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)
Date: Mon, 24 Apr 2023 14:21:13 +0100 [thread overview]
Message-ID: <ZEaCSXG4UTGlHDam@casper.infradead.org> (raw)
In-Reply-To: <CACT4Y+YKt-YvQ5fKimXAP8nsV=X81OymPd3pxVXvmPG-51YjOw@mail.gmail.com>
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().
next prev parent reply other threads:[~2023-04-24 13:21 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-24 7:19 [syzbot] [fs?] [mm?] KCSAN: data-race in __filemap_remove_folio / folio_mapping (2) syzbot
2023-04-24 7:38 ` Dmitry Vyukov
2023-04-24 13:21 ` Matthew Wilcox [this message]
2023-04-24 13:49 ` Dmitry Vyukov
2023-04-24 14:10 ` Matthew Wilcox
2023-04-24 14:21 ` Dmitry Vyukov
2024-04-18 4:27 ` Jeongjun Park
2024-04-18 4:27 ` syzbot
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