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

[Linus Torvalds]

On Thu, Apr 1, 2021 at 11:17 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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.

        Linus