From: Matthew Wilcox <willy@infradead.org>
To: Pavel Begunkov <asml.silence@gmail.com>
Cc: syzbot <syzbot+77efce558b2b9e6b6405@syzkaller.appspotmail.com>,
axboe@kernel.dk, io-uring@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk
Subject: Re: KASAN: use-after-free Read in __io_uring_files_cancel
Date: Fri, 9 Oct 2020 13:35:37 +0100 [thread overview]
Message-ID: <20201009123537.GR20115@casper.infradead.org> (raw)
In-Reply-To: <6da35bc9-d072-c18b-2268-15d37fa786df@gmail.com>
On Fri, Oct 09, 2020 at 03:28:54PM +0300, Pavel Begunkov wrote:
> On 09/10/2020 15:12, Matthew Wilcox wrote:
> >> It seems this fails on "node->shift" in xas_next_entry(), that would
> >> mean that the node itself was freed while we're iterating on it.
> >>
> >> __io_uring_files_cancel() iterates with xas_next_entry() and creates
> >> XA_STATE once by hand, but it also removes entries in the loop with
> >> io_uring_del_task_file() -> xas_store(&xas, NULL); without updating
> >> the iterating XA_STATE. Could it be the problem? See a diff below
> >
> > No, the problem is that the lock is dropped after calling
> > xas_next_entry(), and at any point after calling xas_next_entry(),
> > the node that it's pointing to can be freed.
>
> Only the task itself clears/removes entries, others can only insert.
> So, could it be freed even though there are no parallel erases?
Not with today's implementation, but that's something that might
change in the future. I agree it's probably the task itself that's
deleting the entry and causing the node to be deleted.
next prev parent reply other threads:[~2020-10-09 12:35 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-09 8:02 KASAN: use-after-free Read in __io_uring_files_cancel syzbot
2020-10-09 11:10 ` Pavel Begunkov
2020-10-09 12:12 ` Matthew Wilcox
2020-10-09 12:28 ` Pavel Begunkov
2020-10-09 12:35 ` Matthew Wilcox [this message]
2020-10-09 12:48 ` Pavel Begunkov
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