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From:   Brian Geffon <bgeffon@google.com>
Date:   Wed, 6 May 2020 10:32:18 -0700
Message-ID: <CADyq12xRzr53hM7_ejEG-O8kDF5XBdrLM7yWVpbByd8pOWsTrA@mail.gmail.com>
Subject: Re: [PATCH] userfaultfd: fix remap event with MREMAP_DONTUNMAP.
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Sonny Rao <sonnyrao@google.com>,
        Minchan Kim <minchan@kernel.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>
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It hasn't landed in a stable kernel yet, 5.7 is rc4 so I don't think
it needs to cc stable, right?

Andrew, I'd be happy to mail a new patch if necessary, otherwise here
is the fixes:

Fixes: e346b38 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")

Brian


On Wed, May 6, 2020 at 10:28 AM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Wed, May 6, 2020 at 1:22 PM Brian Geffon <bgeffon@google.com> wrote:
> >
> > A user is not required to set a new address when using
> > MREMAP_DONTUNMAP as it can be used without MREMAP_FIXED.
> > When doing so the remap event will use new_addr which may not
> > have been set and we didn't propagate it back other then
> > in the return value of remap_to.
> >
> > Because ret is always the new address it's probably more
> > correct to use it rather than new_addr on the remap_event_complete
> > call, and it resolves this bug.
> >
>
> Does it need Fixes: tag, and CC to stable? Going into a stable kernel
> will mean the stable kernel merges into ChromeOS also gets it.
>
> thanks,
>
>  - Joel
>
> > Signed-off-by: Brian Geffon <bgeffon@google.com>
> > ---
> >  mm/mremap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index c881abeba0bf..6aa6ea605068 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -794,7 +794,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
> >         if (locked && new_len > old_len)
> >                 mm_populate(new_addr + old_len, new_len - old_len);
> >         userfaultfd_unmap_complete(mm, &uf_unmap_early);
> > -       mremap_userfaultfd_complete(&uf, addr, new_addr, old_len);
> > +       mremap_userfaultfd_complete(&uf, addr, ret, old_len);
> >         userfaultfd_unmap_complete(mm, &uf_unmap);
> >         return ret;
> >  }
> > --
> > 2.26.2.526.g744177e7f7-goog
> >