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

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Date:   Wed, 21 Apr 2021 16:05:23 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Peter Xu <peterx@...hat.com>, stable <stable@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, Shaohua Li <shli@...com>,
        Nadav Amit <namit@...are.com>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"

On Wed, Apr 21, 2021 at 3:59 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 4/21/21 10:01 PM, Suren Baghdasaryan wrote:
> > On Wed, Apr 7, 2021 at 2:53 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
> >>
> >> On Wed, Apr 7, 2021 at 12:23 PM Linus Torvalds
> >> <torvalds@...ux-foundation.org> wrote:
> >> >
> >> > On Wed, Apr 7, 2021 at 11:47 AM Mikulas Patocka <mpatocka@...hat.com> wrote:
> >> > >
> >> > > So, we fixed it, but we don't know why.
> >> > >
> >> > > Peter Xu's patchset that fixed it is here:
> >> > > https://lore.kernel.org/lkml/20200821234958.7896-1-peterx@redhat.com/
> >> >
> >> > Yeah, that's the part that ends up being really painful to backport
> >> > (with all the subsequent fixes too), so the 4.14 people would prefer
> >> > to avoid it.
> >> >
> >> > But I think that if it's a "requires dax pmem and ptrace on top", it
> >> > may simply be a non-issue for those users. Although who knows - maybe
> >> > that ends up being a real issue on Android..
> >>
> >> A lot to digest, so I need to do some reading now. Thanks everyone!
> >
> > After a delay due to vacation I prepared backports of 17839856fd58
> > ("gup: document and work around "COW can break either way" issue") for
> > 4.14 and 4.19 kernels. As Linus pointed out, uffd-wp was introduced
> > later in 5.7, so is not an issue for 4.x kernels. The issue with THPs
> > is still unresolved, so with or without this patch it's still there
> > (Android is not affected by this since we do not use THPs with older
> > kernels).
>
> Which THP issue do you mean here? The race that was part of the same Project
> zero report and was solved by a different patch adding some locking? Or the
> vmsplice info leak but applied to THP's? Because if it's the latter then I
> believe 17839856fd58 did solve that too. It was the later switch of approach to
> rely just on page_count() that left THP side unfixed.

I meant the "vmsplice info leak applied to THP's" but now I realize
that 17839856fd58 does not use elevated reference count, so indeed
that should not be a problem. Thanks for the note!

>
> > Andrea pointed out that there are other issues and to properly fix
> > them his COR approach is needed. However it has not been accepted yet,
> > so I can't really backport it. I'll be happy to do that though if it
> > is accepted in the future.
> >
> > Peter, you mentioned https://lkml.org/lkml/2020/8/10/439 patch to
> > distinguish real writes vs enforced COW read requests, however I also
> > see that you had a later version of this patch here:
> > https://lore.kernel.org/patchwork/patch/1286506/. Which one should I
> > backport? Or is it not needed in the absence of uffd-wp support in the
> > earlier kernels?
> > Thanks,
> > Suren.
> >
> >>
> >> >
> >> >             Linus
> >
>