Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"

From: Suren Baghdasaryan <surenb@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>, stable <stable@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jann Horn <jannh@google.com>, Kirill Tkhai <ktkhai@virtuozzo.com>,
	Shaohua Li <shli@fb.com>, Nadav Amit <namit@vmware.com>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"
Date: Thu, 1 Apr 2021 12:43:51 -0700	[thread overview]
Message-ID: <CAJuCfpE48zkcM_2GABBpXssjmivKLt+r8+CEeXafqK3VNMMjDw@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wg8MDMLi8x+u-dee-ai0KiAavm6+JceV00gRXQRFG=Cgw@mail.gmail.com>

On Thu, Apr 1, 2021 at 11:59 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Apr 1, 2021 at 11:17 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > We received a report that the copy-on-write issue repored by Jann Horn in
> > https://bugs.chromium.org/p/project-zero/issues/detail?id=2045 is still
> > reproducible on 4.14 and 4.19 kernels (the first issue with the reproducer
> > coded in vmsplice.c).
>
> Gaah.
>
> > I confirmed this and also that the issue was not
> > reproducible with 5.10 kernel. I tracked the fix to the following patch
> > introduced in 5.9 which changes the do_wp_page() logic:
> >
> > 09854ba94c6a 'mm: do_wp_page() simplification'
>
> The problem here is that there's a _lot_ more patches than the few you
> found that fixed various other cases (THP etc).
>
> > I backported this patch (#2 in the series) along with 2 prerequisite patches
> > (#1 and #4) that keep the backports clean and two followup fixes to the main
> > patch (#3 and #5). I had to skip the following fix:
> >
> > feb889fb40fa 'mm: don't put pinned pages into the swap cache'
> >
> > because it uses page_maybe_dma_pinned() which does not exists in earlier
> > kernels. Because pin_user_pages() does not exist there as well, I *think*
> > we can safely skip this fix on older kernels, but I would appreciate if
> > someone could confirm that claim.
>
> Hmm. I think this means that swap activity can now break the
> connection to a GUP page (the whole pre-pinning model), but it
> probably isn't a new problem for 4.9/4.19.
>
> I suspect the test there should be something like
>
>         /* Single mapper, more references than us and the map? */
>         if (page_mapcount(page) == 1 && page_count(page) > 2)
>                 goto keep_locked;
>
> in the pre-pinning days.
>
> But I really think that there are a number of other commits you're
> missing too, because we had a whole series for THP fixes for the same
> exact issue.
>
> Added Peter Xu to the cc, because he probably tracked those issues
> better than I did.
>
> So NAK on this for now, I think this limited patch-set likely
> introduces more problems than it fixes.

Thanks for confirming my worries. I'll be happy to add additional
backports if Peter can point me to them.
Thanks,
Suren.

>
>         Linus