linux-kernel - Re: [REPORT] Use-after-free Read in __fdget

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Date:   Tue, 17 May 2022 13:24:12 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [REPORT] Use-after-free Read in __fdget_raw in v5.10.y

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@...nel.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@...nel.dk>
> > 
> >     fs/io-wq.c | 30 ++++++++++++++++++++++--------
> >     1 file changed, 22 insertions(+), 8 deletions(-)
> 
> Does this fix it:
> 
> commit 886d0137f104a440d9dfa1d16efc1db06c9a2c02
> Author: Jens Axboe <axboe@...nel.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.

I'll have a go at back-porting it.  Please bear with me.

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