Re: [PATCH] mm/swap: abort swapoff after disk error

[Konstantin Khlebnikov]

Hugh Dickins wrote:
> On Fri, 14 Dec 2012, Konstantin Khlebnikov wrote:
>
> > Content of non-uptodate pages completely random, we cannot expose them into
> > userspace. This leads to information leak and will crash userspace for sure.
>
> Good find, yes, it's very wrong as is.  But, sorry, I don't like your fix
> - better than ignoring the issue as at present, but not the right answer.
>
> > Probably we can reuse hwpoison entries here, but tmpfs already too complex.
>
> HWpoison entries?  They're for when that page of RAM is bad, but this is
> quite a different case: the page is fine and can perfectly well be freed
> and reused - what's bad is the data currently in it.
>
> > Signed-off-by: Konstantin Khlebnikov<khlebnikov@xxxxxxxxxx>
> > Original-patch-by: Alexey Kuznetsov<kuznet@xxxxxxxxxxxxx>
> > Cc: Andrew Morton<akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Hugh Dickins<hughd@xxxxxxxxxx>
> > Cc: Andi Kleen<andi@xxxxxxxxxxxxxx>
> > ---
> >   mm/swapfile.c |   16 ++++++++++++++++
> >   1 file changed, 16 insertions(+)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index e97a0e5..98fc2fd 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1127,6 +1127,22 @@ int try_to_unuse(unsigned int type, bool frontswap,
> >   		wait_on_page_writeback(page);
> >
> >   		/*
> > +		 * If read failed we cannot map not-uptodate page to
> > +		 * user space. Actually, we are in serious troubles,
> > +		 * we do not even know what process to kill. So, the only
>
> try_to_unuse() is all about locating exactly where this page belongs;
> and if the user is lucky, the page in question won't even be needed again
> before the process exits, so nothing should be killed at this point.
>
>
> > +		 * variant remains: to stop swapoff() and allow someone
> > +		 * to kill processes to zap invalid pages.
>
> No, we should not abort swapoff: there's every reason to continue,
> to make sure that this unreliable area can be taken out of service.
>
> > +		 *
> > +		 * TODO replace page with hwpoison entry in pte and shmem.
>
> Instead of blindly going ahead and inserting ptes pointing to the
> !PageUptodate page, unuse_pte() and shmem_unuse_inode() should insert
> a substitute bad swapentry, to generate SIGBUS if it's accessed.
>
> swp_entry(1, 0) might serve, but there's probably a few mods needed
> here and there; and getting the details right (e.g. memcg charges)
> will need care.
>
> Not as straightforward as your block below, I admit.  I wonder if you
> posted that just to stir me to do better: or can you take it further?

I found this patch in our kernel tree. For some reason it wasn't sent
to mainline. So I decided to send it as is to not lose it for a few more
years. Using here hwpoison was just a guess. Your bad-swap-entry is much
more accurate solution. Seems like here is no rush, this bug was here from
the beginning, so I'll handle it. Thanks for your advice.

> Thanks,
> Hugh
>
> > +		 */
> > +		if (unlikely(!PageUptodate(page))) {
> > +			unlock_page(page);
> > +			page_cache_release(page);
> > +			retval = -EIO;
> > +			break;
> > +		}
> > +
> > +		/*
> >   		 * Remove all references to entry.
> >   		 */
> >   		swcount = *swap_map;
--
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