Re: [PATCH] mm/swapfile: unuse_pte can map random data if swap read fails

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 04, 2022 at 03:37:36PM +0200, David Hildenbrand wrote:
> On 01.04.22 09:29, Miaohe Lin wrote:
> > There is a bug in unuse_pte(): when swap page happens to be unreadable,
> > page filled with random data is mapped into user address space. The fix
> > is to check for PageUptodate and fail swapoff in case of error.
> > 
> > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > ---
> >  mm/swapfile.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 63c61f8b2611..e72a35de7a0f 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1795,6 +1795,10 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
> >  		ret = 0;
> >  		goto out;
> >  	}
> > +	if (unlikely(!PageUptodate(page))) {
> > +		ret = -EIO;
> > +		goto out;
> > +	}
> 
> Yeah, we have the same handling in do_swap_page(), whereby we send a
> SIGBUS because we're dealing with an actual access.
> 
> Interestingly, folio_test_uptodate() states:
> 
> "Anonymous and CoW folios are always uptodate."
> 
> @Willy, is that true or is the swapin case not documented there?

Why do we keep a !Uptodate page in the swap cache?  If it can't be
read in from swap, I thought we just freed the page.  Since Miaohe
has observed that not happening, I guess it doesn't work that way,
but why not make it work that way?