_page = NULL;
> > +
> > +     WARN_ON_ONCE(!pfn_valid(pfn) && (flags & MF_COUNT_INCREASED));
>
> Did you see any scenario where this could happen? I understand that you are
> adding this because we will leak a reference in case pfn is not valid anymore.
>

I did not, more future proofing / documenting against refactoring that
fails to consider that case.