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Date:   Fri, 22 Nov 2019 11:34:11 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Jens Axboe <axboe@kernel.dk>,
        syzbot+21cfe1f803e0e158acf1@syzkaller.appspotmail.com,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] block: Replace bio_check_ro()'s WARN_ON()
Message-ID: <201911221131.A34DFAA49@keescook>
References: <20180824211535.GA22251@beast>
 <201911221052.0FDE1A1@keescook>
 <20191122190707.GA2136@infradead.org>
 <94976fb5-12d3-557d-7f31-347d6116b18c@kernel.dk>
 <20191122191434.GA10150@infradead.org>
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On Fri, Nov 22, 2019 at 11:14:34AM -0800, Christoph Hellwig wrote:
> On Fri, Nov 22, 2019 at 12:09:14PM -0700, Jens Axboe wrote:
> > On 11/22/19 12:07 PM, Christoph Hellwig wrote:
> > > On Fri, Nov 22, 2019 at 10:53:22AM -0800, Kees Cook wrote:
> > >> Friendly ping! I keep tripping over this. Can this please get applied so
> > >> we can silence syzbot and avoid needless WARNs? :)
> > > 
> > > What call stack reaches this?  Upper layers should never submit a write
> > > bio on a read-only queue, and we need to fix that in the upper layer.
> > 
> > It's an fsync, the trace is here:
> > 
> > https://syzkaller.appspot.com/x/log.txt?x=159503d2e00000
> 
> Oh.  I think this is a bug in the block layer, we should not treat
> a sync as write for the purposes of is read-only checks, as it never
> writes data to the device.  At the request layer we alread use
> the proper REQ_OP_FLUSH, but at the bio layer we are still abusing
> empty writes apparently.  I'll try to cook up something over the
> weekend.

Cool; thanks! Note that syzbot has a reproducer for it:
https://syzkaller.appspot.com/text?tag=ReproC&x=117ccc8c400000

If that doesn't work for your own testing, you can ask syzbot to test
patches itself:
https://goo.gl/tpsmEJ#testing-patches

-- 
Kees Cook