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Date:   Mon, 24 Apr 2023 14:21:13 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     syzbot <syzbot+606f94dfeaaa45124c90@syzkaller.appspotmail.com>,
        djwong@kernel.org, hch@infradead.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-xfs@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [fs?] [mm?] KCSAN: data-race in __filemap_remove_folio
 / folio_mapping (2)
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 <CACT4Y+YKt-YvQ5fKimXAP8nsV=X81OymPd3pxVXvmPG-51YjOw@mail.gmail.com>
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On Mon, Apr 24, 2023 at 09:38:43AM +0200, Dmitry Vyukov wrote:
> On Mon, 24 Apr 2023 at 09:19, syzbot
> <syzbot+606f94dfeaaa45124c90@syzkaller.appspotmail.com> wrote:
> If I am reading this correctly, it can lead to NULL derefs in
> folio_mapping() if folio->mapping is read twice. I think
> folio->mapping reads/writes need to use READ/WRITE_ONCE if racy.

You aren't reading it correctly.

        mapping = folio->mapping;
        if ((unsigned long)mapping & PAGE_MAPPING_FLAGS)
                return NULL;

        return mapping;

The racing write is storing NULL.  So it might return NULL or it might
return the old mapping, or it might return NULL.  Either way, the caller
has to be prepared for NULL to be returned.

It's a false posiive, but probably worth silencing with a READ_ONCE().