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Date:   Tue, 17 May 2022 13:36:43 +0100
From:   Lee Jones <lee.jones@linaro.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [REPORT] Use-after-free Read in __fdget_raw in v5.10.y
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On Tue, 17 May 2022, Jens Axboe wrote:

> On 5/17/22 6:24 AM, Lee Jones wrote:
> > On Tue, 17 May 2022, Jens Axboe wrote:
> > 
> >> On 5/17/22 5:41 AM, Lee Jones wrote:
> >>> Good afternoon Jens, Pavel, et al.,
> >>>
> >>> Not sure if you are presently aware, but there appears to be a
> >>> use-after-free issue affecting the io_uring worker driver (fs/io-wq.c)
> >>> in Stable v5.10.y.
> >>>
> >>> The full sysbot report can be seen below [0].
> >>>
> >>> The C-reproducer has been placed below that [1].
> >>>
> >>> I had great success running this reproducer in an infinite loop.
> >>>
> >>> My colleague reverse-bisected the fixing commit to:
> >>>
> >>>   commit fb3a1f6c745ccd896afadf6e2d6f073e871d38ba
> >>>   Author: Jens Axboe <axboe@kernel.dk>
> >>>   Date:   Fri Feb 26 09:47:20 2021 -0700
> >>>
> >>>        io-wq: have manager wait for all workers to exit
> >>>
> >>>        Instead of having to wait separately on workers and manager, just have
> >>>        the manager wait on the workers. We use an atomic_t for the reference
> >>>        here, as we need to start at 0 and allow increment from that. Since the
> >>>        number of workers is naturally capped by the allowed nr of processes,
> >>>        and that uses an int, there is no risk of overflow.
> >>>
> >>>        Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>
> >>>     fs/io-wq.c | 30 ++++++++++++++++++++++--------
> >>>     1 file changed, 22 insertions(+), 8 deletions(-)
> >>
> >> Does this fix it:
> >>
> >> commit 886d0137f104a440d9dfa1d16efc1db06c9a2c02
> >> Author: Jens Axboe <axboe@kernel.dk>
> >> Date:   Fri Mar 5 12:59:30 2021 -0700
> >>
> >>     io-wq: fix race in freeing 'wq' and worker access
> >>
> >> Looks like it didn't make it into 5.10-stable, but we can certainly
> >> rectify that.
> > 
> > Thanks for your quick response Jens.
> > 
> > This patch doesn't apply cleanly to v5.10.y.
> 
> This is probably why it never made it into 5.10-stable :-/

Right.  It doesn't apply at all unfortunately.

> > I'll have a go at back-porting it.  Please bear with me.
> 
> Let me know if you into issues with that and I can help out.

I think the dependency list is too big.

Too much has changed that was never back-ported.

Actually the list of patches pertaining to fs/io-wq.c alone isn't so
bad, I did start to back-port them all but some of the big ones have
fs/io_uring.c changes incorporated and that list is huge (256 patches
from v5.10 to the fixing patch mentioned above).

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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